Newborn animals were subjected to a two-day postnatal fast immediately after birth. These animals and control littermates matched for birth weight were killed at 60 days of age. The brain was weighed, divided into regional brain areas, and analyzed for DNA, RNA, protein, and cholesterol content. Those fasted animals whose body weight was more than 25% below the control littermate at 12 days of age exhibited a significant reduction of brain weight and constituents at 60 days of age (group II). Cholesterol accumulation was slowed to a greater extent than other constituents (84% of control values) but DNA, RNA, and protein levels were also diminished in a statistically significant manner. Those fasted animals whose body weight nearly caught up to the controls after the fast did not manifest differences of brain size and composition at 60 days of age (group I). When adjusted for life span, the timetable of resumption of weight gain in fasted rabbits is comparable to the time required for regaining birth weight of very low birth weight human infants. These data indicate that the degree of body weight catch-up after a brief postnatal fast will affect subsequent brain development in the rabbit, a species which, like the human, is a perinatal brain developer.

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