|The Remarkable Calorie
By Carole A. Conn, Ph.D., R.D. & Len Kravitz, Ph.D.
Energy represents the capacity to do work. Completing a Pilates class, aerobics workout, resistance training session, or a yoga lesson are examples of activities in which foods are being converted to chemical energy in the muscle cells and then transformed into mechanical energy for the physical exercise. In the United States, the most common term used to express energy is the calorie.
The number of calories is listed on the label of any energy bar you pick up. Broccoli has calories even though it has no label telling you how many. Somewhere along the line most people learned that calories are used by the body for energy and if you eat too many you get fat and if you eat none you eventually starve. But have you ever wondered just what a calorie is, how it got into your food, and how your body goes about using it? This article will review these aspects of the remarkable calorie.
Just what is a calorie anyway?
A calorie is a measure of energy. It is defined as the heat energy required to raise the temperature of one gram of water by one degree Celsius. It is also defined as 4.184 joules where one joule is the heat energy given off when an ampere flows through the resistance of one ohm for one second (Stedmans). The energy used in physical activity and the energy stored in foods is actually given in kilocalories (the heat energy required to raise the temperature of one kilogram of water by one degree Celsius). Often kilocalories are referred to as kcals or as large calories or as Calories, where the capital C indicates kilocalories. However, because a calorie is such a small unit of energy the word calorie to define a small calorie is mainly used in scientific literature. Most of the time calorie spelled with the small c actually refers to the kilocalories provided in food and used during exercise. In this article, we follow the common custom and use calorie to refer to kilocalorie.
Why do foods have calories?
Foods have calories because foods come either from plants or from animals that have eaten plants. It is actually the plants that create the primary molecules in food that contain the energy quantified as calories (Taiz and Zeiger). Green plants create these molecules from carbon dioxide and water by capturing energy from the sun in a process called photosynthesis. The green plant pigment chlorophyll absorbs radiant energy from the sun which is then converted to chemical energy in the bonds that link the carbon from carbon dioxide (CO2) to water (H2O), creating carbohydrates, (CH2O)n or hydrates of carbon and freeing oxygen (O2) into the atmosphere. From carbohydrates, plants can create other molecules that contain captured energy; these are fats and proteins. Humans can use carbohydrates to synthesize most fatty acids, fats, non-essential amino acids and proteins just like the plants. However, the primary source of all calories are carbohydrates created by plants from carbon dioxide and water by capturing the energy of the sun.
Why do foods have different calorie levels?
There are six classes of nutrients in foods: carbohydrates, fats, proteins, vitamins, minerals and water. Only the carbohydrates, fats and proteins can provide energy. Because these three classes are consumed in large quantities in the range of 50 to 500 grams per day, they are called macronutrients. In contrast, the micronutrient classes of vitamins and minerals need to be consumed in very small quantities of 1 to 100 milligrams per day. Vitamins, minerals and water provide no calories but they are essential in our ability to use the calories stored in the macronutrients.
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, a little protein, vitamins, minerals and water, but very little carbohydrate. Meat contains a lot of protein and water, some fat, vitamins and minerals, and little or no carbohydrate, while whole wheat bread contains a lot of carbohydrate, a little protein and fat, many vitamins and minerals, but not much water. So part of the reason that foods have different calorie levels is that a usual serving of each food contains different amounts of the three classes of the energy-providing nutrients: carbohydrates, proteins and fats.
Another part of the reason that foods have different calorie levels is that the energy-providing nutrients supply different amounts of energy per gram. Fats supply the most energy at 9 calories per gram. Carbohydrates and proteins each provide 4 calories per gram for use as energy in the body. We know this because of the careful work of W. O. Atwater and his colleagues done in the late 1800s. These scientists pioneered the analysis of the nutrient classes in foods and the differing ability of each macronutrient class to supply energy (Merrill and Watt, 1973). From their work, we know that more calories will come from the peanut butter, which is higher in fat content, than from the jelly, which contains more carbohydrate, on your P B and J.
How do the calories in food become available for use by the body?
The energy stored in the carbohydrates, fats and proteins in foods becomes available to the body when the energy stored in the chemical bonds of the macronutrients has been transformed into the high-energy phosphate bonds that are usable in the myriad metabolic processes of the body (Groff and Gropper). The main molecule that carries these high-energy bonds is adenosine triphosphate (ATP). The transformation of food in the mouth to ATP in the muscle involves digestion, absorption and metabolic catabolism (chemical breakdown of large molecules into smaller ones). Digestion results in the breakdown of carbohydrates into the simple sugars called glucose (mainly), fructose and galactose. Proteins in food are broken down to amino acids and dietary fats to fatty acids and glycerol. These small molecules are absorbed by the cells lining the intestines, passed into the bloodstream and then circulate in the blood until they enter the cells of the rest of the body. Creation of ATP from metabolic catabolism of glucose, fatty acids and amino acids occurs within each cell. ATP is composed of high-energy bonds which, when split with the help of enzymes, releases energy for use by the muscles for movement, by the liver for protein synthesis, by the brain for neural transmission and by all of the bodys metabolic systems that need energy. So, it is important to emphasize that the energy that is liberated during the breakdown of food is not directly used for exercise, but to manufacture ATP. ATP is often referred to as a high-energy compound that is stored in small amounts of the tissues. PC or phosphocreatine, another high-energy compound is also stored in the tissue in limited amounts. However, it is meaningful to note that the breakdown of PC is not used as source of energy, but to quickly replenish ATP.
How do the energy systems work in the body to burn calories?
Although you may just think of caloric energy needs in terms of exercise, it is important to realize that every movement you make in your daily life requires ATP breakdown. Therefore, to sustain life ATP is consistently being used and renewed. Since there is such a limited supply of ATP and PC stored in the body, perhaps only lasting for up to 30 seconds, the body depends on stored carbohydrate, fat and sometimes protein as back-up stores for the synthesis of ATP. This ability to store these foodstuffs for energy production allows for the successful completion of numerous physical activities, such as completing a 10-kilometer race and a marathon.
The high-energy and rapid delivering ATP-PC system (referred to as the phosphagen system) provides a very short supply of energy, for use in physical activities like a set in resistance exercise or performing sprints. Continued muscular exercise requires the use of the glycolytic and aerobic energy systems.
The glycolytic system provides energy from the partial breakdown of glucose (found in the blood) and glycogen (stored glucose molecules in the liver and muscles). Glucose used by active muscles is incompletely broken down into pyruvate through a series of enzyme-mediated steps called glycolysis. Glycolysis occurs in the intracellular fluid of the cell, or cytoplasm. Glycolysis is sometimes referred to as anaerobic glycolysis because this process takes place without the need for oxygen in any of the metabolic steps. However, for every metabolic step, specialized enzymes are needed to speed up the reactions. Activities lasting between 30 seconds to 3 minutes, such as running 400 and 800 meters, depend heavily on glycolysis. In summary, glycolysis uses only carbohydrates in the form of glucose to yield ATP, which happens without the presence of oxygen.
Aerobic metabolism is the bodys third, and longest-lasting energy system. It is referred to as mitochondrial respiration because the reactions of this system occur in specialized organelles of the cells known as mitochondria. The term respiration is used because breakdown products from carbohydrates, in the presence of oxygen, can now be completely broken down into carbon dioxide (CO2), water (H2O) and energy for ATP synthesis. Mitochondria are dispersed richly throughout muscles cells to supply ATP to actively working muscles. All physical activities lasting 3 minutes or more depend primarily on mitochondria respiration for the synthesis of ATP.
Up to this point, the discussion has been focused on the bodys break down of carbohydrates to yield ATP, in the absence or presence of oxygen However, fats which also liberate ATP, can only be metabolized in the presence of oxygen. Fatty acids from triglycerides in dietary fat can be broken down into two-carbon compounds, preparing them for entry into the mitochondrial respiration energy system. Proteins play a very minor role in ATP production at rest and may only supply up to 10% of the bodys energy need during exercise.
What regulates the bodys ATP production during calorie burning?
Although it is essential to emphasize the concept that the bodys three energy systems interact with each other simultaneously to produce ATP, their relative roles are dependent on the 1) the duration of exercise; short such as in sprints, versus prolonged as in exercise maintained for over 10 minutes, 2) the intensity of exercise, 3) the persons fitness level and body composition and 4) a persons diet. What tells the cells to use more of the phosphagen system or to transition into a predominant use of fats and carbohydrates within the mitochondrial respiration system? In other words, how do the cells control and regulate which macronutrients will supply the caloric needs of the exercise?
This complex but intriguing question is answered by two methods of metabolic control during exercise. One method operates within the cells and one operates outside the cell. Both of these regulatory control systems are activated or inhibited by specific regulatory hormones. Intracellular regulation depends on key enzymes that monitor levels of ATP and ADP (adenosine diphosphate) and other molecules, and inhibit or activate the production of ATP to meet the bodys energy needs depending on the levels present (or absent) of these molecules. Intracellular regulation is quick responding and thus tightly linked to the phosphagen system and glycolysis. The second major regulatory system is extracellular regulation by hormones. Hormones like epinephrine and glucagon may activate enzymes if the muscle cell is in a lowered energy state to break down more glycogen for glycolysis. Also, during prolonged exercise, epinephrine and other hormones may activate hormone sensitive lipase and lipoprotein lipase to begin the breakdown of stored triglycerides for metabolism in mitochondrial respiration.
Can dietary supplements enhance calorie burning?
Many dietary supplements are sold with the promise that they will enhance calorie burning and cause weight loss without the need for changes in diet and activity. The major constituent in these supplements touted for burning calories is ephedra or its synthetic equivalent, ephedrine. Ephedra is the name for the alkaloid substances found in the extract of the Ephedra sinica plant and several other Ephedra species (Betz 1997; Nat Med database, p 400). Alkaloids are nitrogen-containing molecules made by plants that have significant actions in the body; for example, morphine is an alkaloid. Ephedra is also known as Ma Huang or Chinese Ephedra and this is the designation often found on the supplement label that cues you into the ephedrine alkaloids contained by the product. Another herb found on the labels that contains the ephedra alkaloids is Sida cordifoila. Just because Ma Huang is labeled as natural doesnt mean that it is safe. It has the same effects as the synthetic ephedrine found in over-the-counter decongestant medications. In folk medicine, ephedra was used short-term for nose colds and asthma and, in the early 1900s, American physicians prescribed it as a central nervous system stimulant (Foster & Tyler, 1999). It is a newer idea to use ephedra several times a day over several weeks to promote weight loss. This newer use is traced to 1972 when a Danish general practitioner noticed unintentional weight loss in his asthma patients who were taking ephedrine as part of their medication (Greenway, 2001).
The ephedra in dietary supplements that claim to increase energy and improve weight loss stimulates the sympathetic nervous system. In combination with another sympathetic stimulant, caffeine, ephedrine has been shown to increase oxygen consumption and therefore calorie burning in humans (Greenway 2000). Several studies have shown that the ephedrine/caffeine combination is effective in enhancing weight loss (Boozer, 2002; Greenway, 2001). Synthetic caffeine or several different herbs that contain caffeine can be included in the various weight-loss supplements. Names of herbs to look for on the label are guarana (Paullinia cupana or Brazilian cocoa or Zoom), kola nut (Cola acuminata, Cola nitida or Bissey Nut or Cola Seed; avoid confusion with gotu cola which does not contain caffeine), green tea (Camilla sinensis), Yerba maté (Ilex paraguariensis, maté or Paraguay Tea or St. Bartholemews Tea (Nat Med database). All of these herbs contain caffeine, which augments the action of the ephedra alkaloids in Ma Huang.
The safety of the ephedrine/caffeine combination, whether the combination is synthetic ephedrine and caffeine or the natural products found in herbal extracts, has been questioned. Although several clinical trials for weight loss have reported few adverse effects (Greenway, 2001), there are a sufficient number of severe cardiovascular and nervous system problems (such as agitation, dizziness, insomnia, headache, weakness, sweating, heart palpitations, tremor) and deaths attributed to the intake of ephedra to warrant concern (Palevitz, 2002; Haller & Benowitz, 2000). The US Health and Human Services has recently called for an evaluation of ephedra products and has recommended that the strongest possible mandatory warning label to protect the public who can buy these products freely in the marketplace. Ephedrine has been banned by the International Olympic Committee, National Football League and the National Collegiate Athletic Association and consuming a product containing Ma Huang or Chinese Ephedra is likely to make an athlete test positive. Health Canada has asked for a halt on sales of products containing more than 8 mg ephedrine per dose (website). However, it is not easy to know how much active ephedrine is actually present in a dietary supplement. Label claims for ephedrine in dietary supplements have been found to differ from the actual contents substantially. In one study, the content varied from the label by more than 20% in half of the 20 supplements measured. For some products tested, no ephedrine was present. In others, variation from lot to lot of the same product was as much as 1000% (Gurley, 2000). Despite these difficulties, some argue that the risks of being obese outweigh the risks of taking these stimulant substances, which have been shown to enhance calorie burning and weight loss (Greenway, 2001). Thus, the safety of dietary supplements containing ephedrine/caffeine is very controversial (Palevitz, 2002).
Aspirin is another substance often added to supplements sold for calorie burning. The ephedrine/caffeine/aspirin stack with synthetic compounds has been used by bodybuilders while cutting weight for competition. Aspirin prevents the formation of prostaglandin, a molecule that normally is formed to prevent the release of too much norepinephrine in response to anything that stimulates release of norepinephrine. Therefore, the effects of both ephedrine and caffeine last longer when aspirin is added (Dulloo, 1993). The active molecule in aspirin is derived from a molecule originally isolated from willow bark (several Salix species). Therefore any herb that contains natural aspirin-like molecules can exacerbate the effects of herbal Ma Huang and any of the caffeine-containing herbs like guarana, cola, or tea. Look for these aspirin-like herbs on the label: willow, white willow, aspen bark, black cohosh, poplar, sweet birch, wintergreen (Natural Med database).
Likely because of the adverse publicity surrounding ephedra, some newer weight loss or calorie burning supplements contain synephrine and claim to be nonstimulating to the nervous system. Synephrine is similar to ephedrine but little has been published regarding its effects in humans. It is derived from the Seville or bitter orange (Citrus aurantium) and appears to have minimal effects in healthy adults according to one recent study (Penzak, 2001). However, individuals with hypertension or rapid heartbeat and those taking decongestant-containing cold tablets are currently warned to avoid bitter orange.
Conjugated Linoleic Acid
Conjugated linoleic acid is a different supplement sold for weight loss. This polyunsaturated fatty acid is found naturally in beef and beef fat, so many Americans are eating less of it now than in the past. It has several different forms and there is substantial evidence that certain forms of it can significantly decrease body fat in animals (Evans, 2002). However, data for humans are conflicting and the mechanism of action in animals has not yet been identified. So at this point, whether conjugated linoleic acid promotes enhanced calorie burning is not known.
Current Status on Calorie Burning Supplements
None of the dietary supplements sold to promote calorie burning can currently be recommended for maintaining a healthy body weight, either because they have not yet been shown to be effective in humans or because the risks of heart or nervous system problems may outweigh the benefits. This is particularly true because there is already known to be a better way to enhance your ability to burn calories without harming your health. Regular exercise actually promotes many known health benefits (such as lower blood pressure, improved blood glucose control, less risk for heart disease, maintenance of weight loss) in concert with its ability to help you burn calories better.
How does aerobic exercise enhance calorie burning?
It is well understood that duration and intensity of any aerobic session will contribute directly to the amount of calories burned by the body in that bout of exercise. In this section, a number of the metabolic adaptations in the muscle that enhance calorie burning with regular aerobic exercise will be discussed.
Aerobic activities rely primarily on slow-twitch muscles. In response to aerobic training, research has shown that there is a 7% to 22% increase in size of the slow-twitch fibers (Wilmore and Costill, 1999). Capillaries are the blood vessels that form the complex networks within muscle tissue for the exchange of oxygen, carbon dioxide, water and other cells products. Endurance exercise has been shown to increase the number of capillaries surrounding the muscle fibers from 5% to 15%. Oxygen entering the muscle binds to myoglobin, which is a molecule similar to hemoglobin. Myoglobin transports oxygen in the cell to the mitochondria for mitochondria respiration. Aerobic training has been shown to increase myoglobin content by 75% to 80% (Wilmore and Costill, 1999). The mitochondria also increase in size (35%), number (15%) and efficiency from regular endurance exercise (Wilmore and Costill, 1999). Finally, aerobic exercise enhances the efficiency of the mitochondrial oxidative enzymes that facilitate the breakdown reactions of nutrients. Research has shown that oxidation of free fatty acids is 30% higher in cycle-trained males compared with their pre-training status (Wilmore and Costill, 1999). All of these metabolic changes contribute considerably to the bodys improved ability to burn calories more efficiently during aerobic exercise.
How does resistance exercise enhance calorie burning?
The largest component of total body calorie expenditure is the energy needed to maintain resting metabolic rate (RMR). The RMR represents the calories needed by the body at rest to maintain equilibrium of all vital processes and systems, such as the nervous, cardiovascular, respiratory, digestive and endocrine systems. Various factors such as age, gender, thyroid activity, medications, and diet influence RMR. Muscle tissue is one of the most metabolically active tissues that contribute to RMR. A well-designed and meaningful study by Campbell and colleagues (1994) showed a 7% increase in RMR in elderly (56 80 yrs) men and women after 12 weeks of resistance exercise. The exact mechanisms that contribute to the increase in RMR are complex, but may include an increase in protein turnover, increased activity of various enzymatic reactions, the replenishment of glycogen stores, the repair of muscle tissue, and the increased concentration of metabolic hormones (Campbell et al).
What are the best exercises to do to burn calories?
It is clear from the previous discussion that both cardiovascular and resistance training programs are essential to optimize caloric expenditure. For aerobic exercise, advise students to select a mode of aerobic exercise that uses the large muscles of the body in a continuous, rhythmical fashion, and that is relatively easy for them to maintain at various workout intensities. For exercise adherence, select a mode (or preferably modes) of exercise that satisfies your clients personal interests, while always being sensitive to possible injury risk from problems such as overuse.
A major way to optimize energy expenditure in aerobic exercise is to vary the intensity of the exercise with various interval training schemes (See Side Bar 1 on Interval Training). Using modes of exercise that can be adjusted or graded easily to overload the cardiorespiratory system is quite beneficial. For instance, treadmill walking can be made much more challenging by heightening the grade on the treadmill. Cycling intensity can be made more demanding by simply increasing the pedaling resistance. Elliptical cross-training can be graded by increasing the speed, grade and/or resistance.
With resistance training, the best type of resistance training program to optimize caloric expenditure is presently unknown, however, recent research with periodized programs has shown very favorable results (Marx et al, 2001). The reader is suggested to read the November/December 2002 edition of IDEA Personal Trainer, which extensively reviews a contemporary periodized training program.
The optimum way to enhance the burning of calories is by regular use is with properly designed and prescribed cardiovascular and resistance training programs. Hopefully this article has enabled you to appreciate and better realize the important concepts regarding the development of these programs as well as to understand the current controversies regarding the use of calorie burning supplements and the food-based origins of the remarkable calorie.
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