Calories: Total Macronutrient Intake.

Carbohydrates, protein, fats, and alcohol—the dietary macrocomponents—are the sources of energy in the diet. Under normal circumstances, more than 95% of this food energy is digested and absorbed from the gastrointestinal tract to provide the body's energy needs. Studies of normal and overweight subjects have not shown any significant differences in the proportion of food energy absorbed. In some underweight subjects, however, malabsorption of nutrients is an important factor. Food energy is used to meet the body's needs, including protein synthesis; maintenance of body temperature, cardiac output, respiration, and muscle function; and storage and metabolism of food sources of energy. When more energy is consumed than is needed for metabolism and physical activity, the excess is stored, primarily as adipose tissue.

Energy is the capacity to do work. In biologic systems it is usually measured in kilocalories (kcal) or kilojoules (kJ). One kilocalorie (equivalent to 4.184 kJ) is the amount of heat required to raise the temperature of 1 kg of water 1°C (e.g., from 15 to 16°C) at standard atmospheric pressure (760 mm Hg).

Energy balance refers to the relationship of energy intake to energy expenditure and energy storage. Less energy expenditure than energy intake results in a positive energy balance and storage of energy primarily as body fat. Increased fat storage is appropriate during pregnancy and lactation, during some periods of growth and development, and during recovery from trauma or malnutrition, but it may not be desirable under other conditions. When energy expenditure exceeds energy intake, energy balance is negative and leads to weight loss. When intake equals expenditure, equilibrium results and body fat is maintained, regardless of whether the body weight is at, above, or below normal. Even at stable body weight, however, the percentage of body fat frequently increases with age unless regular physical activity is maintained. At the same body weight, some sedentary people have relatively more body fat than those who exercise. Thus even at normal weights, these individuals may have more adiposity than desirable.

Prolonged insufficient energy intake results in malnutrition, which is observed in many developing countries and is also a public health concern for a minority of the U.S. population. In most cases, insufficient food intake results from a lack of economic ability to obtain food, from an illness, or from physical or mental disorders that prevent sufficient ingestion or utilization of food to meet energy expenditure, or in some cases from voluntary restriction of food intake and dieting.
(Diet and Health: Implications for Reducing Chronic Disease Risk (1989)
Commission on Life Sciences)