Reduced Urinary Calcium on a Vegetarian Diet May Reflect Reduced Calcium Absorption With Phytic Acid, Not Calcium Conservation With Less Meat Protein

Abstract

LEARNING OUTCOME: To consider phytic acid, not just meat protein, as a dietary variable that influences urinary calcium excretion in vegetarians. Dietary protein, especially meat protein, is frequently cited as a risk factor for osteoporosis. Although protein from purified sources is calciuretic, protein from meat is not (Spencer, et al., Am J Clin Nutr 31: 2167, 1978; Hunt, et al., Am J Clin Nutr 62: 621, 1995), probably because the phosphorus from meat reduces urinary calcium. A recent balance study comparing mineral balance on a lacto-ovo-vegetarian and nonvegetarian diet emphasized the influence of phytic acid, rather than meat protein, in reducing urinary calcium. Twenty-one women, age 33±7 (20 to 42) y, consumed both weighed diets for 8 wk each in a cross-over design. Calcium balance was determined during the last 2 wk by inductively coupled argon plasma spectrophotometry. The lacto-ovo-vegetarian and nonvegetarian diets provided, respectively, (by analysis) 973 and 995 mg calcium, 1457 and 1667 mg phosphorus, and (by calculation) 12 and 16% protein energy, 40 and 16 g dietary fiber, and 2.5 and 0.8 mmol phytic acid. The lacto-ovo-vegetarian, compared to the nonvegetarian diet, reduced apparent calcium absorption (dietary calcium minus fecal calcium) (211 vs. 292 mg/d, pooled SD=130, p<0.06) and urinary calcium excretion (119 vs. 155 mg/d, SD=14, p<0.0001). Because urinary calcium excretion partially compensated for the difference in calcium absorption, calcium balance (92 vs. 136 mg/d, SD=133, NS) was unaffected. These results are consistent with reduced calcium absorption by persons consuming sodium phytate, or brown rather than white bread, as described in the classic report of McCance and Widdowson (J Physiol 101: 44, 1942). The lower urinary calcium on the lacto-ovo-vegetarian diet in the present study probably resulted from lower calcium absorption associated with phytic acid, rather than from reduced meat consumption. Phytic acid, rather than meat protein, may be the primary dietary variable that reduces urinary calcium on a lacto-ovo-vegetarian diet with adequate calcium content.