Calcium Effects on Phosphorus Absorption: Implications for the Prevention and Co-therapy of Osteoporosis

Objective: To evaluate the effect of calcium intake on absorption of dietary phosphorus, with special reference to typical calcium intakes and to those likely to be encountered in prevention and treatment of osteoporosis.

Setting: Two academic health sciences centers; inpatient metabolic research unit.

Methods: Evaluation of calcium and phosphorus balance data obtained in two data sets, the first, 543 studies of healthy women aged 35-65, and the second, 93 men and women aged 19-78; development of multiple regression models predicting fecal phosphorus (the complement of net absorbed phosphorus); data from the two centers analyzed separately as a check on the consistency of the findings.

Results: Mean net absorption of phosphorus was 60.3% (+/- 18.1) for data set 1 and 53.0% (+/-9.4) for data set 2. Just two variables, fecal calcium and diet phosphorus, were positively and independently associated with fecal phosphorus. These variables explained 73% of the variance in fecal phosphorus in data set 1 and 33% in data set 2. Fecal calcium alone explained the lion's share of the relationship. The coefficients of the fecal calcium term in the models fitted to the data were 0.332+/-0.022 and 0.155+/-0.039, for data sets I and 2, respectively. Adjusting for the relationship between fecal calcium and calcium intake and using the parameters of the larger data set, it follows that each increase in calcium intake of 0.5 g (12.5 mmol) decreases phosphorus absorption by 0.166 g (5.4 mmol).

Conclusions: As calcium intake increases without a corresponding increase in phosphorus intake, phosphorus absorption falls and the risk of phosphorus insufficiency rises. Intakes with high Ca:P ratios can occur with use of supplements or food fortificants consisting of non-phosphate calcium salts. Older patients with osteoporosis treated with current generation bone active agents should receive at least some of their calcium co-therapy in the form of a calcium phosphate preparation.