Energy is needed to sustain life and to maintain optimal growth and health. In addition, certain macrominerals, trace substances, vitamins, and specific fatty acids also are important. Energy serves as an appropriate denominator for all the other nutrients, when expressed as calories expended to maintain optimal bodily composition and promote growth through infancy, childhood, and adolescence. After cessation of growth, maintenance of optimal composition becomes the goal of good nutrition.

Table 14.1 gives basal or resting expenditures at various ages. Assuming average activity, the basal expenditure should be multiplied by 1.5 to meet the requirement under usual circumstances. Increased body temperature (13% per degree C), ventilating rate, or muscular activity each can increase the expenditure to twice basal; the simultaneous combination of all these may produce a threefold basal expenditure.

The energy requirement is met from protein, fat, and carbohydrate. The only other potential source of calories for humans is ethyl alcohol. The calorie used here, called the kilocalorie, is the nutritionist’s unit (1,000 times the calorie of the physicist) and is defined, in heat terms, as the energy required to raise the temperature of a kilogram of water from 15 °C to 16°C.

The protein intake, providing approximately 4 cal/kg, is of particular importance. Not all proteins are nutritionally equal for the human diet. A specific group of amino acids, known as the essential amino acids, must be present in the diet because they are not synthesized at all or in sufficient quantities. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophane, and valine. Arginine, cystine, and probably taurine also are required by low-birth-weight infants. The remaining amino acids can be synthesized from these three for tissue and cellular needs.

The animal protein found in meat, eggs, and milk supply these amino acids requirements in appropriate proportion. Using an optimal dietary protein source requires that a minimum of about 6% of total caloric intake be from protein. (Human milk contains approximately 8% protein.) It is appropriate to have at least 10% of calories from protein after infancy to allow for inefficiencies of digestion and absorption. If the source of calories is only vegetable, a higher percentage of protein (13% to 15%) is needed to include all the essential amino acids without producing toxicity from an excess of some of them. No single vegetable provides all the essential amino acids. Note that, as the expenditure of energy increases, the percentage requirement for protein remains constant. For this reason, giving protein requirements in g/kg at varying ages underestimates the need when the energy expenditure is unusually high (e.g., manual labor, athletic participation).

Fat has an important place in the diet as a provider of energy, yielding approximately 9 cal/g. Two fatty acids, linolenic acid and linoleic acid, are important membrane constituents. These essential dietary components need constitute only 1% to 2% of calories. In infants younger than about 2 years of age, it is important to keep 50% of calories as fat; this keeps the volume of food needed within the range of the infant’s capability to ingest and thus assists in normal growth. As the child grows, concern about cholesterol levels and related adult heart disease has led to the recommendation to limit fat to no more than 30% of calories. This aim may be achieved readily by a gradual dietary adjustment between 22 and 30 months of life.

Carbohydrate, the third appropriate source of energy, is required to limit the amounts of protein and fat, either of which may be toxic when ingested in significant excess of the recommended proportions. Small excesses of either protein or fat will be converted to carbohydrate, and the latter may be converted to some fats. Thus, carbohydrate from 40% to 60% of calories is optimal for nutrition and for taste quality in the diet.

Water also is an essential nutrient. Water constitutes 70% of the lean body mass, which is about 60% of the weight when 10% is fat (largely adipose tissue). Normal physiologic processes result in a daily turnover of approximately 10% of weight in the infant and 2% of weight in the older adolescent. This turnover is induced by energy (heat) expended, thus being greater per unit of weight in the more rapidly metabolizing infant with a greater ratio of surface area to weight. The sites of obligatory water loss are evaporation from the skin and lungs (45 mL/100 cal expended), urine formation required to keep a constant composition of extracellular fluid (50 mL/100 cal expended at a concentration of 300 mOsm/Kg), and a small amount in the feces, making the losses 100 mL/100 cal or 1 mL/cal as the obligatory replacement requirement. (The ability to concentrate the urine reduces the requirement, although less so in the early months of life than later.)