The composition of an infant's growth, so far as mineral substances and nitrogen are concerned, can be studied by determining the retentions (intake in food minus the output in urine and stools) of these substances. By dividing the retention of a substance by the total gain in weight during the period of observation one arrives at a value for the composition of growth in respect to this substance.
A table on this subject has been compiled by Lindberg1 from data published by other investigators. Lindberg pointed out that although definite conclusions cannot be drawn, it would seem that the composition of growth after birth was very different from the known composition of the body at birth, the growth after birth being poor in calcium and phosphorus and rich in sodium, potassium and magnesium. This conclusion of Lindberg agrees well with the results obtained in the present work. The composition