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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.

Leslie Sue Lieberman