Using the baboon as a model, we tested the hypothesis that preweaning food intake influences the number of adipocytes at weaning. Two groups of 12 newborn baboons each were fed either a concentrated or a diluted Similac formula from birth to 18 weeks of age. Baboons fed the concentrated Similac were 38% heavier (P less than 0.01) and had 87% more fat mass (P less than 0.01) than the baboons fed diluted Similac. The mean adipocyte volume and adipocyte number were measured directly in 10 individual fat depots, and the total number of adipocytes were estimated for each baboon. The difference in fat mass was due to differences (P less than 0.01) in mean adipocyte volume, which was 0.22 nl in overfed baboons and 0.09 nl in underfed baboons. There was no significant diet effect on the estimated total number of fat cells; nor on 8 of 10 depots in which adipocyte number was directly measured. These results indicate that, in baboons, preweaning caloric intake has little or no influence on the number of fat cells at the age of weaning.

The hypothesis that energy intake influences differently the composition of mass gain during the preweaning period was tested in male and female baboons. Infant baboons were fed either a high (92 kcal/100 g formula) or a low (49 kcal/100 g formula) caloric formula from birth. Body mass, lean body mass and fat mass of each baboon were measured directly at weaning (18 weeks of age). Lean mass and fat mass gained from birth to 18 weeks was estimated by using body composition data from eight newborn baboons. Both energy intake and gender significantly influenced gain of total mass, lean mass and fat mass. There was a significant gender by diet interaction effect on the gain of fat mass. When fed the high caloric formula, males gained 145 g more lean mass than females, but the increase of fat mass was approximately the same. In contrast, on the low caloric formula, males gained 150 g more lean mass than females but 74 g less fat mass than females. These results indicate that fundamental differences in the development of lean mass and fat mass exist between male and female baboons during preweaning life.

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