The amount of calcium individuals are able to absorb from their diet may influence their calcium balance more than the amount of calcium consumed. The high protein and phosphorus content of the typical Western diet could contribute to poor calcium absorption, thus aggravating the problems of calcium deficiency and osteoporosis. In research published in The American Journal of Clinical Nutrition, Dr. Robert Heaney studied the diets of a large group of nuns and concluded that protein and phosphorous intake have no role in the absorption of dietary calcium.
The 191 nuns had entered the study in 1967, when they were between 35 and 45 years of age. Each subject was evaluated 1 to 5 times at 5 year intervals over 32 years. At each study interval, subjects were fed duplicate weighed diets which had been analyzed for calcium, phosphorus and nitrogen (the last used to calculate protein intake). Calcium absorption was estimated using the double-tracer method of both oral and intravenous calcium administration. A statistical analysis of the full set of 567 observations showed no relation between relative calcium absorption and either phosphorus or protein intake. The same results were obtained when the group was segregated by estrogen status ("estrogen replete" or "estrogen deprived") , or by high versus low protein intake. Two advantages of this study were its large sample size and the nutritionally steady state the nuns maintained. Heaney adds, however, that the lack of influence of protein on calcium absorption could be considered maladaptive because increased dietary protein has been found to increase urinary calcium loss, which can result in a negative calcium balance if calcium intakes are inadequate.