EFFECTS OF A HIGH PROTEIN-INTAKE FROM COMMON FOODS ON CALCIUM-METABOLISM IN A COHORT OF POSTMENOPAUSAL WOMEN

The effect of a high protein intake on Ca metabolism in a cohort of postmenopausal women consuming approximately 650 mg Ca/day was investigated. Eight subjects, aged 50-64 yr, were fed a control diet of common foods providing 58 g protein/day for 15 days. After a 2-wk interval, they were fed a diet providing 92 g protein/day for another 15 days. All foods, as served, and complete feces and urine collections made over the final 12 days of each period were analyzed for N, Ca and P. There was no significant effect of the increase in protein intake on the apparent absorption of Ca (14.2% vs. 10.3%), urinary Ca (119 vs 142 mg/day) or Ca balance (-26 vs. -78 mg/day). Mean Ca balance was not significantly different from zero on either diet, but there was greater variability in urinary Ca on the high protein diet (P = .03) and 5 of the 8 subjects were in negative Ca balance by > 100 mg/day. Mean P balance was significantly less than zero (P = .03) on the low protein diet (-134 mg/day). The natural association of protein and P in foodstuffs was reflected in a 46% increment in P intake that accompanied the 58% increase in protein intake. This increment was reflected in a depression in serum Ca (P < .05) and a rise in urinary cyclic AMP (P < .01), both indicators of increased parathyroid activity. Phosphate-induced, parathyroid hormone-mediated stimulation of renal Ca reabsorption is of major importance in counteracting the calciuria that otherwise is associated with excretion of the endogenous acid generated on a high protein intake. The present results suggest that some elderly subjects may have difficulty in adapting to this acid load at a Ca intake of 650 mg/day.