The regulation of eating and of body energy involves numerous redundant mechanisms and complex homeostatic mechanisms. Overall, the system for controlling food intake and energy balance is significantly more complex than those controlling thermoregulation and fluid balance. One important reason for this greater complexity is that we need food to supply not only energy, but also crucial nutrients (chemicals required for the effective functioning, growth, and maintenance of the body). We do not know all the nutritional requirements of the body—even for humans. Of the 20 amino acids found in our bodies, 9 are difficult or impossible for us to manufacture, so we must find these essential amino acids in our diet. From food we must also obtain a few fatty acids, as well as about 15 vitamins and a variety of minerals.

No animal can afford to run out of energy or nutrients; there must be a reserve on hand at all times. If the reserves are too large, though, mobility (for avoiding predators or securing prey) will be compromised. For this reason, the nervous system not only monitors nutrient and energy levels and controls digestion (the process of breaking down ingested food), but also has complex mechanisms for anticipating future requirements.

Most of our food is used to provide us with energy

All the energy that we need to move, think, breathe, and maintain body temperature is derived in the same way: it is released when the chemical bonds of complex molecules are broken and smaller, simpler compounds form as a result. In a sense we “burn” food for energy just as a car burns gasoline. To raise body temperature, we release chemical-bond energy as heat. For other bodily processes, such as those in the brain, the energy is utilized by more-sophisticated biochemical processes.

Metabolic studies indicate that lab animals lose about 33% of the energy in food during digestion (through excretion of indigestible material or the digestive process itself). Another 55% of food energy in a meal is consumed by basal metabolism—processes such as heat production, maintenance of membrane potentials, and all the other basic life-sustaining functions of the body. The remainder, only about 12% of the total, is used for active behavioral processes, although this proportion is higher in more-complex environments or during intense activity.

In general, the rate of basal metabolism follows a rule, devised by Max Kleiber (1947), that relates energy expenditure to body weight:

kcal/day = 70 × weight0.75

where weight is expressed in kilograms. This relationship applies across a vast range of body sizes (see Figure 1). However, although Kleiber’s equation fits nicely at the population level, it is not very accurate for individuals within a species, because body weight is only one factor affecting metabolic rate. For example, food-deprived people experience a significant decrease in basal metabolism. In fact, severe food restriction affects metabolic rate much more than it affects body weight (Keesey and Corbett, 1984), presumably reflecting the operation of an evolved homeostatic mechanism for conserving energy when food is scarce (Keesey and Powley, 1986).

Figure 1  The Relation between Body Size and Metabolism
Basal metabolic rate increases in a very regular, predictable fashion over a wide range of body weights. However, endotherms have a higher metabolic rate than ectotherms of a similar body weight. (After Hemmingsen, 1960.)

References

Hemmingsen, A. M. (1960). Energy metabolism as related to body size and respiratory surfaces, and its evolution. Reports of Steno Memorial Hospital, Copenhagen 9: 1–110.

Keesey, R. E., and Corbett, S. W. (1984). Metabolic defense of the body weight set-point. Research Publications—Association for Research in Nervous and Mental Disease 62: 87–96.

Keesey, R. E., and Powley, T. L. (1986). The regulation of body weight. Annual Review of Psychology 37: 109–133.

Kleiber, M. (1947). Body size and metabolic rate. Physiological Reviews 15: 511–541.