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-³H 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-¹⁴C was administered, when brain growth diminishes but total body weight gain continues to be rapid. At 35 days of age the ratio of ³H:¹⁴C in cerebrum, cerebellum, and brain stem protein averaged between 1.63 and 1.82. In skeletal muscle, liver, myocardium, and intestinal mucosa the mean ³H:¹⁴C was 1.07 or less. Then, a diet containing either 26% or 3.4% protein was administered. In animals fed the 26% protein diet, ³H:¹⁴C in the three brain segments remained essentially unchanged over a 42 day period. In contrast, in the 3.4% protein group ³H:¹⁴C 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.

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