NUTRITION, CALORIC RESTRICTION

The first work that linked caloric restriction to an extension of life span was published in 1935 by Clive McCay and his colleagues, nutritionists at Cornell University in Ithaca, New York. Their studies showed that restricting the food intake of rats soon after weaning increases their length of life. Since then, this finding has been confirmed many times in rats, mice, and hamsters. In most of these studies, animals fed ad libitum (allowed to eat as much as they want) were compared to those restricted to 50 to 60 percent of that amount of food, with care being taken to provide a sufficient diet to avoid malnutrition.

Decreasing food intake has also been found to extend the length of life of nonmammalian species such as fish, flies, nematodes, and water fleas. It has yet to be established that restricting food intake has such an effect on all, or most, species, however. This is because the effect of reducing food intake on longevity has yet to be assessed in most species, and also because the nutritional requirements of most species are not sufficiently known to be certain that one is not dealing with the effects of malnutrition.

Studies on rats and mice

Most research on the life-extending effects of long-term dietary restriction has been done on rats and mice. These studies have shown that the effects on longevity do not relate to a decrease in the intake of a specific nutrient, such as vitamins, minerals, and protein, or a dietary contaminant, but rather result from a decrease in intake of calories. It is for this reason that this phenomenon is referred to as caloric restriction.

Caloric restriction has been found to markedly increase the length of life when initiated in the young adult. It also does so when started as late as early middle age, but less markedly than in younger animals.

With increasing age, there is an increase in mortality. Analysis of age-specific mortality (the fraction of the population that dies during a specific age interval) reveals that caloric restriction reduces the age-associated increase in age-specific mortality of adult rats and mice. This finding strongly suggests that caloric restriction extends the length of life by slowing the rate of aging, a conclusion supported by the fact that it retards the age-associated deterioration of physiological functions. These functions range from fundamental cellular processes, such as DNA repair, apoptosis, proteolysis, signal transduction, gene expression, and many others, to integrated organismic functions, such as negotiating a maze. Caloric restriction also delays the occurrence or slows the progression of most age-associated diseases in rodents, including many different cancers, as well as degenerative diseases, such as nephropathy, cardiomyopathy, cataracts, and autoimmune diseases.

Many studies have aimed at discovering the mechanisms underlying the antiaging action of caloric restriction. Such knowledge would provide insights in the quest to understand the basic nature of aging and to develop interventions in aging. In their 1935 publication, McCay and his colleagues proposed that food restriction extended length of life in rats by retarding development and growth. However, the finding that life extension occurs when food restriction is started in adult life invalidates this hypothesis.

Caloric restriction is known to decrease the content of body fat, and excess body fat is associated with premature death in humans. Thus, some have hypothesized that caloric restriction increases the length of life by decreasing body fat. However, studies on rats by Helen Bertrand and her associates in Texas, and on mice by David Harrison and his associates in Maine, found that caloric restriction's action on body fat is not related to the antiaging effect.

Caloric restriction also decreases the body temperature of rats and mice. It is known that reducing the body temperature of poikilotherms (species in which body temperature varies with the environmental temperature) increases their life span. It was hypothesized that caloric restriction increases the length of life of rodents by decreasing body temperature, but several lines of evidence indicate this is not a valid hypothesis. For example, caloric restriction causes the body temperature in mice to decrease much more than in rats, but the increase in length of life is similar in both species. Also, restricting food intake in fish increases length of life without lowering body temperature. Finally, while the body temperature of mice maintained in a warm environment is not decreased by caloric restriction, these mice continue to exhibit most antiaging effects.

It is known that biological molecules, such as DNA, proteins, and lipids, are damaged by reactive oxygen molecules such as hydroxyl and superoxide radicals. Reactive oxygen molecules are generated by intrinsic living processes as well as environmental factors. In 1996, Rajindar Sohal and Richard Weindruch suggested that caloric restriction retards aging by decreasing oxidative damage. Caloric restriction does, indeed, retard the age-associated accumulation of oxidatively damaged molecules. It is often stated that this protective action results from a lowered specific metabolic rate (metabolic rate per unit of body mass); however, studies on both rats and mice have shown that caloric restriction can have life-extending and antiaging actions without decreasing the specific metabolic rate. Of course, a decreased production of reactive oxygen molecules is not dependent on a reduction in metabolic rate. Furthermore, enhancement of antioxidant defenses would also protect against damage from reactive oxygen molecules even if their rate of production did not change. Caloric restriction may well decrease the rate of production of reactive oxygen molecules, and/or increase the level of protection against their damaging effect, but there are not yet sufficient data to judge the importance of either. Indeed, the fact that caloric restriction increases the repair or removal and replacement of damaged molecules may play the major role in its ability to reduce the accumulation of oxidatively damaged molecules. However, the question of whether this ability of caloric restriction protects against the accumulation of oxidative damage is the reason for its antiaging action cannot be answered until the importance of oxidative damage in aging is clearly established.

Food restriction has been found to result in sustained reductions in blood levels of glucose and insulin without decreasing the ability to use glucose as a fuel. It does so, at least in part, by increasing insulin sensitivity. It has long been known that elevated blood levels of glucose and/ or insulin cause damage similar to aging, and it has been suggested that maintenance of low levels of these substances underlies the antiaging action. Although much has been learned about the mechanisms responsible for the decreased levels of glucose and insulin, the hypothesis of their playing a major role in the antiaging action remains to be tested.

In 1998, this author proposed the hormesis hypothesis. Hormesis is defined as the beneficial action resulting from the response to a low-intensity stressor. Caloric restriction in rats and mice meets the criteria of hormesis. It causes a daily moderate elevation of blood glucocorticoid level, a characteristic action of a low-intensity stressor, and, in regard to beneficial action, it increases the ability of rats and mice to cope with the damaging actions of acute, intense stressors such as surgery, toxic chemicals, and high environmental temperature. Since aging appears to be the result of the accumulation of unrepaired damage due to intrinsic processes, as well as environmental agents and their interactions, this increased ability to cope with stressors may well be the basis for the retardation of aging by caloric restriction. Indeed, genetic manipulations that retard aging in fruit flies, nematodes, and yeast have also been shown to increase the ability of these organisms to cope with acute, intense stressors. Moreover, the increased daily levels of blood glucocorticoids may well play a major role in the increased resistance to acute, intense stressors and in the antiaging action. Caloric restriction also enhances expression of stress response genes, thereby increasing the production of proteins that protect cells against damaging agents, possibly including agents that promote aging. Although many actions of caloric restriction fall within the realm of hormesis, this hypothesis has yet to be rigorously tested.

Studies on humans and nonhuman primates

Does caloric restriction retard aging and extend life in humans? It is not yet possible to answer this often-asked question because a carefully designed and executed study has not been done with humans—nor is it likely that one will ever be done. In lieu of this, studies on the effects of caloric restriction on surrogates—nonhuman primates—were started in the 1980s and early 1990s.

The rhesus monkey is the primate species most studied. Since the maximum life span of this species is estimated to be forty years, information on the effect of caloric restriction on the length of life of the rhesus monkey will not be available until the year 2025 at the earliest. Thus far, however, many of the effects of caloric restriction found in rats and mice have also been observed in the rhesus monkey. For example, as in rodent species, caloric restriction in the rhesus monkey decreases body temperature, decreases blood glucose and insulin levels, and increases insulin sensitivity. This kind of information indicates that caloric restriction may have an antiaging action in the rhesus monkey similar to that in rats and mice. However, one must be cautious about jumping to any conclusions since the relationship between a particular physiological effect of caloric restriction and its antiaging action still remains to be established in rodents.

In rhesus monkeys, caloric restriction decreases the risk factors for age-associated cardiovascular diseases. It decreases both body fat and its age-associated increased distribution to the abdominal region, as well as decreasing the blood level LDL ("bad") cholesterol and increasing that HDL ("good") cholesterol. It also improves glucose tolerance and prevents the development of Type II diabetes. Nevertheless, a definitive answer regarding the antiaging action of caloric restriction in the rhesus monkey awaits the findings of the longevity studies.

Future research directions

Calorically restricted rats and mice are important tools for learning about basic biological processes that underlie aging. Indeed, the findings to date have focused research on the potential importance of oxidative stress in aging, and have also drawn attention to a possible role of blood glucose and insulin levels. Additionally, this research has provided further evidence of a relationship between resistance to stressors and retardation of aging.

Understanding the mechanisms underlying the antiaging action of caloric restriction could well yield insights on possible interventions for the retardation of human aging. The development of pharmacological agents with antiaging actions similar to those of caloric restriction is a particularly promising possibility. Even if long-term caloric restriction were conclusively demonstrated to extend life in humans, however, it is not likely many would undertake such a Spartan existence. Thus, there may be need for a more palatable regimen, such as a daily pill.

It is anticipated that research in the twenty-first century will focus on defining the mechanisms responsible for the antiaging action of caloric restriction. Genetically engineered mice are increasingly being used in this endeavor. In such mice, specific genes are either underexpressed or overexpressed. Although conceptually appealing, interpretations of such studies are problematic for at least two reasons. One, it will be a Herculean task for an investigator to establish that the mouse has been altered only in regard to the particular characteristic or characteristics being assessed. Moreover, the antiaging action of caloric restriction may be due to the action of more than one gene; if so, the manipulation of a single gene might provide little or no insight. The probability is infinitesimal that a mouse could be designed with the appropriate level of expression of several genes involved in the antiaging process.

Another new approach being increasingly used is gene expression profile methodology, in which a broad array of genes are assessed. In one such study, although only 5 to 10 percent of the genome was monitored, caloric restriction was found to affect the expression of more than one hundred genes. Thus assessment of the entire genome may find the expression of well over one thousand genes to be modified by caloric restriction. With so many genes influenced, it will be incredibly difficult to interpret the findings in regard to the mechanism(s) responsible for the antiaging action of caloric restriction.

For these reasons, it will be a formidable undertaking to determine the mechanisms of the antiaging action of caloric restriction, though the thoughtful use of currently available methodologies, as well the employment of new technologies, should eventually bring success.

Edward Masoro

See also DNA Damage and Repair; Genetics: Gene Expression; Life Span Extension; Longevity; Nutrition; Stress.

BIBLIOGRAPHY

Duffy, P. H.; Feuers, R. J.; Pipkin, J. L.; Berg, T. F.; Leakey, J. E. A.; Turturro, A.; and Hart, R. W. "The Effect of Dietary Restriction and Aging on Physiological Response of Rodents to Drugs." In Dietary Restriction: Implications for the Design and Interpretation of Toxicity and Carcinogenicity Studies. Edited by R. W. Hart, D. A. Neuman, and R. T. Robertson. Washington, D.C.: ILSI Press, 1995. Pages 125–140.

Holehan, A. M., and Merry, B. J. "The Experimental Manipulation of Ageing by Diet." Biological Reviews 61 (1986): 329–368.

Holliday, R. "Food, Reproduction, and Longevity: Is the Extended Lifespan of Calorie-Restricted Animals an Evolutionary Adaptation." BioEssays 10 (1989): 125–127.

Lee, C-K.; Klopp, R. G.; Weindruch, R.; and Prolla, T. A. "Gene Expression Profile of Aging and Its Retardation by Caloric Restriction." Science 285 (1999): 1390–1393.

Masoro, E. J. "Hormesis and the Antiaging Action of Dietary Restriction: A Hypothesis." Experimental Gerontology 33 (1998): 61–66.

Masoro, E. J. "Caloric Restriction and Aging: An Update." Experimental Gerontology 35 (2000): 299–305.

McCarter R. J. M. "Caloric Restriction, Exercise, and Aging." In Handbook of Oxidants and Antioxidants in Exercise. Edited by C. K. Sen, L. Packer, and O. Hanninen. Amsterdam: Elsivier Science, 2000. Pages 797–829.

Pahlavani, M. A. "Intervention in the Aging Immune System: Influence of Dietary Restriction, Dehydroepiandrosterone, Melatonin, and Exercise." Age 21 (1998): 153–173.

Roth, G. S.; Ingram, D. K.; and Lane, M. A. "Caloric Restriction in Primates: Will It Work and How Will We Know?" Journal of the American Geriatrics Society 47 (1999): 896–903.

Sabatino, F.; Masoro, E. J.; McMahan, C. A.; and Kuhn, R. W. "An Assessment of the Role of the Glucocorticoid System in Aging Processes and in the Action of Food Restriction." Journal of Gerontology: Biological Sciences 46 (1991): B171–B179.

Sohal, R. S., and Weindruch, R. "Oxidative Stress, Caloric Restriction, and Aging." Science 273 (1996): 59–63.

Weindruch, R., and Walford, R. The Retardation of Aging and Diseases by Dietary Restriction. Springfield, Ill.: Thomas, 1988.

Weraarchakul, N.; Strong, R.; Wood, W. G.; and Richardson, A. "Effect of Aging and Dietary Restriction on DNA Repair." Experimental Cell Research 181 (1989): 197–204.

Yu, B. P. "Aging and Oxidative Stress: Modulation by Dietary Restriction." Free Radical Biology and Medicine 21 (1996): 651–668.

CALORIC INTAKE

CALORIC INTAKE. The calorie is a unit of heat energy required to raise the temperature of 1 gram (1 milliliter) of water 1°C from 14.5° to 15.5°C. The calorie is a very small unit; and although it is used colloquially, the energy measured is 1,000 gram calories or kilocalories (kcal). Another unit of energy is the joule: 1 kcal = 4.184 kJ. Calories are used as a unit to measure the energy in food as well as the energy produced, stored, and utilized by living organisms.

Calories in Food

Foods are comprised of carbohydrates, fats, proteins, vitamins, minerals, and water. The energy-yielding nutrients

are carbohydrates, fats, and proteins. When foods are oxidized or burned, they yield approximately 4.0 calories/gram for proteins and carbohydrates and 9.0 calories/gram for fats. Most foods contain mixtures of the three macronutrients but are classified by the predominant nutrient. For example, protein-riched foods such as beef also contain fat. Table 1 presents a sample of ten common foods, their weights, serving sizes, and calories. Daily caloric consumption is assessed in a number of ways: 24-hour dietary recalls, food intake diaries, weighing food before it is eaten, and using food labels on packaged foods.

In the early to mid-1990s, world food supply estimates of available calories per person per day ranged from 2,099 kcal in sub-Saharan African countries to 3,600 kcal in North America with averages of 2,573 kcal for developing countries and 3,356 kcal for industrialized countries. The percentages of calories are estimated to be 70.4 and 52.3 from carbohydrate, 19.6 and 35.4 from fat, and 10.0 and 12.3 from protein for developing and industrialized countries, respectively.

The energy requirements for humans are established by national and international organizations (for example, World Health Organization). The Recommended Dietary Allowances (RDAs), or more recently, the Dietary Reference Intakes (DRIs) are shown in Table 2. These caloric recommendations are based on the average needs for an individual based on age and sex with additional allowances for pregnancy and lactation. Restricted intakes lead to growth faltering in weight and height; and conversely, a surfeit of calories leads to excess energy stores and obesity. For example, a short and thin, preadolescent child with a slow rate of growth along the fifth percentile requires about 17 percent less dietary energy than a child following the average growth trajectory at the fiftieth percentile.

Energy Expenditure

Caloric needs are based on energetic demands related to body size and activity. Overall caloric demands per kilogram of body weight are extremely high from conception through infancy, and decrease thereafter. Total Daily Energy Expenditure (TDEE) is a combination of the Basal Metabolic Rate (BMR) sometimes called the Resting Metabolic Rate (RMR) and Physical Activity Level (PAL). The BMR is the minimum energy expenditure for maintenance of respiration, circulation, body temperature, and other vegetative functions. It is based on body size, growth, reproduction, diurnal hormonal variation, other physiological conditions, and thermoregulation related to environmental temperature. The BMR accounts for approximately two-thirds of the TDEE. In addition, about 10 percent of energy intake is expended in dietary thermogenesis or the energy costs of processing food.

Metabolic rate can be obtained by direct calorimetry that measures the total quantity of heat liberated from the body in a specially constructed chamber. With direct calorimetry, a young male with an average American diet generates 4.82 calories for every liter of oxygen consumed. Because direct measurements involve expensive equipment in a laboratory setting, most of the calculations for caloric needs are done with indirect calorimetry. Indirect calorimetry uses the differences in the concentrations of inhaled and exhaled oxygen to estimate energy use. The law of conservation of energy states that there must be a balance between all forms of energy expended or absorbed.

Reference equations are used most frequently to estimate energy needs and expenditures. Sets of reference

equations for BMR and standard energy expenditures for activities or PAL are routinely used in assessing an individual's TDEE. For adults, PAL ranges from 1.4 to 2.10 for light to very heavy work. For example, daily energy expenditure for a 120-pound (54.4-kg) woman who has a sedentary office job would be approximately:

BMR = 14.7(54.4) + 496 = 1,296 kcal
TDEE = PAL × BMR = 1.56 × 1,296 = 2,022 kcal.

Body weight is a key variable in energy expenditure. Walking at 3.5 miles/hour expends 0.035 kcal/min which would be 3.9, 5.2, and 7.0 kcal/min for individuals weighing 110, 150, and 200 pounds, respectively.

Individual differences in metabolic mechanisms are not well understood. Remarkably, for most adults, the sensitivity of the energy balance system for change is less than one percent per year. The "average" American adult male contains 140,000 kilocalories of energy in body fat, 24,000 kilocalories in protein, and only about 800 kilo-calories in carbohydrate. Consequently, an individual consuming 2,000 kilocalories per day of which 40 percent is carbohydrate will ingest an amount of carbohydrate comparable to body stores, protein intake will average only about one percent, and fat intake will be considerably less than one percent of total body stores.

Weight Control and Energy Balance

Caloric intake and expenditure are factors in the growing worldwide epidemic of obesity. It is estimated that in the United States, 35 percent of adults eighteen years of age and older are overweight or obese based on weight for height standards. The rising prevalence of obesity is due to an increase in intake of calorically dense foods and a decrease of energy expenditure with modernization, including increases in transportation, decreases in subsistence activities, increases in thermally regulated environments, and decreases in energy demands due to chronic illness. Weight reduction hinges on reducing caloric intake while maintaining micronutrient balance and fiber and water intake. Traditionally, recommended diets were labeled by their caloric content, such as the "1,200 calorie diet." Some diets have focused on the reduction of fat because each gram of fat is twice as calorically dense as a gram of carbohydrate or protein. Interventions also emphasize increased voluntary energy expenditure through daily routines such as walking or formal exercise programs. Pharmacological interventions directly influence energy balance by increasing BMR, decreasing absorption of fat calories, or decreasing transit time through the gut, and indirectly by altering hunger and satiety.

Low-energy intake compromises growth in vital tissues, lowers basal metabolic rate, and reduces work capacity relative to individuals with sufficient energy intake. People develop behavioral strategies to reduce muscular activity to conserve energy. For undernourished populations, interventions target increasing caloric intakes and reducing macro-and micronutrient deficiencies.

Finally, energy in kilocalories or joules has been used for measuring the flow of energy through ecological systems. Energy flow analyses measure time and task allocations in various subsistence activities as well as the biological characteristics of human and nonhuman populations in an ecosystem.

See also Assessment of Nutritional Status; Composition of Food; Dietary System: A Historical Perspective; Eating: Anatomy and Physiology of Eating; Nutrient Requirements; Nutrition; Obesity; Physical Activity and Nutrition.

BIBLIOGRAPHY

Bray, George A. "Obesity—A Disease of Nutrient or Energy Balance?" Nutrition Reviews 45 (1987): 33–43.

FAOSTAT. Computerized Information Series: Food Balance Sheets 1961–1994. Rome: Food and Agricultural Organization, 1996.

Food and Agricultural Organization. World Food Supplies and Prevalence of Hunger. Rome: Food and Agricultural Organization, 1992.

Food and Agricultural Organization/World Health Organization/United Nations University. Energy and Protein Requirements. Geneva: WHO Technical Report Series No. 724, 1985.

Food and Nutrition Board. Recommended Dietary Allowances. 10th ed. Washington, D.C.: National Research Council/ National Academy of Science, 1989.

Ulijaszek, Stanley J. Human Energetics in Biological Anthropology. Cambridge: Cambridge University Press, 1995.

U.S. Department of Agriculture. Nutritive Value of Foods. Home and Garden Bulletin No. 72. Washington, D.C.: USDA, 1986.

Leslie Sue Lieberman

CALORIE

CALORIE. The calorie is a unit for measuring heat energy, and it is usually used as the unit for food energy and of energy expenditure. Media and lay attention to food, exercise, and health, as well as the greater prevalence of obesity during the past few decades, has resulted in a cultural preoccupation with caloric intake and expenditure in industrialized nations. Heat is that which produces a change in temperature. Heat was formerly regarded as a substance called "caloric," but it came to be viewed as the random motion of molecules.

The calorie has traditionally been defined as the amount of heat required to raise the temperature of 1 gram of water by 1.8°F (1.0°C), usually defined as from 58.1°F to 59.9°F (14.5°C to 15.5°C), under normal atmospheric conditions. Because electrical measurements can be standardized more accurately than heat measurements, a calorie is officially defined as equivalent to 4.186 joule. A joule is defined, in "force × distance" units, as 1 Newton meter, which is equal to (1 kg m/s²) × (1m) or 1 kg m²/s². Energy values are expressed as joules when the Système International d'Unités, which is recommended for all scientific purposes, is required.

Food energy values and energy expenditures are commonly expressed as the number of kilocalories (kcal). One kcal is equal to 1000 calories or 4.186 kJ or 0.004186 MJ. Although the terms "calorie" and "large calorie" have frequently been used in place of kilocalorie in the nutrition literature and for food labeling purposes, these alternative terms are confusing, and their use is discouraged.

Measurement of Energy Values of Foods

The energy in foods is present as chemical energy; it can be measured by the heat evolved when the food is oxidized or combusted. Although energy transformations normally involve friction and heat conduction, which cause the changes of one form of energy to another to be incomplete, various forms of energy normally can be converted completely to heat. The caloric value of a food may be determined by burning weighed samples of the food in an oxygen atmosphere in an apparatus called a calorimeter, which is designed to allow measurement of the heat released by combustion of the fuel or food. The total amount of heat produced or consumed when a chemical system changes from an initial state to a final state is independent of the way this change is brought about (the law of Hess or the law of constant heat sums). Thus the complete oxidation of a compound, such as glucose, to CO₂ and H₂O produces the same amount of heat whether the process is carried out in a calorimeter or by metabolism within the body.

Heats of combustion are not accurate reflections of the amount of energy available to the body, however, because the body does not completely absorb and metabolize ingested nutrients. The energy lost in the excreta (feces and urine) must be subtracted from the total energy value of the food to obtain the amount of energy available to the body from consumption of the food. The caloric values of foods reported in food composition tables are "physiological fuel values," also referred to as "available energy" or "metabolizable energy" values. They are not total energy values.

Physiological Fuel Values of Foods

The physiological fuel value of a food or a food component may be determined by measuring the heat of combustion of the food in a calorimeter and then multiplying the heat of combustion by correction factors for incomplete digestion and incomplete oxidation of the food in the body. In about 1900, Wilbur Olin Atwater and his associates at the Connecticut (Storrs) Agriculture Experiment Station used this approach to determine the physiological fuel values of a number of food components (i.e., the protein, fat, and carbohydrate isolated from various foods). They determined factors appropriate for individual foods or groups of foods, and they proposed the general physiological fuel equivalents of 4.0, 8.9, and 4.0 kcal per gram of dietary protein, fat, and carbohydrate respectively for application to the mixed American diet. These factors are commonly rounded to 4, 9, and 4 kcal per gram (17, 36, and 17 kJ per gram) respectively for protein, fat, and carbohydrate. The conversion factors determined by Atwater and his associates remain in use in the twenty-first century, and energy values of foods are calculated using these factors. The energy values (physiological fuel values) reported in food composition tables are commonly estimated by determination of the proximate composition of each food (i.e., the water, protein, fat, carbohydrate, and ash contents) followed by multiplication of the amount of each energy-yielding component by the appropriate conversion factor.

See also Caloric Intake; Dietary Assessment; Nutrition.

BIBLIOGRAPHY

Kleiber, Max. The Fire of Life: An Introduction to Animal Energetics. New York: Wiley, 1961.

Kriketos, Adamandia D., John C. Peters, and James O. Hill. "Cellular and Whole-Animal Energetics." In Biochemical and Physiological Aspects of Human Nutrition, edited by Martha H. Stipanuk. Philadelphia: Saunders, 2000.

Merrill, A. L., and B. K. Watt. Energy Values of Foods . . . Basis and Derivations. USDA Agriculture Handbook No. 74. Washington, D.C.: U.S. Government Printing Office, 1973.

Martha H. Stipanuk

Caloric Intake

Caloric intake is defined as the amount of heat energy the body receives from eating food. The unit used to measure this amount of energy is called the calorie. It is also used to measure how much energy the body expends during human activity or inactivity such as running, walking, sitting, thinking, or sleeping.

The calorie is the metric unit of measurement for energy. Specifically called the food calorie (sometimes the nutritional or dietary calorie) within the food industry, it is equivalent to the kilogram calorie, or kilocalorie (symbolized as kcal). The kilocalorie is defined as the amount of heat energy needed to increase the temperature of one kilogram of water by one degree within the Celsius temperature scale. (As a note of clarity, outside the food industry the gram calorie, or the small calorie, is also referred to as the calorie and is defined as the amount of heat energy necessary to increase the temperature of one gram of water by 1° Celsius.) Thus, one food calorie is equal to one kilogram calorie, which is equal to 1,000 gram calories.

The intake of food calories is an important part of how well any person maintains a healthy lifestyle. Food calories are especially important in how effectively an athlete trains and ultimately competes. Eating a balanced diet is a key part to sports nutrition. The correct combination of fuel (calories ingested), especially from carbohydrates, proteins, and fats, gives a person plenty of energy for top performance whether it is running, walking, sitting, thinking, or sleeping.

Carbohydrates are a very important source of caloric intake for the human body. They come in the form of such foods as fruits, vegetables, pastas, breads, cereals, rice, and honey. For a balanced diet, proper health, and peak performance, they should provide about 60-70% of a person's daily caloric intake. Carbohydrates provide energy when the body changes the starches and sugars within carbohydrates into glycogen, which is then stored in the liver and muscles. Glycogen is easily converted into glucose in order to provide endurance and strength for short-duration/high-intensity sports activities and exercises. A person's performance level will drop rapidly if the body eliminates its supply of carbohydrates and it is forced instead to intake proteins and fats for energy, which are less efficient fuels (calories) for the body. For maximum performance in such activities and exercises, a person should eat plenty of carbohydrates several days before the sporting event so that the body will possess plenty of glycogen within the muscles. During the event, if the competition lasts for more than an hour, the athlete should replace lost carbohydrates with additional ones. For instance, many bicyclists love to eat spaghetti before a long race because it is filled with carbohydrates. They then fill up with sports energy bars and drinks containing carbohydrates throughout the day of competition.

Proteins should consist of from about 12% to 15% of a person's daily caloric intake. They come in various foods such as beans, dairy products, eggs, and nuts. Proteins provide the human body with power to build new tissues and generate other essential functions. They are not stored in the body but are turned immediately into energy or changed into fat. Physically active people need more proteins then inactive people. Endurance athletes such as body builders often consume plenty of proteins as a way to produce fuel to build strong muscles. However, it is a falsehood that by eating plenty of protein and/or taking protein supplements a person can become a muscular person. Building muscles depends on heredity, the intensity of the workout, the amount of training, and getting sufficient calories in a daily diet. The average diet of a United States citizen contains sufficient proteins for regular muscle building. Thus, if a person does not eat enough carbohydrates, then proteins are consumed rather than allowed to perform their regular function of building new tissues. When a body has enough carbohydrates, then proteins produce about 5% of energy. If not enough carbohydrates are available, then proteins can produce up to 10% of energy. The caloric intake of fats are needed only in small amounts. They come as saturated fats in such foods as cheese, eggs, meats, and milk, and as unsaturated fats in such foods as canola oil, corn oil, and palm oil. Fats are used as the energy source primarily when a person is inactive or during low to medium rates of exercise. When carbohydrates are used up, however, fats are burned instead, which decreases the performance level of an athlete because energy is produced less efficiently.

For adults and for children over the age of two years, the American Heart Association (AHA) provides general dietary recommendations in order to maintain a normal weight-to-height ratio and to provide an adequate caloric intake of food. The AHA suggests that a diet should consist primarily of whole grains, beans, fruits and vegetables, low-fat and nonfat dairy products, fish, and lean meats. The diet should also consist of low caloric intakes of saturated fats, trans fatty acids (trans fats), partially hydrogenated oils, cholesterol, and supplemental sugar and salt.

Caloric intake is important. The caloric intake of healthy foods is even more important to the health and well-being of all persons.

see also Calories; Carbohydrates; Fat intake; Protein ingestion and recovery from exercise.

Calorie

A calorie is the amount of energy required to raise the temperature of 1 gram of pure water by 34°F(1°C) under standard conditions. These conditions include an atmospheric pressure of one atmosphere, and a temperature change from 60° to 62°F (15.5-16.5°C).

The calorie is also sometimes designated as a gram-calorie or small calorie (abbreviated: cal), to distinguish it from the calorie of dieticians (abbreviated: Cal), also known as a large calorie, or kilocalorie (kcal), which is equal to 1,000 (small) calories.

One calorie is equivalent to 3.968 British thermal units (btu), a non-metric measure of energy content. A calorie is also equivalent to 4.187 joules (also known as an International Table calorie), which is now the unit of energy that is most commonly used in science.

Scientists are often interested in the energy contents of organic materials. These data are usually obtained by completely oxidizing (burning) a known quantity of a substance by igniting it in an oxygen-rich atmosphere inside of a device known as a bomb-calorimeter. The quantity of energy released is determined by measuring the increase in temperature of a known quantity of water contained within the bomb.

Dieticians are interested in the calorie contents of foods of various sorts. The potential energy of food is utilized metabolically by animals to drive their physiological processes, and to achieve growth and reproduction. Foods vary tremendously in their energy contents, so careful planning of food intake requires an understanding of the balance of the nutrients, such as vitamins and amino acids.

On average, pure carbohydrates have a calorific content of about 4,600 cal/g (or 4.6 Cal/g), while proteins contain about 4,800 cal/g, and fats or lipids about 6,000-9,000 cal/g. Because fats are so energy-dense, they are commonly used by organisms as a compact material in which to store potential energy for future use. Of course, some of us store more of this potential energy of fat than others.

Engineers are often concerned with the energy contents of petroleum, coal, and natural gas, and of distillates or synthetic materials refined from any of these fossil fuels. Knowledge of the amounts of energy that are liberated through the complete oxidation of these materials is important in the design of engines, fossil-fueled generating stations, and other machines that we use to achieve mechanical work. In order to maximize the amount of useful work that is achieved per unit of fuel consumed, that is, the energy-conversion efficiency, engineers are constantly re-designing machines of these sorts, and tuning their operating parameters, such as fuel: oxygen ratios.

Ecologists are also interested in the energy contents of organic materials, and how these change over time. Although ecologists commonly measure biomass and productivity in terms of weight, these are often converted into energy units, in order to account for the greatly varying calorific contents of different sorts of biomass, as was described above for carbohydrates, proteins, and fats. In parallel with the interests of engineers, ecologists are concerned with the efficiency of ecosystems in converting solar energy into plant productivity, as well as the transfers of the energy of plants to herbivores and carnivores. These efficiencies are best determined through knowledge of the amounts and transfers of energy, as expressed in calorific units.