Plant vs animal protein digestibility?

The protein scoring methodologies ( rate plant proteins of a lower quality than animal proteins. Now I can understand the reasoning that proteins have a lesser quantity of essential amino acids (allegedly) making them "lower quality" in terms of protein; but I don't see why there should be a difference in digestibility between plant and animal proteins (I can give links but it's common knowledge that plant proteins are less digestible). What is the basis for this difference in digestibility ? Whatever the amino acid composition, I don't see how that would affect digestion of proteins. So why are plant proteins considered less digestible ?

The Skeletal Muscle Anabolic Response to Plant- versus Animal-Based Protein Consumption

Clinical and consumer market interest is increasingly directed toward the use of plant-based proteins as dietary components aimed at preserving or increasing skeletal muscle mass. However, recent evidence suggests that the ingestion of the plant-based proteins in soy and wheat results in a lower muscle protein synthetic response when compared with several animal-based proteins. The possible lower anabolic properties of plant-based protein sources may be attributed to the lower digestibility of plant-based sources, in addition to greater splanchnic extraction and subsequent urea synthesis of plant protein-derived amino acids compared with animal-based proteins. The latter may be related to the relative lack of specific essential amino acids in plant- as opposed to animal-based proteins. Furthermore, most plant proteins have a relatively low leucine content, which may further reduce their anabolic properties when compared with animal proteins. However, few studies have actually assessed the postprandial muscle protein synthetic response to the ingestion of plant proteins, with soy and wheat protein being the primary sources studied. Despite the proposed lower anabolic properties of plant vs. animal proteins, various strategies may be applied to augment the anabolic properties of plant proteins. These may include the following: 1) fortification of plant-based protein sources with the amino acids methionine, lysine, and/or leucine 2) selective breeding of plant sources to improve amino acid profiles 3) consumption of greater amounts of plant-based protein sources or 4) ingesting multiple protein sources to provide a more balanced amino acid profile. However, the efficacy of such dietary strategies on postprandial muscle protein synthesis remains to be studied. Future research comparing the anabolic properties of a variety of plant-based proteins should define the preferred protein sources to be used in nutritional interventions to support skeletal muscle mass gain or maintenance in both healthy and clinical populations.

Keywords: aging animal protein exercise muscle mass plant protein vegetarian.

The Role of the Anabolic Properties of Plant- versus Animal-Based Protein Sources in Supporting Muscle Mass Maintenance: A Critical Review

Plant-sourced proteins offer environmental and health benefits, and research increasingly includes them in study formulas. However, plant-based proteins have less of an anabolic effect than animal proteins due to their lower digestibility, lower essential amino acid content (especially leucine), and deficiency in other essential amino acids, such as sulfur amino acids or lysine. Thus, plant amino acids are directed toward oxidation rather than used for muscle protein synthesis. In this review, we evaluate the ability of plant- versus animal-based proteins to help maintain skeletal muscle mass in healthy and especially older people and examine different nutritional strategies for improving the anabolic properties of plant-based proteins. Among these strategies, increasing protein intake has led to a positive acute postprandial muscle protein synthesis response and even positive long-term improvement in lean mass. Increasing the quality of protein intake by improving amino acid composition could also compensate for the lower anabolic potential of plant-based proteins. We evaluated and discussed four nutritional strategies for improving the amino acid composition of plant-based proteins: fortifying plant-based proteins with specific essential amino acids, selective breeding, blending several plant protein sources, and blending plant with animal-based protein sources. These nutritional approaches need to be profoundly examined in older individuals in order to optimize protein intake for this population who require a high-quality food protein intake to mitigate age-related muscle loss.

Keywords: animal-based proteins critical review muscle protein synthesis older people plant-based proteins skeletal muscle.

Conflict of interest statement

The authors declare no conflict of interest.


The percentage of dietary protein…

The percentage of dietary protein intake derived from plant and animal protein sources…

What is the difference between animal and plant proteins?

Protein is an essential part of the diet. It helps to build, repair, and maintain the body’s structures. Foods derived from plants and animals can both provide protein, but there are some differences.

Protein exists throughout the body, in everything from the muscles and organs to the bones, skin, and hair. The body does not store protein like it does other macronutrients, so this protein has to come from the diet.

Proteins are made up of amino acids. A person’s body needs a balance of all 22 types of amino acids to function correctly.

The body cannot produce nine of these acids, called essential amino acids.

A complete protein source refers to a type of food that contains all nine.

Having the right balance of amino acids can build muscle and help the body to recover from exercise quickly. Understanding the differences between plant and animal proteins is important for anyone who wants to ensure that their diet is healthful.

In this article, we look at the differences between animal and plant proteins. We also investigate the effects on health, describe which type is better for bodybuilding, and list the best sources of each.

Share on Pinterest Plant and animal proteins vary in the number of amino acids they contain.

One of the main differences between plant and animal proteins involves their amino acid contents.

Amino acids are the building blocks of protein. When the body digests the proteins in food, it breaks them down into amino acids.

The body may need different amino acids at different times. Many people believe that the diet should include complete sources of protein, which contain all nine essential amino acids.

Some animal products are complete sources of protein, such as:

  • fish
  • various types of eggs
  • dairy products, such as cheese, milk, and whey
  • red meat from cows, bison, and deer
  • poultry from sources such as chickens, turkeys, and quails
  • meat from less common sources, including boars, hares, and horses

Most plant proteins are incomplete, which means that they are missing at least one of the essential amino acids.

However, some plant-based foods, such as quinoa and buckwheat, are complete sources of protein.

It is important for vegetarians and vegans to mix their protein sources and ensure that they are getting all of the essential amino acids.

Also, keep in mind that some sources of plant protein may take longer for the body to digest and use.

The following are examples of plant-based foods rich in protein:

  • grains
  • lentils
  • nuts
  • beans
  • legumes
  • certain fruits, such as avocados
  • soy
  • hemp
  • rice
  • peas

Many other nuts, grains, and vegetables also contain high amounts of protein.

When choosing between plant and animal sources of protein, it is important to factor in the other nutrients that the foods provide.

Foods rich in protein can have widely ranging nutritional profiles.

Certain sources of animal protein can contain high levels of heme iron and vitamin B-12, while some plant-based foods lack these nutrients.

On the other hand, plant-specific nutrients, called phytonutrients, and some antioxidants are absent from sources of animal protein.

Animal products contain saturated fat and higher levels of cholesterol than sources of plant protein. A person may wish to avoid animal products for these reasons.

Many used to believe that dietary cholesterol was associated with cardiovascular disease. While recent evidence suggests no significant link, the Institute of Medicine (IOM) still recommends limiting dietary cholesterol.

Fiber is another important factor. Only plant-based foods contain fiber, which helps to keep the digestive system balanced.

Eating more plant protein may also improve a person’s overall health.

Results of a 2016 meta-analysis suggested that eating more animal protein, especially that derived from processed red meat, may increase the risk of dying from cardiovascular disease.

However, researchers noted that they only found the link between animal protein and cardiovascular disease in people with at least one lifestyle-related risk factor, such as smoking, heavy alcohol intake, or being overweight or obese.

The results also indicated that eating more plant protein may help to reduce this risk and others.

In general, the best way to cover a person’s dietary needs is to eat a wide variety of foods.


The protein requirements for athletic populations have been the subject of much scientific debate. Only recently has the notion that both strength/power and endurance athletes require a greater protein consumption than the general population become generally accepted. In addition, high protein diets have also become quite popular in the general population as part of many weight reduction programs. Despite the prevalence of high protein diets in athletic and sedentary populations, information available concerning the type of protein (e.g. animal or vegetable) to consume is limited. The purpose of this paper is to examine and analyze key factors responsible for making appropriate choices on the type of protein to consume in both athletic and general populations.

Role of Protein

Proteins are nitrogen-containing substances that are formed by amino acids. They serve as the major structural component of muscle and other tissues in the body. In addition, they are used to produce hormones, enzymes and hemoglobin. Proteins can also be used as energy however, they are not the primary choice as an energy source. For proteins to be used by the body they need to be metabolized into their simplest form, amino acids. There have been 20 amino acids identified that are needed for human growth and metabolism. Twelve of these amino acids (eleven in children) are termed nonessential, meaning that they can be synthesized by our body and do not need to be consumed in the diet. The remaining amino acids cannot be synthesized in the body and are described as essential meaning that they need to be consumed in our diets. The absence of any of these amino acids will compromise the ability of tissue to grow, be repaired or be maintained.

Protein and Athletic Performance

The primary role of dietary proteins is for use in the various anabolic processes of the body. As a result, many athletes and coaches are under the belief that high intensity training creates a greater protein requirement. This stems from the notion that if more protein or amino acids were available to the exercising muscle it would enhance protein synthesis. Research has tended to support this hypothesis. Within four weeks of protein supplementation (3.3 versus 1.3 g·kg -1 럚y -1 ) in subjects’ resistance training, significantly greater gains were seen in protein synthesis and body mass in the group of subjects with the greater protein intake (Fern et al., 1991). Similarly, Lemon et al. (1992) also reported a greater protein synthesis in novice resistance trained individuals with protein intakes of 2.62 versus 0.99 g·kg -1 럚y -1 . In studies examining strength-trained individuals, higher protein intakes have generally been shown to have a positive effect on muscle protein synthesis and size gains (Lemon, 1995 Walberg et al., 1988). Tarnapolsky and colleagues (1992) have shown that for strength trained individuals to maintain a positive nitrogen balance they need to consume a protein intake equivalent to 1.8 g·kg -1 럚y -1 . This is consistent with other studies showing that protein intakes between 1.4 – 2.4 g·kg -1 럚y -1 will maintain a positive nitrogen balance in resistance trained athletes (Lemon, 1995). As a result, recommendations for strength/power athletes’ protein intake are generally suggested to be between 1.4 - 1.8 g·kg -1 럚y -1 .

Similarly, to prevent significant losses in lean tissue endurance athletes also appear to require a greater protein consumption (Lemon, 1995). Although the goal for endurance athletes is not necessarily to maximize muscle size and strength, loss of lean tissue can have a significant detrimental effect on endurance performance. Therefore, these athletes need to maintain muscle mass to ensure adequate performance. Several studies have determined that protein intake for endurance athletes should be between 1.2 – 1.4 g·kg -1 럚y -1 to ensure a positive nitrogen balance (Freidman and Lemon, 1989 Lemon, 1995 Meredith et al., 1989 Tarnopolsky et al., 1988). Evidence is clear that athletes do benefit from increased protein intake. The focus then becomes on what type of protein to take.

Protein Assessment

The composition of various proteins may be so unique that their influence on physiological function in the human body could be quite different. The quality of a protein is vital when considering the nutritional benefits that it can provide. Determining the quality of a protein is determined by assessing its essential amino acid composition, digestibility and bioavailability of amino acids (FAO/WHO, 1990). There are several measurement scales and techniques that are used to evaluate the quality of protein.

Protein Rating Scales

Numerous methods exist to determine protein quality. These methods have been identified as protein efficiency ratio, biological value, net protein utilization, and protein digestibility corrected amino acid score.

Protein Efficiency Ratio

The protein efficiency ratio (PER) determines the effectiveness of a protein through the measurement of animal growth. This technique requires feeding rats a test protein and then measuring the weight gain in grams per gram of protein consumed. The computed value is then compared to a standard value of 2.7, which is the standard value of casein protein. Any value that exceeds 2.7 is considered to be an excellent protein source. However, this calculation provides a measure of growth in rats and does not provide a strong correlation to the growth needs of humans.

Biological Value

Biological value measures protein quality by calculating the nitrogen used for tissue formation divided by the nitrogen absorbed from food. This product is multiplied by 100 and expressed as a percentage of nitrogen utilized. The biological value provides a measurement of how efficient the body utilizes protein consumed in the diet. A food with a high value correlates to a high supply of the essential amino acids. Animal sources typically possess a higher biological value than vegetable sources due to the vegetable source’s lack of one or more of the essential amino acids. There are, however, some inherent problems with this rating system. The biological value does not take into consideration several key factors that influence the digestion of protein and interaction with other foods before absorption. The biological value also measures a protein’s maximal potential quality and not its estimate at requirement levels.

Net Protein Utilization

Net protein utilization is similar to the biological value except that it involves a direct measure of retention of absorbed nitrogen. Net protein utilization and biological value both measure the same parameter of nitrogen retention, however, the difference lies in that the biological value is calculated from nitrogen absorbed whereas net protein utilization is from nitrogen ingested.

Protein Digestibility Corrected Amino Acid Score

In 1989, the Food & Agriculture Organization and World Health Organization (FAO/WHO) in a joint position stand stated that protein quality could be determined by expressing the content of the first limiting essential amino acid of the test protein as a percentage of the content of the same amino acid content in a reference pattern of essential amino acids (FAO/WHO, 1990). The reference values used were based upon the essential amino acids requirements of preschool-age children. The recommendation of the joint FAO/WHO statement was to take this reference value and correct it for true fecal digestibility of the test protein. The value obtained was referred to as the protein digestibility corrected amino acid score (PDCAAS). This method has been adopted as the preferred method for measurement of the protein value in human nutrition (Schaafsma, 2000). Table 1 provides a measure of the quantity of various proteins using these protein rating scales.

Table 1.

Protein TypeProtein Efficiency RatioBiological ValueNet Protein UtilizationProtein Digestibility Corrected Amino Acid Score
Black Beans0 00.75
Peanuts1.8 0.52
Soy protein2.274611.00
Wheat gluten0.864670.25
Whey protein3.2104921.00

Adapted from: U.S Dairy Export Council, Reference Manual for U.S. Whey Products 2nd Edition, 1999 and Sarwar, 1997.

Although the PDCAAS is currently the most accepted and widely used method, limitations still exist relating to overestimation in the elderly (likely related to references values based on young individuals), influence of ileal digestibility, and antinutritional factors (Sarwar, 1997).

Amino acids that move past the terminal ileum may be an important route for bacterial consumption of amino acids, and any amino acids that reach the colon would not likely be utilized for protein synthesis, even though they do not appear in the feces (Schaarfsma, 2000). Thus, to get truly valid measure of fecal digestibility the location at which protein synthesis is determined is important in making a more accurate determination. Thus, ileal digestibility would provide a more accurate measure of digestibility. PDCAAS, however, does not factor ileal digestibility into its equation. This is considered to be one of the shortcomings of the PDCAAS (Schaafsma 2000).

Antinutritional factors such as trypsin inhibitors, lectins, and tannins present in certain protein sources such as soybean meal, peas and fava beans have been reported to increase losses of endogenous proteins at the terminal ileum (Salgado et al., 2002). These antinutritional factors may cause reduced protein hydrolysis and amino acid absorption. This may also be more effected by age, as the ability of the gut to adapt to dietary nutritional insults may be reduced as part of the aging process (Sarwar, 1997).

Protein Sources

Protein is available in a variety of dietary sources. These include foods of animal and plant origins as well as the highly marketed sport supplement industry. In the following section proteins from both vegetable and animal sources, including whey, casein, and soy will be explored. Determining the effectiveness of a protein is accomplished by determining its quality and digestibility. Quality refers to the availability of amino acids that it supplies, and digestibility considers how the protein is best utilized. Typically, all dietary animal protein sources are considered to be complete proteins. That is, a protein that contains all of the essential amino acids. Proteins from vegetable sources are incomplete in that they are generally lacking one or two essential amino acids. Thus, someone who desires to get their protein from vegetable sources (i.e. vegetarian) will need to consume a variety of vegetables, fruits, grains, and legumes to ensure consumption of all essential amino acids. As such, individuals are able to achieve necessary protein requirements without consuming beef, poultry, or dairy. Protein digestibility ratings usually involve measuring how the body can efficiently utilize dietary sources of protein. Typically, vegetable protein sources do not score as high in ratings of biological value, net protein utilization, PDCAAS, and protein efficiency ratio as animal proteins.

Animal Protein

Proteins from animal sources (i.e. eggs, milk, meat, fish and poultry) provide the highest quality rating of food sources. This is primarily due to the 𠆌ompleteness’ of proteins from these sources. Although protein from these sources are also associated with high intakes of saturated fats and cholesterol, there have been a number of studies that have demonstrated positive benefits of animal proteins in various population groups (Campbell et al., 1999 Godfrey et al., 1996 Pannemans et al., 1998).

Protein from animal sources during late pregnancy is believed to have an important role in infants born with normal body weights. Godfrey et al. (1996) examined the nutrition behavior of more than 500 pregnant women to determine the effect of nutritional intake on placental and fetal growth. They reported that a low intake of protein from dairy and meat sources during late pregnancy was associated with low birth weights.

In addition to the benefits from total protein consumption, elderly subjects have also benefited from consuming animal sources of protein. Diets consisting of meat resulted in greater gains in lean body mass compared to subjects on a lactoovovegetarian diet (Campbell et al., 1999). High animal protein diets have also been shown to cause a significantly greater net protein synthesis than a high vegetable protein diet (Pannemans et al., 1998). This was suggested to be a function of reduced protein breakdown occurring during the high animal protein diet.

There have been a number of health concerns raised concerning the risks associated with protein emanating primarily from animal sources. Primarily, these health risks have focused on cardiovascular disease (due to the high saturated fat and cholesterol consumption), bone health (from bone resorption due to sulfur-containing amino acids associated with animal protein) and other physiological system disease that will be addressed in the section on high protein diets.

Whey is a general term that typically denotes the translucent liquid part of milk that remains following the process (coagulation and curd removal) of cheese manufacturing. From this liquid, whey proteins are separated and purified using various techniques yielding different concentrations of whey proteins. Whey is one of the two major protein groups of bovine milk, accounting for 20% of the milk while casein accounts for the remainder. All of the constituents of whey protein provide high levels of the essential and branched chain amino acids. The bioactivities of these proteins possess many beneficial properties as well. Additionally, whey is also rich in vitamins and minerals. Whey protein is most recognized for its applicability in sports nutrition. Additionally, whey products are also evident in baked goods, salad dressings, emulsifiers, infant formulas, and medical nutritional formulas.

Varieties of Whey Protein

There are three main forms of whey protein that result from various processing techniques used to separate whey protein. They are whey powder, whey concentrate, and whey isolate. Table 2 provides the composition of Whey Proteins.

Table 2.

Composition (%) of whey protein forms.

ComponentWhey PowderWhey ConcentrateWhey Isolate
Protein11 – 14.525 – 8990 +
Lactose63 – 7510 – 550.5
Milk Fat1 – 1.52 – 100.5
Whey Protein Powder

Whey protein powder has many applications throughout the food industry. As an additive it is seen in food products for beef, dairy, bakery, confectionery, and snack products. Whey powder itself has several different varieties including sweet whey, acid whey (seen in salad dressings), demineralized (seen primarily as a food additive including infant formulas), and reduced forms. The demineralized and reduced forms are used in products other than sports supplements.

Whey Protein Concentrate

The processing of whey concentrate removes the water, lactose, ash, and some minerals. In addition, compared to whey isolates whey concentrate typically contains more biologically active components and proteins that make them a very attractive supplement for the athlete.

Whey Protein Isolate (WPI)

Isolates are the purest protein source available. Whey protein isolates contain protein concentrations of 90% or higher. During the processing of whey protein isolate there is a significant removal of fat and lactose. As a result, individuals who are lactose-intolerant can often safely take these products (Geiser, 2003). Although the concentration of protein in this form of whey protein is the highest, it often contain proteins that have become denatured due to the manufacturing process. The denaturation of proteins involves breaking down their structure and losing peptide bonds and reducing the effectiveness of the protein.

Whey is a complete protein whose biologically active components provide additional benefits to enhance human function. Whey protein contains an ample supply of the amino acid cysteine. Cysteine appears to enhance glutathione levels, which has been shown to have strong antioxidant properties that can assist the body in combating various diseases (Counous, 2000). In addition, whey protein contains a number of other proteins that positively effect immune function such as antimicrobial activity (Ha and Zemel, 2003). Whey protein also contains a high concentration of branched chain amino acids (BCAA) that are important for their role in the maintenance of tissue and prevention of catabolic actions during exercise. (MacLean et al., 1994).


Casein is the major component of protein found in bovine milk accounting for nearly 70-80% of its total protein and is responsible for the white color of milk. It is the most commonly used milk protein in the industry today. Milk proteins are of significant physiological importance to the body for functions relating to the uptake of nutrients and vitamins and they are a source of biologically active peptides. Similar to whey, casein is a complete protein and also contains the minerals calcium and phosphorous. Casein has a PDCAAS rating of 1.23 (generally reported as a truncated value of 1.0) (Deutz et al. 1998).

Casein exists in milk in the form of a micelle, which is a large colloidal particle. An attractive property of the casein micelle is its ability to form a gel or clot in the stomach. The ability to form this clot makes it very efficient in nutrient supply. The clot is able to provide a sustained slow release of amino acids into the blood stream, sometimes lasting for several hours (Boirie et al. 1997). This provides better nitrogen retention and utilization by the body.

Bovine Colostrum

Bovine colostrum is the “pre” milk liquid secreted by female mammals the first few days following birth. This nutrient-dense fluid is important for the newborn for its ability to provide immunities and assist in the growth of developing tissues in the initial stages of life. Evidence exists that bovine colostrum contains growth factors that stimulate cellular growth and DNA synthesis (Kishikawa et al., 1996), and as might be expected with such properties, it makes for interesting choice as a potential sports supplement.

Although bovine colostrum is not typically thought of as a food supplement, the use by strength/power athletes of this protein supplement as an ergogenic aid has become common. Oral supplementation of bovine colostrum has been demonstrated to significantly elevate insulin-like-growth factor 1 (IGF-1) (Mero et al., 1997) and enhance lean tissue accruement (Antonio et al., 2001 Brinkworth et al., 2004). However, the results on athletic performance improvement are less conclusive. Mero and colleagues (1997) reported no changes in vertical jump performance following 2-weeks of supplementation, and Brinkworth and colleagues (2004) saw no significant differences in strength following 8-weeks of training and supplementation in both trained and untrained subjects. In contrast, following 8-weeks of supplementation significant improvements in sprint performance were seen in elite hockey players (Hofman et al., 2002). Further research concerning bovine colostrum supplementation is still warranted.

Vegetable Protein

Vegetable proteins, when combined to provide for all of the essential amino acids, provide an excellent source for protein considering that they will likely result in a reduction in the intake of saturated fat and cholesterol. Popular sources include legumes, nuts and soy. Aside from these products, vegetable protein can also be found in a fibrous form called textured vegetable protein (TVP). TVP is produced from soy flour in which proteins are isolated. TVP is mainly a meat alternative and functions as a meat analog in vegetarian hot dogs, hamburgers, chicken patties, etc. It is also a low-calorie and low-fat source of vegetable protein. Vegetable sources of protein also provide numerous other nutrients such as phytochemicals and fiber that are also highly regarded in the diet diet.

Soy is the most widely used vegetable protein source. The soybean, from the legume family, was first chronicled in China in the year 2838 B.C. and was considered to be as valuable as wheat, barley, and rice as a nutritional staple. Soy’s popularity spanned several other countries, but did not gain notoriety for its nutritional value in The United States until the 1920s. The American population consumes a relatively low intake of soy protein (5g럚y -1 ) compared to Asian countries (Hasler, 2002). Although cultural differences may be partly responsible, the low protein quality rating from the PER scale may also have influenced protein consumption tendencies. However, when the more accurate PDCAAS scale is used, soy protein was reported to be equivalent to animal protein with a score of 1.0, the highest possible rating (Hasler, 2002). Soy’s quality makes it a very attractive alternative for those seeking non-animal sources of protein in their diet and those who are lactose intolerant. Soy is a complete protein with a high concentration of BCAA’s. There have been many reported benefits related to soy proteins relating to health and performance (including reducing plasma lipid profiles, increasing LDL-cholesterol oxidation and reducing blood pressure), however further research still needs to be performed on these claims.

Soy Protein Types

The soybean can be separated into three distinct categories flour, concentrates, and isolates. Soy flour can be further divided into natural or full-fat (contains natural oils), defatted (oils removed), and lecithinated (lecithin added) forms (Hasler, 2002). Of the three different categories of soy protein products, soy flour is the least refined form. It is commonly found in baked goods. Another product of soy flour is called textured soy flour. This is primarily used for processing as a meat extender. See Table 3 for protein composition of soy flour, concentrates, and isolates.

Table 3.

Protein composition of soy protein forms.

Soy concentrate was developed in the late 1960s and early 1970s and is made from defatted soybeans. While retaining most of the bean’s protein content, concentrates do not contain as much soluble carbohydrates as flour, making it more palatable. Soy concentrate has a high digestibility and is found in nutrition bars, cereals, and yogurts.

Isolates are the most refined soy protein product containing the greatest concentration of protein, but unlike flour and concentrates, contain no dietary fiber. Isolates originated around the 1950s in The United States. They are very digestible and easily introduced into foods such as sports drinks and health beverages as well as infant formulas.

Nutritional Benefits

For centuries, soy has been part of a human diet. Epidemiologists were most likely the first to recognize soy’s benefits to overall health when considering populations with a high intake of soy. These populations shared lower incidences in certain cancers, decreased cardiac conditions, and improvements in menopausal symptoms and osteoporosis in women (Hasler, 2002). Based upon a multitude of studies examining the health benefits of soy protein the American Heart Association issued a statement that recommended soy protein foods in a diet low in saturated fat and cholesterol to promote heart health (Erdman, 2000). The health benefits associated with soy protein are related to the physiologically active components that are part of soy, such as protease inhibitors, phytosterols, saponins, and isoflavones (Potter, 2000). These components have been noted to demonstrate lipid-lowering effects, increase LDL-cholesterol oxidation, and have beneficial effects on lowering blood pressure.


Of the many active components in soy products, isoflavones have been given considerably more attention than others. Isoflavones are thought to be beneficial for cardiovascular health, possibly by lowering LDL concentrations (Crouse et al., 1999) increasing LDL oxidation (Tikkanen et al., 1998) and improving vessel elasticity (Nestel et al., 1999). However, these studies have not met without conflicting results and further research is still warranted concerning the benefits of isoflavones.

Soy Benefits for Women

An additional focus of studies investigating soy supplementation has been on women’s health issues. It has been hypothesized that considering that isoflavones are considered phytoestrogens (exhibit estrogen- like effects and bind to estrogen receptors) they compete for estrogen receptor sites in breast tissue with endogenous estrogen, potentially reducing the risk for breast cancer risk (Wu et al. 1998). Still, the association between soy intake and breast cancer risk remains inconclusive. However, other studies have demonstrated positive effects of soy protein supplementation on maintaining bone mineral content (Ho et al., 2003) and reducing the severity of menopausal symptoms (Murkies et al., 1995).

High Protein Diets

Increased protein intakes and supplementation have generally been focused on athletic populations. However, over the past few years high protein diets have become a method used by the general population to enhance weight reduction. The low-carbohydrate, high protein, high fat diet promoted by Atkins may be the most popular diet used today for weight loss in the United States (Johnston et al., 2004). The basis behind this diet is that protein is associated with feelings of satiety and voluntary reductions in caloric consumption (Araya et al., 2000 Eisenstein et al., 2002). A recent study has shown that the Atkins diet can produce greater weight reduction at 3 and 6 months than a low-fat, high carbohydrate diet based upon U.S. dietary guidelines (Foster et al., 2003). However, potential health concerns have arisen concerning the safety of high protein diets. In 2001, the American Heart Association published a statement on dietary protein and weight reduction and suggested that individuals following such a diet may be at potential risk for metabolic, cardiac, renal, bone and liver diseases (St. Jeor et al., 2001).

Protein Intake and Metabolic Disease Risk

One of the major concerns for individuals on high protein, low carbohydrate diets is the potential for the development of metabolic ketosis. As carbohydrate stores are reduced the body relies more upon fat as its primary energy source. The greater amount of free fatty acids that are utilized by the liver for energy will result in a greater production and release of ketone bodies in the circulation. This will increase the risk for metabolic acidosis and can potentially lead to a coma and death. A recent multi-site clinical study (Foster et al., 2003) examined the effects of low-carbohydrate, high protein diets and reported significant elevation in ketone bodies during the first three months of the study. However, as the study duration continued the percentage of subjects with positive urinary ketone concentrations became reduced, and by six months urinary ketones were not present in any of the subjects.

Dietary Protein and Cardiovascular Disease Risk

High protein diets have also been suggested to have negative effects on blood lipid profiles and blood pressure, causing an increase risk for cardiovascular disease. This is primarily due to the higher fat intakes associated with these diets. However, this has not been proven in any scientifically controlled studies. Hu et al., (1999) have reported an inverse relationship between dietary protein (animal and vegetable) and risk of cardiovascular disease in women, and Jenkins and colleagues (2001) reported a decrease in lipid profiles in individuals consuming a high protein diet. Furthermore, protein intake has been shown to often have a negative relationship with blood pressure (Obarzanek et al., 1996). Thus, the concern for elevated risk for cardiovascular disease from high protein diets appears to be without merit. Likely, the reduced body weight associated with this type of diet is facilitating these changes.

In strength/power athletes who consume high protein diets, a major concern was the amount of food being consumed that was high in saturated fats. However, through better awareness and nutritional education many of these athletes are able to obtain their protein from sources that minimizes the amount of fat consumed. For instance, removing the skin from chicken breast, consuming fish and lean beef, and egg whites. In addition, many protein supplements are available that contain little to no fat. It should be acknowledged though that if elevated protein does come primarily from meats, dairy products and eggs, without regard to fat intake, there likely would be an increase in the consumption of saturated fat and cholesterol.

Dietary Protein and Renal Function

The major concern associated with renal function was the role that the kidneys have in nitrogen excretion and the potential for a high protein diet to over-stress the kidneys. In healthy individuals there does not appear to be any adverse effects of a high protein diet. In a study on bodybuilders consuming a high protein (2.8 g·kg -1 ) diet no negative changes were seen in any kidney function tests (Poortsman and Dellalieux, 2000). However, in individuals with existing kidney disease it is recommended that they limit their protein intake to approximately half of the normal RDA level for daily protein intake (0.8 g·kg -1 럚y -1 ). Lowering protein intake is thought to reduce the progression of renal disease by decreasing hyperfiltration (Brenner et al., 1996).

Dietary Protein and Bone

High protein diets are associated with an increase in calcium excretion. This is apparently due to a consumption of animal protein, which is higher in sulfur-based amino acids than vegetable proteins (Remer and Manz, 1994 Barzel and Massey, 1998). Sulfur-based amino acids are thought to be the primary cause of calciuria (calcium loss). The mechanism behind this is likely related to the increase in acid secretion due to the elevated protein consumption. If the kidneys are unable to buffer the high endogenous acid levels, other physiological systems will need to compensate, such as bone. Bone acts as a reservoir of alkali, and as a result calcium is liberated from bone to buffer high acidic levels and restore acid-base balance. The calcium released by bone is accomplished through osteoclast-mediated bone resorption (Arnett and Spowage, 1996). Bone resorption (loss or removal of bone) will cause a decline in bone mineral content and bone mass (Barzel, 1976), increasing the risk for bone fracture and osteoporosis.

The effect of the type of protein consumed on bone resorption has been examined in a number of studies. Sellmeyer and colleagues (2001) examined the effects of various animal-to- vegetable protein ratio intakes in elderly women (> 65 y). They showed that the women consuming the highest animal to vegetable protein ratio had nearly a 4-fold greater risk of hip fractures compared with women consuming a lower animal to vegetable protein ratio. Interestingly, they did not report any significant association between the animal to vegetable protein ratio and bone mineral density. Similar results were shown by Feskanich et al (1996), but in a younger female population (age range = 35 – 59 mean 46). In contrast, other studies examining older female populations have shown that elevated animal protein will increase bone mineral density, while increases in vegetable protein will have a lowering effect on bone mineral density (Munger et al., 1999 Promislow et al., 2002). Munger and colleagues (1999) also reported a 69% lower risk of hip fracture as animal protein intake increased in a large (32,000) postmenopausal population. Other large epidemiological studies have also confirmed elevated bone density following high protein diets in both elderly men and women (Dawson-Hughes et al., 2002 Hannan et al., 2000). Hannon and colleagues (2000) demonstrated that animal protein intake in an older population, several times greater than the RDA requirement, results in a bone density accruement and significant decrease in fracture risk. Dawson-Hughes et al (2002), not only showed that animal protein will not increase urinary calcium excretion, but was also associated with higher levels of IGF-I and lower concentrations of the bone resorption marker N-telopeptide.

These conflicting results have contributed to the confusion regarding protein intake and bone. It is likely that other factors play an important role in further understanding the influence that dietary proteins have on bone loss or gain. For instance, the intake of calcium may have an essential function in maintaining bone. A higher calcium intake results in more absorbed calcium and may offset the losses induced by dietary protein and reduce the adverse effect of the endogenous acidosis on bone resorption (Dawson-Hughes, 2003). Furthermore, it is commonly assumed that animal proteins have a higher content of sulfur-containing amino acids per g of protein. However, examination of Table 4 shows that this may not entirely correct. If protein came from wheat sources it would have a mEq of 0.69 per g of protein, while protein from milk contains 0.55 mEq per g of protein. Thus, some plant proteins may have a greater potential to produce more mEq of sulfuric acid per g of protein than some animal proteins (Massey, 2003). Finally, bone resorption may be related to the presence or absence of a vitamin D receptor allele. In subjects that had this specific allele a significant elevation in bone resorption markers were present in the urine following 4-weeks of protein supplementation, while in subjects without this specific allele had no increase in N-telopeptide (Harrington et al., 2004). The effect of protein on bone health is still unclear, but it does appear to be prudent to monitor the amount of animal protein in the diet for susceptible individuals. This may be more pronounced in individuals that may have a genetic endowment for this. However, if animal protein consumption is modified by other nutrients (e.g. calcium) the effects on bone health may be lessened.

Table 4.

Potential acid as sulfate from sulfur-containing amino acids.

FoodmEq per g of protein
Wheat (whole).69
White Rice.68

Protein Intake and Liver Disease Risk

The American Heart Association has suggested that high protein diets may have detrimental effects on liver function (St. Jeor et al., 2001). This is primarily the result of a concern that the liver will be stressed through metabolizing the greater protein intakes. However, there is no scientific evidence to support this contention. Jorda and colleagues (1988) did show that high protein intakes in rats produce morphological changes in liver mitochondria. However, they also suggested that these changes were not pathological, but represented a positive hepatocyte adaptation to a metabolic stress.

Protein is important for the liver not only in promoting tissue repair, but to provide lipotropic agents such as methionine and choline for the conversion of fats to lipoprotein for removal form the liver (Navder and Leiber, 2003a). The importance of high protein diets has also been acknowledged for individuals with liver disease and who are alcoholics. High protein diets may offset the elevated protein catabolism seen with liver disease (Navder and Leiber, 2003b), while a high protein diet has been shown to improve hepatic function in individuals suffering from alcoholic liver disease (Mendellhall et al., 1993).

Comparisons between Different Protein Sources on Human Performance

Earlier discussions on protein supplementation and athletic performance have shown positive effects from proteins of various sources. However, only limited research is available on comparisons between various protein sources and changes in human performance. Recently, there have been a number of comparisons between bovine colostrum and whey protein. The primary reason for this comparison is the use by these investigators of whey protein as the placebo group in many of the studies examining bovine colostrum (Antonio et al., 2001 Brinkworth et al., 2004 Brinkworth and Buckley, 2002 Coombes et al., 2002 Hofman et al., 2002). The reason being that whey protein is similar in taste and texture as bovine colostrum protein.

Studies performed in non-elite athletes have been inconclusive concerning the benefits of bovine colostrum compared to whey protein. Several studies have demonstrated greater gains in lean body mass in individuals supplementing with bovine colostrum than whey, but no changes in endurance or strength performance (Antonio et al., 2001 Brinkworth et al., 2004). However, when performance was measured following prolonged exercise (time to complete 2.8 kJ·kg -1 of work following a 2-hour ride) supplement dosages of 20 g럚y -1 and 60 g럚y -1 were shown to significantly improve time trial performance in competitive cyclists (Coombes et al., 2002). These results may be related to an improved buffering capacity following colostrum supplementation. Brinkworth and colleagues (2004) reported that although no performance changes were seen in rowing performance, the elite rowers that were studied did demonstrate an improved buffering capacity following 9-weeks of supplementation with 60 g럚y -1 of bovine colostrum when compared to supplementing with whey protein. The improved buffering capacity subsequent to colostrum supplementation may have also influenced the results reported by Hofman et al., (2002). In that study elite field hockey players supplemented with either 60 g럚y -1 of either colostrum or whey protein for 8-weeks. A significantly greater improvement was seen in repeated sprint performance in the group supplementing with colostrum compared to the group supplementing with whey protein. However, a recent study has suggested that the improved buffering system seen following colostrum supplementation is not related to an improved plasma buffering system, and that any improved buffering capacity occurs within the tissue (Brinkworth et al., 2004).

In a comparison between casein and whey protein supplementation, Boirie and colleagues (1997) showed that a 30-g feeding of casein versus whey had significantly different effects on postprandial protein gain. They showed that following whey protein ingestion the plasma appearance of amino acids is fast, high and transient. In contrast, casein is absorbed more slowly producing a much less dramatic rise in plasma amino acid concentrations. Whey protein ingestion stimulated protein synthesis by 68%, while casein ingestion stimulated protein synthesis by 31%. When the investigators compared postprandial leucine balance after 7-hours post ingestion, casein consumption resulted in a significantly higher leucine balance, whereas no change from baseline was seen 7-hours following whey consumption. These results suggest that whey protein stimulates a rapid synthesis of protein, but a large part of this protein is oxidized (used as fuel), while casein may result in a greater protein accretion over a longer duration of time. A subsequent study showed that repeated ingestions of whey protein (an equal amount of protein but consumed over a prolonged period of time [4 hours] compared to a single ingestion) produced a greater net leucine oxidation than either a single meal of casein or whey (Dangin et al., 2001). Interestingly, both casein and whey are complete proteins but their amino acid composition is different. Glutamine and leucine have important roles in muscle protein metabolism, yet casein contains 11.6 and 8.9 g of these amino acids, respectively while whey contains 21.9 and 11.1 g of these amino acids, respectively. Thus, the digestion rate of the protein may be more important than the amino acid composition of the protein.

In a study examining the effects of casein and whey on body composition and strength measures, 12 weeks of supplementation on overweight police officers showed significantly greater strength and lean tissue accruement in the subjects ingesting casein compared to whey (Demling and DeSanti, 2000). Protein supplementation provided a relative protein consumption of 1.5 g·kg럚y -1 . Subjects supplemented twice per day approximately 8� hours apart.

Only one study known has compared colostrum, whey and casein supplementation (Fry et al., 2003). Following 12-weeks of supplementation the authors reported no significant differences in lean body mass, strength or power performances between the groups. However, the results of this study should be examined with care. The subjects were comprised of both males and females who were resistance training for recreational purposes. In addition, the subject number for each group ranged from 4𠄶 subjects per group. With a heterogeneous subject population and a low subject number, the statistical power of this study was quite low. However, the authors did analyze effect sizes to account for the low statistical power. This analysis though did not change any of the observations. Clearly, further research is needed in comparisons of various types of protein on performance improvements. However, it is likely that a combination of different proteins from various sources may provide optimal benefits for performance.

Urinary calcium excretion is strongly related to net renal acid excretion. The catabolism of dietary protein generates ammonium ion and sulfates from sulfur-containing amino acids. Bone citrate and carbonate are mobilized to neutralize these acids, so urinary calcium increases when dietary protein increases. Common plant proteins such as soy, corn, wheat and rice have similar total S per g of protein as eggs, milk and muscle from meat, poultry and fish. Therefore increasing intake of purified proteins from either animal or plant sources similarly increases urinary calcium. The effects of a protein on urinary calcium and bone metabolism are modified by other nutrients found in that protein food source. For example, the high amount of calcium in milk compensates for urinary calcium losses generated by milk protein. Similarly, the high potassium levels of plant protein foods, such as legumes and grains, will decrease urinary calcium. The hypocalciuric effect of the high phosphate associated with the amino acids of meat at least partially offsets the hypercalciuric effect of the protein. Other food and dietary constituents such as vitamin D, isoflavones in soy, caffeine and added salt also have effects on bone health. Many of these other components are considered in the potential renal acid load of a food or diet, which predicts its effect on urinary acid and thus calcium. “Excess” dietary protein from either animal or plant proteins may be detrimental to bone health, but its effect will be modified by other nutrients in the food and total diet.

An increase in protein consumption increases urinary calcium excretion over the entire range of protein intakes, from marginal to excess ( 1). Each 10-g increase in dietary protein increases urinary calcium by 16 mg, and doubling protein increases urinary calcium by 50%.

Osteoporotic fracture rates increase as cultures become “Westernized.” Many lifestyle changes occur during cultural development, typically a decrease in physical activity and change in diet. Dietary change usually includes an increase in animal foods at the expense of plant foods. Because increases in dietary animal protein are associated with increases in urinary calcium excretion, the increase in osteoporotic fractures has frequently been attributed to the increase in dietary animal protein. Frassetto et al. ( 2) found the cross-cultural relationship between hip fracture rates and dietary protein was positively related to animal protein intake and inversely related to vegetable protein intake. Even when non-Caucasian populations were removed from the data set, these relationships were still seen. When they plotted the relationship between the ratio of vegetable to animal protein vs. hip fracture rate, the ratio was exponentially inversely related. However, 19 of the 33 countries had a vegetable:animal protein source ratio between 0.3 and 1.0 typical of U.S. ( 3) and similar Western diets, and in that range hip fracture rates varied over 3-fold, from 19 to 57. Obviously, factors other than source of dietary protein have major influence on fracture rate.

Prospective epidemiological evidence is conflicting regarding the role of animal protein vs. plant protein in bone loss. Six prospective studies examining the effect of dietary protein on bone health in older women have been published since 1996 [ Table 1, modified from Bell and Whiting ( 4)]. All six were done on populations of predominantly European ancestry. One study reported lower fracture rates with higher animal protein intakes, whereas two analyses ( 5, 6) reported higher rates. Feskanich et al. ( 5) reported that higher total and animal protein intakes were associated with the 12-y incidence of hip fracture in the Nurses' Health Study. Sellmeyer et al. ( 6) found that elderly women with a high dietary ratio of animal to vegetable protein intake have more rapid femoral bone loss and a greater risk of hip fracture in a 7-y prospective study. Unlike Promislow et al. ( 7), Sellmeyer et al. ( 6) found no difference in bone mineral density associated with source of dietary protein at the beginning of their study. Promislow et al. ( 7) found a positive association of animal protein consumption with bone mineral density (BMD) in the elderly Rancho Bernardo cohort. Hannan et al. ( 8) found lower, not higher, total and animal protein intakes to be associated with higher rates of bone loss in the Framingham cohort, with no adverse effect of higher intakes. Munger et al. ( 9) found similar results in the Iowa Women's Study. Overall, two studies reported higher fracture rates as animal protein increased, whereas one reported a decreased rate. Two others found that BMD was higher with increased animal protein, whereas one found no effect. Total protein intake was found to be associated with greater bone density in four studies, increased fracture rate in one and decreased fracture rate in another. Mean protein intakes ranged from 50 to 80 g daily, and calcium was generally adequate at 718–1346 mg/d. The women in the Feskanich study were younger at baseline than in any of the other studies, and had the lowest calcium to protein ratio. Overall, no pattern of the effect of animal vs. plant protein seems to emerge from these studies. However, the range of protein intakes in these studies included women who had inadequate intakes, and five of the six studies showed beneficial effects of bone with higher total protein.

Summary of recent studies examining the effect of total protein, animal protein and plant protein intakes on bone health in elderly women and men 1

Study . Promislow et al. (2002) ( 7) . Munger et al. (1999) ( 9) . Hannan et al. (2000) ( 8) . Dawson-Hughes et al. (2002) ( 20) . Sellmeyer et al. (2001) ( 6) . Feskanich et al. (1996) ( 5) .
Duration 4 y 3 y 4 y 3 y ∼7 y 12 y
Number of subjects 572 women 32,006 women 391 women, 224 men 138 men and women in Ca-supplement group 1035 women 85,900 women
Subject description Rancho Bernardo cohort 2 Iowa (randomly selected) Framingham cohort 2 Boston volunteers 2 Study of Osteoporosis Fractures cohort Nurses' Health Study cohort
Age at baseline, yMean 71 Mean 61 Mean 75 ≥ 65, mean 70 > 65, mean 73 35–59, mean 46
Mean protein intake, g/d71 78 68 79 50 79.6
Mean calcium intake, mg/d985 1150 810 1346 853 718
Calcium:protein 14:1 15:1 11:1 17:1 17:1 9:1
Conclusions ↑ap ↑BMD ↑pp ↓BMD ↑ap ↓Fx ↑p ↓Fx ↑ap ↓Fx ↑p ↑BMD ↑p ↑BMD (protein source irrelevant) ↑ap:pp ↑hip Fx ↑p ↑Fx (> 95g p)
Study . Promislow et al. (2002) ( 7) . Munger et al. (1999) ( 9) . Hannan et al. (2000) ( 8) . Dawson-Hughes et al. (2002) ( 20) . Sellmeyer et al. (2001) ( 6) . Feskanich et al. (1996) ( 5) .
Duration 4 y 3 y 4 y 3 y ∼7 y 12 y
Number of subjects 572 women 32,006 women 391 women, 224 men 138 men and women in Ca-supplement group 1035 women 85,900 women
Subject description Rancho Bernardo cohort 2 Iowa (randomly selected) Framingham cohort 2 Boston volunteers 2 Study of Osteoporosis Fractures cohort Nurses' Health Study cohort
Age at baseline, yMean 71 Mean 61 Mean 75 ≥ 65, mean 70 > 65, mean 73 35–59, mean 46
Mean protein intake, g/d71 78 68 79 50 79.6
Mean calcium intake, mg/d985 1150 810 1346 853 718
Calcium:protein 14:1 15:1 11:1 17:1 17:1 9:1
Conclusions ↑ap ↑BMD ↑pp ↓BMD ↑ap ↓Fx ↑p ↓Fx ↑ap ↓Fx ↑p ↑BMD ↑p ↑BMD (protein source irrelevant) ↑ap:pp ↑hip Fx ↑p ↑Fx (> 95g p)

Abbreviations: p, total protein ap, animal protein pp, plant protein BMD, change in bone mineral density Fx, fracture.

Chronic Studies of Plant-Based Protein Intake and Muscle Mass

Acute measurements of MPS are often assumed to predict longer-term phenotypic outcomes (i.e., greater skeletal muscle maintenance/gain) to nutritional and/or exercise training interventions ( 78). However, acute measurements of MPS are not a quantitative estimation of muscle hypertrophy/maintenance. Instead, the acute MPS response to various nutritional and/or exercise interventions should better be viewed as an indicator of skeletal muscle reconditioning (e.g., muscle repair and remodeling), with the potential to provide insight into differences in skeletal muscle hypertrophy and/or muscle mass maintenance when performed chronically.

It has been well established that the ingestion of soy protein results in lower postprandial MPS rates than does the ingestion of beef ( 31), whey ( 19, 21), or milk ( 32), both at rest and during recovery from exercise. This begets the question as to whether chronic intake of plant- vs. animal-based proteins would result in divergent phenotypic outcomes (i.e., differences in muscle mass).

Hartman et al. ( 16) observed that the habitual consumption of 17.5 g milk protein during a 12-wk resistance exercise training intervention resulted in greater gains in lean body mass (LBM 3.9 vs. 2.8 kg) than an isonitrogenous amount of soy protein. In agreement, Volek et al. ( 79) demonstrated that the ingestion of 24 g whey as opposed to soy protein resulted in greater gains in LBM (3.3 vs. 1.8 kg) after 36 wk of resistance exercise training in young men. Similarly, Campbell et al. ( 80) showed that the consumption of an omnivorous diet during a 12-wk resistance exercise training program induced greater gains in LBM and increases in type II fiber size than did the consumption of a predominantly lactovarian diet. Later it was demonstrated that increasing daily dietary protein intake from 0.78 g/(kg body weight · d) to 1.15 g/(kg body weight · d) eliminated the differences between the groups who consumed an omnivorous diet vs. a lactovarian diet ( 81). Overall, these findings imply that the ingestion of higher amounts of protein may reduce the proposed differences in the capacity of different protein sources (plant vs. animal) to modulate the gains in skeletal muscle mass during prolonged exercise interventions ( 82).

Indeed, it could be hypothesized that the ingestion of greater quantities of plant-based proteins may compensate for the lower EAA content, thereby improving the potential of plant-based proteins to support skeletal muscle mass gains. Joy et al. ( 33) recently observed that the ingestion of either 48 g rice protein or an isonitrogenous and isoenergetic amount of whey protein, immediately after resistance exercise, promoted similar increases in LBM (2.5 vs. 3.2 kg) during an 8-wk training intervention in healthy young men. Brown et al. ( 37) provided their subjects with either 33 g soy or whey protein and showed similar increases in muscle mass after prolonged resistance exercise training. Collectively, the studies that provided greater amounts of plant-based proteins showed minimized differences in lean mass gain with resistance exercise when compared with the ingestion of animal-based proteins. Although more evidence is required, we argue that plant-based protein supplementation can be successfully applied to support muscle mass accretion during prolonged resistance exercise, provided that greater amounts of plant-based proteins (>30 g/meal) are being consumed. Although the data above are only applicable to the specific population engaged in resistance exercise, divergent phenotypic outcomes with regard to plant- vs. animal-based protein intake have also been observed in other population groups.

A greater proportion of daily protein intake derived from animal- vs. plant-based sources is associated with better muscle maintenance in older and more clinically compromised individuals ( 83– 86). For instance, long-term vegetarianism in older women has been reported to compromise muscle mass maintenance when compared with consumers of an omnivorous diet (18.2 vs. 22.6 kg LBM) ( 85). Although more long-term studies are required, it appears that prolonged (lifelong) vegetarianism can result in lower muscle mass maintenance across the life span. Aging has been associated with a progressive decline in skeletal muscle mass and appears to be driven in part by a greater anabolic resistance of skeletal muscle tissue to dietary protein ingestion ( 87). Given the importance of skeletal muscle mass for metabolic health and physical functioning ( 88), strategies to improve the sensitivity of skeletal muscle tissue to the anabolic properties of plant-based proteins could be of particular interest for aging populations. In addition, strategies to enhance the anabolic response to the ingestion of plant-based proteins may increase consumer demand, thereby supporting global sustainability, and reduce the costs associated with the production of high-quality-protein–dense foods.

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McKenzie is Registered Dietitian Nutritionist for NutriBullet who aims to make the world a healthier, happier place. She believes that living a healthy lifestyle and eating for your health isn't meant to be complicated -- it's meant to be simple, enjoyable, and judgment-free (with room for dessert). When she’s not dishing out nutrition tidbits, you can find McKenzie running after her energetic toddler, hiking along her favorite trails, visiting her local farmers market, or cooking in her sunny kitchen.