Brain response to protein undernutrition. Mechanism of preferential protein retention.

This study was designed to determine how the brain, in contrast to most other tissues, maintains an almost normal protein content during a period of dietary protein deficiency. Administration of leucine-(3)H to rats was started during a period of early development (6-18 days) which is characterized by disproportionately rapid brain growth; later (24-33 days) leucine-(14)C was administered, when brain growth diminishes but total body weight gain continues to be rapid. At 35 days of age the ratio of (3)H:(14)C in cerebrum, cerebellum, and brain stem protein averaged between 1.63 and 1.82. In skeletal muscle, liver, myocardium, and intestinal mucosa the mean (3)H:(14)C was 1.07 or less. Then, a diet containing either 26% or 3.4% protein was administered. In animals fed the 26% protein diet, (3)H:(14)C in the three brain segments remained essentially unchanged over a 42 day period. In contrast, in the 3.4% protein group (3)H:(14)C in brain decreased to values approaching those of other tissues in the body: cerebrum, 1.18; cerebellum, 1.20; and brain stem, 1.16. The results suggest that conservation of brain protein is not due entirely to the long life-span of its cellular components or to efficient reutilization of the products of protein catabolism but through utilization of amino acids from degradation of protein elsewhere in the body.