How efficiently does the human body convert the energy stored in fat into kinetic energy?

I read that one gram fat contains 9 kcal energy. There are many tables around claiming thatxtype of exercise fortduration burns roughlyEkcal energy. I'd like to estimate how much weight one will lose by following a certain training plan based on these two values. I don't know whether I need to calculate with the heat loss too, or not. Most of these tables don't tell anything about the method they used to estimate theEvalues. I could workaround this problem by ignoring efficiency, but I can do that only if it is relative close to 100%. As far as I know fat goes through beta-oxidation and the citric acid cycle. After that ATP is used to move the muscles. It is hard to find anything about how efficient this process is.

this depends on largely how you define it, what we normally measure is efficiency including how much energy is used on metabolic upkeep and heating the body in addition to muscle output. we use oxygen consumption to know exactly how many calories we have "burned" when doing this. In this study we get between 18-26% efficiency, thats calories ingested to work done. Most of the loss is in converting to ATP (~60% loss) and how much ATP we "waste" heating the body. Note efficiency drops quite a lot when you switch to anaerobic exercise.

keep in mind however the body will not start out burning fat, it has glycogen reserves it will start using first.

Can Fats Be Turned Into Glycogen for Muscle?

The amount of fat in the average diet and the amount of stored fat in the average body make the notion of converting that fat into usable energy appealing. Glycogen, a form of energy stored in muscles for quick use, is what the body draws on first to perform movements, and higher glycogen levels result in higher usable energy. It is not possible for fats to be converted directly into glycogen because they are not made up glucose, but it is possible for fats to be indirectly broken down into glucose, which can be used to create glycogen.

Pathway to Bloodstream

Fats are more scientifically known as triglycerides. They are molecules that are made up of three fatty acid chains stuck onto one glycerol molecule. They enter the body in food as this structure, but they have to break up into their individual parts to pass through the digestive wall into the body. On their way out of the digestive wall, they reform into triglycerides. Fat cells and muscle cells then break down these triglycerides again so that the fatty acids and the glycerol can get through the cell membrane into the cell.

The Glycogen Burning Cycle

What is the connection between glycogen and fat burning? Here's the caveat: Your liver and muscle glycogen stores can only hold so much. The total amount of glycogen that you can store in your entire body is approximately 600 grams. Once these stores max out, any excess glycogen is converted into a type of fat called triglycerides. Triglycerides can either enter directly into the bloodstream for energy, or they're stored in your body fat.

If you continuously eat carbohydrates in any form, your body will prioritize them, and the cycle will continue. This means that you'll always be burning glucose and glycogen for energy, and any excess will always get stored as body fat. When you're taking in more carbohydrates than the body can effectively store as glycogen (more calories in than out), it has no choice but to convert some and store it inside the fat cells.

If you consistently overeat, or you eat a lot of sugar and carbohydrates, this can actually cause weight gain over time.


How quickly you metabolize food depends on a few things, all related to your basal metabolic rate. The Mayo Clinic website points out that if you are a muscular person, you will metabolize a meal at a much faster rate than if you had little muscle mass. Your age factors in as well. As people age, they tend to lose muscle mass. Younger people, who tend to have more muscle, generally metabolize food at a faster rate. Men generally have more muscle mass than women and metabolize nutrients more quickly, so gender is also a factor.

Biochemistry. 5th edition.

The metabolic patterns of the brain, muscle, adipose tissue, kidney, and liver are strikingly different. Let us consider how these organs differ in their use of fuels to meet their energy needs:

Brain. Glucose is virtually the sole fuel for the human brain, except during prolonged starvation. The brain lacks fuel stores and hence requires a continuous supply of glucose. It consumes about 120 g daily, which corresponds to an energy input of about 420 kcal (1760 kJ), accounting for some 60% of the utilization of glucose by the whole body in the resting state. Much of the energy, estimates suggest from 60% to 70%, is used to power transport mechanisms that maintain the Na + -K + membrane potential required for the transmission of the nerve impulses. The brain must also synthesize neurotransmitters and their receptors to propagate nerve impulses. Overall, glucose metabolism remains unchanged during mental activity, although local increases are detected when a subject performs certain tasks.

Glucose is transported into brain cells by the glucose transporter GLUT3. This transporter has a low value of KM for glucose (1.6 mM), which means that it is saturated under most conditions. Thus, the brain is usually provided with a constant supply of glucose. Noninvasive 13 C nuclear magnetic resonance measurements have shown that the concentration of glucose in the brain is about 1 mM when the plasma level is 4.7 mM (84.7 mg/dl), a normal value. Glycolysis slows down when the glucose level approaches the KM value of hexokinase (

50 μM), the enzyme that traps glucose in the cell (Section 16.1.1). This danger point is reached when the plasma-glucose level drops below about 2.2 mM (39.6 mg/dl) and thus approaches the KM value of GLUT3.

Fatty acids do not serve as fuel for the brain, because they are bound to albumin in plasma and so do not traverse the blood-brain barrier. In starvation, ketone bodies generated by the liver partly replace glucose as fuel for the brain.

Muscle. The major fuels for muscle are glucose, fatty acids, and ketone bodies. Muscle differs from the brain in having a large store of glycogen (1200 kcal, or 5000 kJ). In fact, about three-fourths of all the glycogen in the body is stored in muscle (Table 30.1). This glycogen is readily converted into glucose 6-phosphate for use within muscle cells. Muscle, like the brain, lacks glucose 6-phosphatase, and so it does not export glucose. Rather, muscle retains glucose, its preferred fuel for bursts of activity.

In actively contracting skeletal muscle, the rate of glycolysis far exceeds that of the citric acid cycle, and much of the pyruvate formed is reduced to lactate, some of which flows to the liver, where it is converted into glucose (Figure 30.12).

These interchanges, known as the Cori cycle (Section 16.4.2), shift part of the metabolic burden of muscle to the liver. In addition, a large amount of alanine is formed in active muscle by the transamination of pyruvate. Alanine, like lactate, can be converted into glucose by the liver. Why does the muscle release alanine? Muscle can absorb and transaminate branched-chain amino acids however, it cannot form urea. Consequently, the nitrogen is released into the blood as alanine. The liver absorbs the alanine, removes the nitrogen for disposal as urea, and processes the pyruvate to glucose or fatty acids. The metabolic pattern of resting muscle is quite different. In resting muscle, fatty acids are the major fuel, meeting 85% of the energy needs.

Unlike skeletal muscle, heart muscle functions almost exclusively aerobically, as evidenced by the density of mitochondria in heart muscle. Moreover, the heart has virtually no glycogen reserves. Fatty acids are the heart's main source of fuel, although ketone bodies as well as lactate can serve as fuel for heart muscle. In fact, heart muscle consumes acetoacetate in preference to glucose.

Adipose tissue. The triacylglycerols stored in adipose tissue are an enormous reservoir of metabolic fuel (see Table 30.1). In a typical 70-kg man, the 15 kg of triacylglycerols have an energy content of 135,000 kcal (565,000 kJ). Adipose tissue is specialized for the esterification of fatty acids and for their release from triacylglycerols. In human beings, the liver is the major site of fatty acid synthesis. Recall that these fatty acids are esterified in the liver to glycerol phosphate to form triacylglycerol and are transported to the adipose tissue in lipoprotein particles, such as very low density lipoproteins (Section 26.3.1). Triacylglycerols are not taken up by adipocytes rather, they are first hydrolyzed by an extracellular lipoprotein lipase for uptake. This lipase is stimulated by processes initiated by insulin. After the fatty acids enter the cell, the principal task of adipose tissue is to activate these fatty acids and transfer the resulting CoA derivatives to glycerol in the form of glycerol 3-phosphate. This essential intermediate in lipid biosynthesis comes from the reduction of the glycolytic intermediate dihydroxyacetone phosphate. Thus, adipose cells need glucose for the synthesis of triacylglycerols (Figure 30.13).

Triacylglycerols are hydrolyzed to fatty acids and glycerol by intracellular lipases. The release of the first fatty acid from a triacylglycerol, the rate-limiting step, is catalyzed by a hormone-sensitive lipase that is reversibly phosphorylated. The hormone epinephrine stimulates the formation of cyclic AMP, the intracellular messenger in the amplifying cascade, which activates a protein kinase𠅊 recurring theme in hormone action. Triacylglycerols in adipose cells are continually being hydrolyzed and resynthesized. Glycerol derived from their hydrolysis is exported to the liver. Most of the fatty acids formed on hydrolysis are reesterified if glycerol 3-phosphate is abundant. In contrast, they are released into the plasma if glycerol 3-phosphate is scarce because of a paucity of glucose. Thus, the glucose level inside adipose cells is a major factor in determining whether fatty acids are released into the blood.

The kidney. The major purpose of the kidney is to produce urine, which serves as a vehicle for excreting metabolic waste products and for maintaining the osmolarity of the body fluids. The blood plasma is filtered nearly 60 times each day in the renal tubules. Most of the material filtered out of the blood is reabsorbed so only 1 to 2 liters of urine is produced. Water-soluble materials in the plasma, such as glucose, and water itself are reabsorbed to prevent wasteful loss. The kidneys require large amounts of energy to accomplish the reabsorption. Although constituting only 0.5% of body mass, kidneys consume 10% of the oxygen used in cellular respiration. Much of the glucose that is reabsorbed is carried into the kidney cells by the sodium-glucose cotransporter. Recall that this transporter is powered by the Na + -K + gradient, which is itself maintained by the Na + -K + ATPase (Section 13.4). During starvation, the kidney becomes an important site of gluconeogenesis and may contribute as much as half of the blood glucose.

Liver. The metabolic activities of the liver are essential for providing fuel to the brain, muscle, and other peripheral organs. Indeed, the liver, which can be from 2% to 4% of body weight, is an organism's metabolic hub (Figure 30.14). Most compounds absorbed by the intestine first pass through the liver, which is thus able to regulate the level of many metabolites in the blood.

Let us first consider how the liver metabolizes carbohydrates. The liver removes two-thirds of the glucose from the blood and all of the remaining monosaccharides. Some glucose is left in the blood for use by other tissues. The absorbed glucose is converted into glucose 6-phosphate by hexokinase and the liver-specific glucokinase. Glucose 6-phosphate, as already stated, has a variety of fates, although the liver uses little of it to meet its own energy needs. Much of the glucose 6-phosphate is converted into glycogen. As much as 400 kcal (1700 kJ) can be stored in this form in the liver. Excess glucose 6-phosphate is metabolized to acetyl CoA, which is used to form fatty acids, cholesterol, and bile salts. The pentose phosphate pathway, another means of processing glucose 6-phosphate, supplies the NADPH for these reductive biosyntheses. The liver can produce glucose for release into the blood by breaking down its store of glycogen and by carrying out gluconeogenesis. The main precursors for gluconeogenesis are lactate and alanine from muscle, glycerol from adipose tissue, and glucogenic amino acids from the diet.

The liver also plays a central role in the regulation of lipid metabolism. When fuels are abundant, fatty acids derived from the diet or synthesized by the liver are esterified and secreted into the blood in the form of very low density lipoprotein (see Figure 30.15). However, in the fasting state, the liver converts fatty acids into ketone bodies. How is the fate of liver fatty acids determined? The selection is made according to whether the fatty acids enter the mitochondrial matrix. Recall that long-chain fatty acids traverse the inner mitochondrial membrane only if they are esterified to carnitine. Carnitine acyltransferase I (also known as carnitine palmitoyl transferase I), which catalyzes the formation of acyl carnitine, is inhibited by malonyl CoA, the committed intermediate in the synthesis of fatty acids (see Figure 30.9). Thus, when malonyl CoA is abundant, long-chain fatty acids are prevented from entering the mitochondrial matrix, the compartment of β-oxidation and ketone-body formation. Instead, fatty acids are exported to adipose tissue for incorporation into triacylglycerols. In contrast, the level of malonyl CoA is low when fuels are scarce. Under these conditions, fatty acids liberated from adipose tissues enter the mitochondrial matrix for conversion into ketone bodies.

The liver also plays an essential role in dietary amino acid metabolism. The liver absorbs the majority of amino acids, leaving some in the blood for peripheral tissues. The priority use of amino acids is for protein synthesis rather than catabolism. By what means are amino acids directed to protein synthesis in preference to use as a fuel? The KM value for the aminoacyl-tRNA synthetases is lower than that of the enzymes taking part in amino acid catabolism. Thus, amino acids are used to synthesize aminoacyl-tRNAs before they are catabolized. When catabolism does take place, the first step is the removal of nitrogen, which is subsequently processed to urea. The liver secretes from 20 to 30 g of urea a day. The α-ketoacids are then used for gluconeogenesis or fatty acid synthesis. Interestingly, the liver cannot remove nitrogen from the branch-chain amino acids (leucine, isoleucine, and valine). Transamination takes place in the muscle.

How does the liver meet its own energy needs? α-Ketoacids derived from the degradation of amino acids are the liver's own fuel. In fact, the main role of glycolysis in the liver is to form building blocks for biosyntheses. Furthermore, the liver cannot use acetoacetate as a fuel, because it has little of the transferase needed for acetoacetate's activation to acetyl CoA. Thus, the liver eschews the fuels that it exports to muscle and the brain.

How efficiently does the human body convert the energy stored in fat into kinetic energy? - Biology

By the end of this section, you will be able to:

  • Explain the human body’s consumption of energy when at rest versus when engaged in activities that do useful work.
  • Calculate the conversion of chemical energy in food into useful work.

Energy Conversion in Humans

Our own bodies, like all living organisms, are energy conversion machines. Conservation of energy implies that the chemical energy stored in food is converted into work, thermal energy, and/or stored as chemical energy in fatty tissue. (See [link].) The fraction going into each form depends both on how much we eat and on our level of physical activity. If we eat more than is needed to do work and stay warm, the remainder goes into body fat.

Energy consumed by humans is converted to work, thermal energy, and stored fat. By far the largest fraction goes to thermal energy, although the fraction varies depending on the type of physical activity.

Power Consumed at Rest

The rate at which the body uses food energy to sustain life and to do different activities is called the metabolic rate . The total energy conversion rate of a person at rest is called the basal metabolic rate (BMR) and is divided among various systems in the body, as shown in [link]. The largest fraction goes to the liver and spleen, with the brain coming next. Of course, during vigorous exercise, the energy consumption of the skeletal muscles and heart increase markedly. About 75% of the calories burned in a day go into these basic functions. The BMR is a function of age, gender, total body weight, and amount of muscle mass (which burns more calories than body fat). Athletes have a greater BMR due to this last factor.

Basal Metabolic Rates (BMR)
Organ Power consumed at rest (W) Oxygen consumption (mL/min) Percent of BMR
Liver & spleen 23 67 27
Brain 16 47 19
Skeletal muscle 15 45 18
Kidney 9 26 10
Heart 6 17 7
Other 16 48 19
Totals 85 W 250 mL/min 100%

Energy consumption is directly proportional to oxygen consumption because the digestive process is basically one of oxidizing food. We can measure the energy people use during various activities by measuring their oxygen use. (See [link].) Approximately 20 kJ of energy are produced for each liter of oxygen consumed, independent of the type of food. [link] shows energy and oxygen consumption rates (power expended) for a variety of activities.

Power of Doing Useful Work

Work done by a person is sometimes called useful work , which is work done on the outside world, such as lifting weights. Useful work requires a force exerted through a distance on the outside world, and so it excludes internal work, such as that done by the heart when pumping blood. Useful work does include that done in climbing stairs or accelerating to a full run, because these are accomplished by exerting forces on the outside world. Forces exerted by the body are nonconservative, so that they can change the mechanical energy ( KE + PE KE + PE size 12 <"KE "+" PE"><> ) of the system worked upon, and this is often the goal. A baseball player throwing a ball, for example, increases both the ball’s kinetic and potential energy.

If a person needs more energy than they consume, such as when doing vigorous work, the body must draw upon the chemical energy stored in fat. So exercise can be helpful in losing fat. However, the amount of exercise needed to produce a loss in fat, or to burn off extra calories consumed that day, can be large, as [link] illustrates.

If a person who normally requires an average of 12,000 kJ (3000 kcal) of food energy per day consumes 13,000 kJ per day, he will steadily gain weight. How much bicycling per day is required to work off this extra 1000 kJ?

[link] states that 400 W are used when cycling at a moderate speed. The time required to work off 1000 kJ at this rate is then

If this person uses more energy than he or she consumes, the person’s body will obtain the needed energy by metabolizing body fat. If the person uses 13,000 kJ but consumes only 12,000 kJ, then the amount of fat loss will be

assuming the energy content of fat to be 39 kJ/g.

A pulse oxymeter is an apparatus that measures the amount of oxygen in blood. Oxymeters can be used to determine a person’s metabolic rate, which is the rate at which food energy is converted to another form. Such measurements can indicate the level of athletic conditioning as well as certain medical problems. (credit: UusiAjaja, Wikimedia Commons) Energy and Oxygen Consumption Rates 1 (Power)
Activity Energy consumption in watts Oxygen consumption in liters O2/min
Sleeping 83 0.24
Sitting at rest 120 0.34
Standing relaxed 125 0.36
Sitting in class 210 0.60
Walking (5 km/h) 280 0.80
Cycling (13󈝾 km/h) 400 1.14
Shivering 425 1.21
Playing tennis 440 1.26
Swimming breaststroke 475 1.36
Ice skating (14.5 km/h) 545 1.56
Climbing stairs (116/min) 685 1.96
Cycling (21 km/h) 700 2.00
Running cross-country 740 2.12
Playing basketball 800 2.28
Cycling, professional racer 1855 5.30
Sprinting 2415 6.90

All bodily functions, from thinking to lifting weights, require energy. (See [link].) The many small muscle actions accompanying all quiet activity, from sleeping to head scratching, ultimately become thermal energy, as do less visible muscle actions by the heart, lungs, and digestive tract. Shivering, in fact, is an involuntary response to low body temperature that pits muscles against one another to produce thermal energy in the body (and do no work). The kidneys and liver consume a surprising amount of energy, but the biggest surprise of all it that a full 25% of all energy consumed by the body is used to maintain electrical potentials in all living cells. (Nerve cells use this electrical potential in nerve impulses.) This bioelectrical energy ultimately becomes mostly thermal energy, but some is utilized to power chemical processes such as in the kidneys and liver, and in fat production.

This fMRI scan shows an increased level of energy consumption in the vision center of the brain. Here, the patient was being asked to recognize faces. (credit: NIH via Wikimedia Commons)

Section Summary

  • The human body converts energy stored in food into work, thermal energy, and/or chemical energy that is stored in fatty tissue.
  • The rate at which the body uses food energy to sustain life and to do different activities is called the metabolic rate, and the corresponding rate when at rest is called the basal metabolic rate (BMR)
  • The energy included in the basal metabolic rate is divided among various systems in the body, with the largest fraction going to the liver and spleen, and the brain coming next.
  • About 75% of food calories are used to sustain basic body functions included in the basal metabolic rate.
  • The energy consumption of people during various activities can be determined by measuring their oxygen use, because the digestive process is basically one of oxidizing food.

Conceptual Questions

Explain why it is easier to climb a mountain on a zigzag path rather than one straight up the side. Is your increase in gravitational potential energy the same in both cases? Is your energy consumption the same in both?

Do you do work on the outside world when you rub your hands together to warm them? What is the efficiency of this activity?

Shivering is an involuntary response to lowered body temperature. What is the efficiency of the body when shivering, and is this a desirable value?

Discuss the relative effectiveness of dieting and exercise in losing weight, noting that most athletic activities consume food energy at a rate of 400 to 500 W, while a single cup of yogurt can contain 1360 kJ (325 kcal). Specifically, is it likely that exercise alone will be sufficient to lose weight? You may wish to consider that regular exercise may increase the metabolic rate, whereas protracted dieting may reduce it.

Problems & Exercises

(a) How long can you rapidly climb stairs (116/min) on the 93.0 kcal of energy in a 10.0-g pat of butter? (b) How many flights is this if each flight has 16 stairs?

(a) What is the power output in watts and horsepower of a 70.0-kg sprinter who accelerates from rest to 10.0 m/s in 3.00 s? (b) Considering the amount of power generated, do you think a well-trained athlete could do this repetitively for long periods of time?

Calculate the power output in watts and horsepower of a shot-putter who takes 1.20 s to accelerate the 7.27-kg shot from rest to 14.0 m/s, while raising it 0.800 m. (Do not include the power produced to accelerate his body.)

Shot putter at the Dornoch Highland Gathering in 2007. (credit: John Haslam, Flickr)

Energy that is not utilized for work or heat transfer is converted to the chemical energy of body fat containing about 39 kJ/g. How many grams of fat will you gain if you eat 10,000 kJ (about 2500 kcal) one day and do nothing but sit relaxed for 16.0 h and sleep for the other 8.00 h? Use data from [link] for the energy consumption rates of these activities.

Using data from [link], calculate the daily energy needs of a person who sleeps for 7.00 h, walks for 2.00 h, attends classes for 4.00 h, cycles for 2.00 h, sits relaxed for 3.00 h, and studies for 6.00 h. (Studying consumes energy at the same rate as sitting in class.)

What is the efficiency of a subject on a treadmill who puts out work at the rate of 100 W while consuming oxygen at the rate of 2.00 L/min? (Hint: See [link].)

Shoveling snow can be extremely taxing because the arms have such a low efficiency in this activity. Suppose a person shoveling a footpath metabolizes food at the rate of 800 W. (a) What is her useful power output? (b) How long will it take her to lift 3000 kg of snow 1.20 m? (This could be the amount of heavy snow on 20 m of footpath.) (c) How much waste heat transfer in kilojoules will she generate in the process?

Very large forces are produced in joints when a person jumps from some height to the ground. (a) Calculate the magnitude of the force produced if an 80.0-kg person jumps from a 0.600–m-high ledge and lands stiffly, compressing joint material 1.50 cm as a result. (Be certain to include the weight of the person.) (b) In practice the knees bend almost involuntarily to help extend the distance over which you stop. Calculate the magnitude of the force produced if the stopping distance is 0.300 m. (c) Compare both forces with the weight of the person.

(c) Ratio of net force to weight of person is 41.0 in part (a) 3.00 in part (b)

Jogging on hard surfaces with insufficiently padded shoes produces large forces in the feet and legs. (a) Calculate the magnitude of the force needed to stop the downward motion of a jogger’s leg, if his leg has a mass of 13.0 kg, a speed of 6.00 m/s, and stops in a distance of 1.50 cm. (Be certain to include the weight of the 75.0-kg jogger’s body.) (b) Compare this force with the weight of the jogger.

(a) Calculate the energy in kJ used by a 55.0-kg woman who does 50 deep knee bends in which her center of mass is lowered and raised 0.400 m. (She does work in both directions.) You may assume her efficiency is 20%. (b) What is the average power consumption rate in watts if she does this in 3.00 min?

Kanellos Kanellopoulos flew 119 km from Crete to Santorini, Greece, on April 23, 1988, in the Daedalus 88, an aircraft powered by a bicycle-type drive mechanism (see [link]). His useful power output for the 234-min trip was about 350 W. Using the efficiency for cycling from [link], calculate the food energy in kilojoules he metabolized during the flight.

The Daedalus 88 in flight. (credit: NASA photo by Beasley)

The swimmer shown in [link] exerts an average horizontal backward force of 80.0 N with his arm during each 1.80 m long stroke. (a) What is his work output in each stroke? (b) Calculate the power output of his arms if he does 120 strokes per minute.

Mountain climbers carry bottled oxygen when at very high altitudes. (a) Assuming that a mountain climber uses oxygen at twice the rate for climbing 116 stairs per minute (because of low air temperature and winds), calculate how many liters of oxygen a climber would need for 10.0 h of climbing. (These are liters at sea level.) Note that only 40% of the inhaled oxygen is utilized the rest is exhaled. (b) How much useful work does the climber do if he and his equipment have a mass of 90.0 kg and he gains 1000 m of altitude? (c) What is his efficiency for the 10.0-h climb?

The awe-inspiring Great Pyramid of Cheops was built more than 4500 years ago. Its square base, originally 230 m on a side, covered 13.1 acres, and it was 146 m high, with a mass of about 7 × 10 9 kg 7 × 10 9 kg . (The pyramid’s dimensions are slightly different today due to quarrying and some sagging.) Historians estimate that 20,000 workers spent 20 years to construct it, working 12-hour days, 330 days per year. (a) Calculate the gravitational potential energy stored in the pyramid, given its center of mass is at one-fourth its height. (b) Only a fraction of the workers lifted blocks most were involved in support services such as building ramps (see [link]), bringing food and water, and hauling blocks to the site. Calculate the efficiency of the workers who did the lifting, assuming there were 1000 of them and they consumed food energy at the rate of 300 kcal/h. What does your answer imply about how much of their work went into block-lifting, versus how much work went into friction and lifting and lowering their own bodies? (c) Calculate the mass of food that had to be supplied each day, assuming that the average worker required 3600 kcal per day and that their diet was 5% protein, 60% carbohydrate, and 35% fat. (These proportions neglect the mass of bulk and nondigestible materials consumed.)

Ancient pyramids were probably constructed using ramps as simple machines. (credit: Franck Monnier, Wikimedia Commons)

(a) How long can you play tennis on the 800 kJ (about 200 kcal) of energy in a candy bar? (b) Does this seem like a long time? Discuss why exercise is necessary but may not be sufficient to cause a person to lose weight.

Internal Energy U

We can think about the internal energy of a system in two different but consistent ways. The first is the atomic and molecular view, which examines the system on the atomic and molecular scale. The internal energy of a system is the sum of the kinetic and potential energies of its atoms and molecules. Recall that kinetic plus potential energy is called mechanical energy. Thus internal energy is the sum of atomic and molecular mechanical energy. Because it is impossible to keep track of all individual atoms and molecules, we must deal with averages and distributions. A second way to view the internal energy of a system is in terms of its macroscopic characteristics, which are very similar to atomic and molecular average values.

Macroscopically, we define the change in internal energy to be that given by the first law of thermodynamics:

Many detailed experiments have verified that where is the change in total kinetic and potential energy of all atoms and molecules in a system. It has also been determined experimentally that the internal energy of a system depends only on the state of the system and not how it reached that state. More specifically, is found to be a function of a few macroscopic quantities (pressure, volume, and temperature, for example), independent of past history such as whether there has been heat transfer or work done. This independence means that if we know the state of a system, we can calculate changes in its internal energy from a few macroscopic variables.


In thermodynamics, we often use the macroscopic picture when making calculations of how a system behaves, while the atomic and molecular picture gives underlying explanations in terms of averages and distributions. We shall see this again in later sections of this chapter. For example, in the topic of entropy, calculations will be made using the atomic and molecular view.

To get a better idea of how to think about the internal energy of a system, let us examine a system going from State 1 to State 2. The system has internal energy in State 1, and it has internal energy in State 2, no matter how it got to either state. So the change in internal energy is independent of what caused the change. In other words, is independent of path. By path, we mean the method of getting from the starting point to the ending point. Why is this independence important? Note that Both and depend on path, but does not. This path independence means that internal energy is easier to consider than either heat transfer or work done.

Example 1: Calculating Change in Internal Energy: The Same Change in U is Produced by Two Different Processes

(a) Suppose there is heat transfer of 40.00 J to a system, while the system does 10.00 J of work. Later, there is heat transfer of 25.00 J out of the system while 4.00 J of work is done on the system. What is the net change in internal energy of the system?

(b) What is the change in internal energy of a system when a total of 150.00 J of heat transfer occurs out of (from) the system and 159.00 J of work is done on the system? (See Figure 3).

In part (a), we must first find the net heat transfer and net work done from the given information. Then the first law of thermodynamics can be used to find the change in internal energy. In part (b), the net heat transfer and work done are given, so the equation can be used directly.

Solution for (a)

The net heat transfer is the heat transfer into the system minus the heat transfer out of the system, or

Similarly, the total work is the work done by the system minus the work done on the system, or

Thus the change in internal energy is given by the first law of thermodynamics:

We can also find the change in internal energy for each of the two steps. First, consider 40.00 J of heat transfer in and 10.00 J of work out, or

Now consider 25.00 J of heat transfer out and 4.00 J of work in, or

The total change is the sum of these two steps, or

Discussion on (a)

No matter whether you look at the overall process or break it into steps, the change in internal energy is the same.

Solution for (b)

Here the net heat transfer and total work are given directly to be and so that

Discussion on (b)

A very different process in part (b) produces the same 9.00-J change in internal energy as in part (a). Note that the change in the system in both parts is related to and not to the individual or involved. The system ends up in the same state in both (a) and (b). Parts (a) and (b) present two different paths for the system to follow between the same starting and ending points, and the change in internal energy for each is the same—it is independent of path.

Figure 3. Two different processes produce the same change in a system. (a) A total of 15.00 J of heat transfer occurs into the system, while work takes out a total of 6.00 J. The change in internal energy is ΔU=Q−W=9.00 J. (b) Heat transfer removes 150.00 J from the system while work puts 159.00 J into it, producing an increase of 9.00 J in internal energy. If the system starts out in the same state in (a) and (b), it will end up in the same final state in either case—its final state is related to internal energy, not how that energy was acquired.

What Is Metabolism?

This fat-burning magic comes from your metabolism, a word you've probably heard tossed around a lot but maybe don't quite understand. What is metabolism? Simply put, it's all the various chemical reactions that happen inside your body, 24-7, that keep you alive. It's food being turned into energy and that energy being burned off to keep your hair growing, your heart beating, your liver pumping out bile, your lungs transferring oxygen into your bloodstream and your intestines turning Amstel Light into urine (not that there's a huge leap there).

It's the engine room of your individual starship, your never-ending calorie burn. And while you may imagine that the majority of your calories get burned while you're engaged in some strenuous activity like riding a bike, diving into a pool or getting jiggy with your honey, you're actually burning most of your calories, well, just keeping the lights on.

In fact, think of metabolism as your caloric 401(K) program. It's not going to give you instant gratification, like hitting a slot machine jackpot. It's a long-term strategy, but it's a sure thing: Invest in it and you'll get slow, steady, effective returns that will keep you happy and healthy for years to come.

Now, like any long-term investment, it needs a little maintenance from time-to-time. Learn the smart ways to tweak your metabolism, improving your burn just enough to gain even more over the long haul. (Or to borrow what they say in financial circles, it's time to work less for your calorie burn and have your calorie burn start working for you!)

Prepare for a few surprises, starting with.

How our bodies turn food into energy

All parts of the body (muscles, brain, heart, and liver) need energy to work. This energy comes from the food we eat.

Our bodies digest the food we eat by mixing it with fluids (acids and enzymes) in the stomach. When the stomach digests food, the carbohydrate (sugars and starches) in the food breaks down into another type of sugar, called glucose.

The stomach and small intestines absorb the glucose and then release it into the bloodstream. Once in the bloodstream, glucose can be used immediately for energy or stored in our bodies, to be used later.

However, our bodies need insulin in order to use or store glucose for energy. Without insulin, glucose stays in the bloodstream, keeping blood sugar levels high.

How the body makes insulin

Insulin is a hormone made by beta cells in the pancreas. Beta cells are very sensitive to the amount of glucose in the bloodstream. Normally beta cells check the blood's glucose level every few seconds and sense when they need to speed up or slow down the amount of insulin they're making and releasing. When someone eats something high in carbohydrates, like a piece of bread, the glucose level in the blood rises and the beta cells trigger the pancreas to release more insulin into the bloodstream.

Insulin opens cell doors

When insulin is released from the pancreas, it travels through the bloodstream to the body's cells and tells the cell doors to open up to let the glucose in. Once inside, the cells convert glucose into energy to use right then or store it to use later.

As glucose moves from the bloodstream into the cells, blood sugar levels start to drop. The beta cells in the pancreas can tell this is happening, so they slow down the amount of insulin they're making. At the same time, the pancreas slows down the amount of insulin that it's releasing into the bloodstream. When this happens, the amount of glucose going into the cells also slows down.

Balancing insulin and blood sugar for energy

The rise and fall in insulin and blood sugar happens many times during the day and night. The amount of glucose and insulin in our bloodstream depends on when we eat and how much. When the body is working as it should, it can keep blood sugar at a normal level, which is between 70 and 120 milligrams per deciliter. However, even in people without diabetes, blood sugar levels can go up as high as 180 during or right after a meal. Within two hours after eating, blood sugar levels should drop to under 140. After several hours without eating, blood sugar can drop as low as 70.

Using glucose for energy and keeping it balanced with just the right amount of insulin — not too much and not too little — is the way our bodies maintain the energy needed to stay alive, work, play, and function even as we sleep.

Insulin helps our bodies store extra glucose

Insulin helps our cells convert glucose into energy, and it helps our bodies store extra glucose for use later. For example, if you eat a large meal and your body doesn't need that much glucose right away, insulin will help your body store it to convert to energy later.

Insulin does this by turning the extra food into larger packages of glucose called glycogen. Glycogen is stored in the liver and muscles.

Insulin also helps our bodies store fat and protein. Almost all body cells need protein to work and grow. The body needs fat to protect nerves and make several important hormones. Fat can also be used by the body as an energy source.

How diabetes changes the way this works

With diabetes, the body has stopped making insulin, has slowed down the amount of insulin it's making, or is no longer able to use its own insulin very well. When this happens, it can lead to several things.

For example, glucose cannot enter the cells where it's needed, so the amount of glucose in the bloodstream continues to rise. This is called hyperglycemia (high blood sugar).

When blood sugar levels reach 180 or higher, the kidneys try to get rid of the extra sugar through the urine. This makes a person urinate more than usual. It also makes a person feel thirstier because of the water he or she is losing by urinating so much.

When a person loses sugar in the urine, it's the same as losing energy because the sugar isn't available for the cells to use or store. When this happens, a person might feel tired, lose weight, and feel hungry all the time.

Other problems caused by high blood sugar include blurry vision and skin infections or injuries that don't heal. Women might have vaginal yeast infections more often.

When the body doesn't have enough insulin to help convert sugar into energy, it often starts burning body fat instead. This sounds like it might work well, but burning too much fat for energy produces a byproduct called ketones. High levels of ketones can lead to a condition called diabetic ketoacidosis (DKA), which can be life threatening if not treated quickly. DKA is more common in type 1 diabetes because the body has stopped making insulin.

Keep blood sugar levels under control

For a person with diabetes, the main focus of treatment is to control the amount of glucose in the body so that blood sugar levels stay as close to normal as possible.

People with type 1 diabetes need insulin shots as part of their care plan to control their blood sugar levels. Some people with type 2 diabetes can control their blood sugar levels with a healthy diet and exercise. However, many people with type 2 diabetes will need to include diabetes pills, insulin shots, or both in their diabetes care plans.

People with either type 1 or type 2 diabetes need to pay close attention to how blood sugar levels change at various times throughout the day in order to keep them as close to normal as possible. When blood sugar levels are close to normal, it means the body is getting the energy it needs to work, play, heal, and stay healthy.