Summary: Body composition and the composition of weight gain in 82 healthy infants during the first 3 months of life were measured in relation to the type of feeding and nutrient intakes by serial measurements of total body potassium (TBK), anthropometry, and formula intake. Infants were exclusively fed either breast milk (n = 34, B fed) or whey-based formula (n = 48, F fed). Formula intakes were substantially lower than WHO-FAO recommendations (although serial weights paralleled standards), and were significantly greater in male infants, who had greater gains in weight and lean tissue than females. In both sexes, tissues low in K (fat and extracellular water) were added faster to the body weight than lean tissue. Overall, compared with B fed infants, F fed infants gained less weight between birth and 10 days and more weight between 10 and 90 days, although mean actual weights at 90 days were similar: formula feeding caused greater fat deposition in males and greater daily gains in lean mass in females. Differences in both quantity and quality of ingested nutrients account for these differences although the biological importance of these findings is speculative. These studies further elucidate the nature of growth in relation to modern feeding practices and the question of optimal versus maximal nutrient intake and growth in early life.
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