Adaptation to restricted intake of protein and energy.

Adult rats that maintained nitrogen balance on a diet containing 5% casein fed ad libitum were restricted to 70% of their normal food intake for 31 days. This resulted in a negative N balance, loss of body weight and increased activities of hepatic glutamic pyruvic transaminase and arginase--all of which persisted for 10 days. After this time there was no further weight loss, the negative N balance returned to equilibrium and the activities of the two enzymes returned to normal indicating adaptation to the dietary restriction. Radio-isotope studies with labelled methionine showed that dietary restriction led to an increase in the radioactivity of the liver and a decrease of that of the muscles indicating maintenance of liver protein at the expense of muscle protein. Resting oxygen consumption decreased by 34% after 20 days of dietary restriction and this would account for the adaptation observed.