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Most adults have built-in controls for appetite: their food intake is governed by physiological needs, and the control is so good that body weight varies but little over long periods of time. The regularity of the growth pattern in infants and children attests to the precision of appetite control in this younger age group as well. Observations on recovery from infantile malnutrition illustrate this beautifully: when ad libitum feedings are given, appetite is voracious until catch-up growth is complete, whereupon it abruptly diminishes to approximate the normal intake for body size.
The controls for protein are not so well defined. Most Americans select a diet providing 10% to 15% of calories from protein. Infants fed at the breast receive about 8% of calories from protein and those who drink plain cow's milk about 20%. Some groups prefer high-protein diets: the men who accompanied Lewis and Clark on their 1804 expedition
FORBES GB. Joint FAO/WHO ad hoc Expert Committee, Energy and Protein Requirements, WHO Technical Report Series 522.. Am J Dis Child. 1974;127(2):296-297. doi:10.1001/archpedi.1974.02110210146027