Pick up any packaged food, and you will see the number of calories listed on the label. Of course, nonpackaged foods, such as fresh produce, have calories, too; they just don’t carry labels telling you how many. Most people know that the body uses the calories contained in foods for energy and that, if they consume more calories than they expend, they will gain weight. (They also know that, if they do the opposite, they will lose weight.) Nonetheless, what exactly is a calorie, why do foods have calories and how does the body use them? This article answers these and other questions.

What Is a Calorie?

In the United States, calorie is the most common term used to express energy expenditure. A calorie is defined either as the heat energy required to raise the temperature of 1 gram
of water by 1 degree Celsius or as 4.184 joules, 1 joule being the heat energy given off when 1 ampere of electrical current flows through a resistance of 1 ohm for 1 second (Dirckx 2001).

In scientific writing, the energy expended during physical activity and stored in foods is actually measured in kilocalories (kcal), the heat energy required to raise the temperature of 1 kilogram of water by 1 degree Celsius. However, because a calorie is such a small unit of energy, the word “calories” is generally used to mean “kilocalories” outside the realm of scientific literature. We follow this custom in this article.

Why Do Foods Have Calories?

All foods come either from plants or from animals that have eaten plants. Plants create the primary food molecules that contain the energy quantified as calories (Taiz & Zeiger 1998). Green plants create these molecules from carbon dioxide and water through the process of photosynthesis. In photosynthesis, the chlorophyll in these plants absorbs radiant energy from the sun. This energy is then converted into chemical energy contained in the bonds that link the carbon from carbon dioxide (CO2) to water (H2O) to create carbohydrates (CH2O); the “loose” oxygen (O2) is freed into the atmosphere. From carbohydrates, plants can create other molecules (including fats and proteins) that
also contain captured energy. Figure 1 shows the Calvin cycle and shikimic acid pathway, two important processes that enable plants to synthesize carbohydrates and the essential amino acids that become parts of plant proteins.

Like plants, humans can use carbohydrates to synthesize most fatty acids, fats, nonessential amino acids and proteins. However, the primary source of all calories remains carbohydrates themselves, which only plants create.

Why Do Different Foods
Have Different Calorie Levels?

Foods contain six classes of nutrients: carbohydrates, fats, proteins, vitamins, minerals and water. Only carbohydrates, fats and proteins provide the substrate that the body needs to break down for energy. Because these three classes of nutrients are consumed in large quantities (50 to 500 grams per day), they are called macronutrients. In contrast, the micronutrient classes of vitamins and minerals need to be consumed in only very small quantities (1 to 100 milligrams per day). Although vitamins, minerals and water provide
no calories, they are essential to the body’s ability to break down macronutrients to release their stored energy for
bodily functions.

Most foods are mixtures of some or all of the six classes
of nutrients, and different foods contain different amounts of each class. For example, butter contains a lot of fat; some protein, vitamins, minerals and water; and very little carbohydrate. Meat contains a lot of protein and water; some fat, vitamins and minerals; and very little carbohydrate. Whole-wheat bread contains a lot of carbohydrate, vitamins and minerals; some protein and fat; and very little water. Different foods have different calorie levels because their regular servings contain different amounts of carbohydrates, proteins and fats and these energy-providing nutrients supply different amounts of energy per gram. Fats supply the most energy
at 9 calories per gram. Carbohydrates provide 4 calories per gram, as do proteins (Merrill & Watt 1973).

How Do Calories Become Available to the Body?

The energy stored in the chemical bonds of macronutrients is transformed into high-energy phosphate bonds usable in the myriad metabolic processes of the body (Groff & Gropper 2000). The main molecule containing these phosphate bonds is adenosine triphosphate (ATP). The transformation of food into ATP involves not only digestion but also absorption and metabolic catabolism (the chemical breakdown of large molecules into smaller ones).

During digestion, carbohydrates are broken down into
the simple sugars glucose (primarily), fructose and galactose. Proteins are broken down first into amino acids and dietary fats and then into free fatty acids and glycerol. These small molecules are absorbed by the cells lining the intestines and passed into the bloodstream, where they circulate until they enter the cells in the rest of the body. The subsequent creation of ATP from the metabolic catabolism of glucose, free fatty acids and amino acids occurs within each cell.


When split with the help of enzymes, the high-energy bonds in ATP release that energy for use by the muscles, liver and brain and all of the body’s metabolic systems. Essentially, the energy harnessed from the breakdown of food is directly used not to exercise but to produce ATP, which the body’s tissues store in very small amounts. Phosphocreatine (PC, also known as creatine phosphate), another high-energy compound, is also stored in tissues in limited amounts. The breakdown of PC is also used not to produce energy directly but to regenerate ATP quickly during high-intensity exercise.

How Do the Energy Systems
of the Body Burn Calories?

Every movement that a person makes in his daily life requires the breakdown of ATP. Therefore, to sustain life, ATP is consistently used and regenerated. However, the body stores such a limited supply of ATP and PC, which lasts only as long as about 30 seconds. Consequently, it depends on stored carbohydrates and fats and sometimes
on proteins as backup to regenerate ATP. The body’s ability to store these macronutrients for energy production makes possible the successful completion of various physical activities (Wilmore & Costill 1999).

The high-energy, rapid-delivery ATP-PC system (also known as the phosphagen system) provides a very short supply of energy for use in physical activities such as a sprint or a single set of a resistance exercise. Continued muscular exercise relies on the glycolytic and aerobic energy systems.

The glycolytic system provides energy through the partial breakdown of glucose (found in the blood) and glycogen (glucose molecules stored in the liver and muscles). Through the process of glycolysis, glucose used by active muscles is incompletely broken down into pyruvate in intracellular fluid (cytoplasm). At every step of glycolysis, specialized enzymes are employed to speed up the chemical reactions to provide energy for activities lasting 30 seconds to 3 minutes, such as a 400-to-800-meter run.

Whereas glycolysis is sometimes called “anaerobic glycolysis” because it does not require oxygen, aerobic metabolism, the body’s third and longest-lasting energy system, is driven by oxygen. Aerobic metabolism is also called “mitochondrial respiration” because it occurs in the mitochondria, specialized organelles within cells and dispersed richly among muscle cells to supply ATP to active muscles. All physical activities lasting 3 minutes or longer depend primarily on mitochondrial respiration. During this process, the products of carbohydrate breakdown are broken down further into carbon dioxide and water, releasing energy used to regenerate ATP.


Whereas carbohydrates can be broken down with or without oxygen, fats, which also help produce ATP, are metabolized only in the presence of oxygen. Free fatty acids, derived from triglycerides in dietary fat, can be broken down into two-carbon compounds for use in mitochondrial respiration.

At rest, proteins play a very minor role in ATP production. However, during prolonged endurance exercise, they may supply as much as 10 percent of the energy needed.

What Regulates ATP Production During Calorie Burning?

Although the body’s three energy systems interact with each other simultaneously to produce ATP, the relative extent of their roles depends on (1) exercise duration and intensity, (2) fitness level and body composition and (3) diet. How do cells control and regulate which macronutrients supply the caloric needs of exercise? What tells cells to use more of the phosphagen system or transition into mitochondrial respiration by predominantly using fats and carbohydrates?

Two methods of metabolic control are at work during exercise. One operates inside cells; the other operates outside cells. Specific regulatory hormones either activate or inhibit both of these control systems.

Intracellular regulation depends on key enzymes that—depending on the levels of ATP, adenosine diphosphate (ADP) and other molecules present (or absent) in cells—inhibit or activate ATP production to meet the body’s energy needs. Because intracellular regulation is quick, it is closely linked to the phosphagen and glycolytic systems.

In extracellular regulation, the second major regulatory system, if the body’s glycogen level is low, the hormone glucagon stimulates gluconeogenesis (glucose production from noncarbohydrate sources) and releases glucose into the blood for use by the muscles. During prolonged exercise, epinephrine and other hormones may activate hormone-sensitive lipase to begin breaking down stored triglycerides for metabolism through mitochondrial respiration.

How Does Endurance Training Enhance Calorie Burning?

Endurance (cardiovascular) training contributes to a number of metabolic adaptations that enhance calorie burning.

Increase in the Size of Slow-Twitch Muscle Fibers. Cardiovascular exercise relies primarily on slow-twitch muscles, and research has demonstrated that these muscle fibers become 7 to 22 percent larger than corresponding fast-twitch fibers in response to cardiovascular training (Wilmore & Costill 1999). This increase in the size of a muscle increases its metabolic demands, improving calorie burning.

Increase in the Number of Capillaries Surrounding Muscle Fibers. Capillaries are the blood vessels that form within muscle tissue the complex networks necessary for the exchange of oxygen, carbon dioxide, water and other cellular products. Cardiovascular exercise has been shown to increase the number of capillaries surrounding muscle fibers by 5 to 15 percent for greater exchange of gases, water and nutrients between the blood and the working muscles to enhance calorie burning (Wilmore & Costill 1999).

Increase in Myoglobin Content. Oxygen entering the muscle is bound to myoglobin, which stores oxygen and transports it to the mitochondria for mitochondrial respiration when needed. Regular endurance training has been shown to increase myoglobin content by 75 to 80 percent
to increase the trained muscle’s local storage capacity for oxygen (Wilmore & Costill 1999).

Increase in the Size, Number and Efficiency of Mitochondria/Improved Efficiency of Mitochondrial Oxidative Enzymes. Research has demonstrated that endurance training contributes to an increase in the size (35%), number (15%) and efficiency of mitochondria. In addition, cardiovascular exercise enhances the efficiency of the mitochondrial oxidative enzymes that facilitate the breakdown reactions of nutrients and subsequent production of ATP. In one study, the oxidation of free fatty acids in cycle-trained males was 30 percent higher than that of the same men in their pretraining status (Wilmore & Costill 1999).

These metabolic changes improve submaximal endurance and maximal aerobic capacity to help the body burn calories more efficiently during cardiovascular exercise (Wilmore
& Costill 1999).

How Does Resistance Training Enhance Calorie Burning?

The largest component of the body’s total calorie expenditure is the energy needed to maintain its resting metabolic rate (RMR). RMR represents the calories needed at rest to maintain all of the body’s vital processes and systems. Therefore, increasing RMR helps enhance calorie burning.

Various factors influence RMR, but the body’s amount
of muscle tissue, one of the most metabolically active kinds of tissue, is particularly important. Therefore, the more muscle one has, the higher one’s RMR, and, as you know, resistance training builds muscle.

One well-designed and meaningful study showed a 7 percent increase in RMR in men and women ages 56 to 80 after 12 weeks of resistance training. The exact reasons for the increase in their RMR are complex but may include an increase in the turnover of proteins, an increase in the activity of various enzymatic reactions, an increase in the concentration of metabolic hormones, the replenishment of glycogen stores and the repair of muscle tissue (Campbell
et al. 1994).

Which Exercises
Are Best for
Burning Calories?

Both endurance and resistance training programs are needed to optimize calorie expenditure. Endurance exercise should use the large muscles of the body in a continuous, rhythmic fashion; be relatively easy to maintain at various workout intensities; and, to encourage adherence, satisfy the exerciser’s personal interests. To avoid overuse injury, vary the mode of endurance training regularly.

A major way to optimize energy expenditure in endurance exercise is to vary the intensity with various interval training schemes. (See “Interval Training Design” on page 32.) Using exercise modes that can be easily adjusted or graded to overload the cardiorespiratory system is quite helpful. For instance, increasing the grade of a treadmill can make walking on it more challenging, and stationary cycling becomes more demanding when the pedaling resistance is increased.

Although the type of resistance training program best for optimal calorie expenditure is unknown, recent research on periodized programs has shown favorable results (Marx et al. 2001). See “Circuit vs. Periodized Resistance Training for Women” (IDEA Personal Trainer, November-December 2002, pp. 28-33) for an extensive review of a contemporary periodized training program.

How Does
This Information
Help Clients?

Understanding the origins of calories, the processes by which calories are turned into energy and the enhanced calorie-burning effects of regular exercise should enable you to help your clients control their weight not by taking potentially risky supplements (as discussed in “Can Dietary Supplements Enhance Calorie Burning?” on page 30) but by eating a sensible diet and participating in properly designed cardiovascular and resistance training.