Effects of nitrogen, phosphorus, and caffeine on calcium balance in women.

The effects of different levels of nitrogen, phosphorus, and caffeine intake on calcium balance and on certain of its components were assessed in 170 studies in normal middle-aged, but still premenopausal women. Statistically significant negative associations with calcium balance were found for nitrogen and for caffeine, but no effect could be found for phosphorus. Higher nitrogen intakes were associated with proportionately higher levels of urinary calcium; higher phosphorus intake was associated with slightly lower levels of urinary calcium but also with slightly more intestinal secretion of calcium. Since these two effects were opposite in direction, there was no net association of different phosphorus intakes with calcium balance. Caffeine intake was associated with higher levels of both urinary calcium and intestinal calcium secretion. None of the three intake variables was associated with differences in calcium absorption efficiency. Both the nitrogen and the caffeine effects were proportional to intake. The magnitude of the effects observed was such that a 50% increase in intake of nitrogen above the group mean intake value would be predicted to result in calcium balance shift of -0.032 gm/day. For caffeine, the corresponding calcium balance shift would be predicted to be -0.006 gm/day.