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Aging is characterized by shrinkage of muscle fibers and loss of protein from them. Bone is also lost, and matrix and mineral are lost together. The predominance of breakdown over synthesis is probably the fundamental cause of both muscle and bone loss. Little can be done to prevent this by dietary means, but physical activity is of vital importance in helping to maintain the integrity of both muscle and bone. McCay's finding that rats underfed from weaning lived longer than those that had adequate food has been confirmed by others, but this is only true if the rats remain free of infections and other hazards, particularly during growth. The mechanisms by which underfeeding delays the aging process in rats may apply to the human body but, if so, this is completely offset by other considerations. For their present and future well-being, children should be fed in the best possible way.

The vitamin-D concentration in human milk is reported to be very low, yet breast-fed infants do not develop rickets. All the earlier assays of vitamin D were made on the lipid fraction of milk, and the aqueous phase was discarded. It is now clear that most of the vitamin D in human milk is present as a water-soluble conjugate of vitamin D with sulphate. The concentration of vitamin-D sulphate in the aqueous fraction of human milk collected at different stages of lactation has been chemically measured. Milk was collected from twenty-two women 3-8 days post partum and fourteen women 4-6 weeks post partum. The vitamin-D sulphate concentration in milk collected between the 3rd and 5th days was 1-78 mug/dl, significantly higher that the 1-00 mug/dl for milk collected between the 6th and 8th days. After the 8th day there was no significant change.

The upper limit for fat in infant formulas in the United States is 6 g per 100 kcal. This concentration of fat, similar to that of human milk, seems appropriate for fats that are well absorbed. The upper limit for linoleic acid recommended by the working party in the United Kingdom was 1200 mg per 100 kcal. However, there seems to be no evidence that a higher percentage does the infant any temporary or permanent harm.