Human Milk. Human milk is recognized as the optimal source of nourishment for infants throughout at least the first year of life, and as a sole nutritional source for infants during the first 4 to 6 months of life (IOM, 1991). Further, there are no reports of full-term infants, who are exclusively and freely fed human milk and are vitamin D replete, who manifest any incidence of calcium deficiency. Therefore, consideration of the AI for calcium for infants is based on mean intake data from infants fed human milk as the principal fluid during the first year of life. This value was derived from studies where intake of human milk was measured by test weighing and where intake of food and formula by dietary records was determined for 3 days or more.

The concentration of calcium in human milk remains relatively constant, with a mean value of 264 mg (6.6 mmol)/liter throughout the first 6 months of nursing and with a small decrease during the second 6 months to about 210 mg (5.2 mmol)/liter (Atkinson et al., 1995). Variations in milk calcium content have been found between population groups. For example, in comparison with the above data from the United States, milk calcium concentrations were lower (by approximately 20 mg (0.5 mmol)/100 ml at 5 months of lactation) in mothers from the Gambia who usually have diets low in calcium (Prentice et al., 1995). Thus, for establishing values of milk calcium to use in setting the AIs, only studies on milk calcium from North America and the United Kingdom were considered to obtain a mean value.

Balance. Recently, calcium absorption using stable isotopes (Abrams et al., 1997a) was measured in 14 human milk-fed infants who were 5 through 7 months of age at the time of the study. Mean absorption was 61 ± 22 percent of intake when approximately 80 percent of the dietary calcium was from human milk. There was no significant relationship between calcium from solid foods and the fractional calcium absorption from human milk. This finding suggests that calcium from solid foods does not negatively affect the bioavailability of calcium from human milk. Using measured urinary calcium and estimates of endogenous excretion, net retention was calculated to be 68 ± 38 mg (1.7 ± 0.95 mmol)/day of calcium for those infants. At 61 percent calcium absorption from intake of the AI, this observed net retention (assuming small urinary and endogenous calcium losses) would easily be attained by infants fed the mixed diet of human milk and solid food.

Accretion. Total body calcium at birth in healthy, full-term infants