The metabolism of the animal is equipped to adapt to changes in both the
internal and the external environment. Among internal factors are activity
versus rest and sleep, and the menstrual cycle in the case of the female.
Metabolism must also respond to variations in the external environment, such
as heat and cold, and notably the availability of food. Metabolic adaptation
to nutrient supply is of two kinds. First, there are transient physiological
adaptations to the intermittent intake of nutritionally adequate meals.
These short-lived adaptations account for a large part of the diurnal
variations that have been observed in the protein metabolism of mammals
(Wurtman, 1969). Secondly, long-term adaptive reactions occur when there is
a decrease in availability of an essential nutrient in the diet. Under such
circumstances, tissue constituents are lost to varying degrees from
different parts of the body. It is proposed to discuss here mainly
short-term physiological adaptations to variations in amino acid supply and
their relevance to the needs of the body for dietary protein. Adaptive
changes resulting from long-term protein deficiency are considered in the
paper by Waterlow & Stephen (1969).