Determinants of Serum Antirachitic Activity

Abstract

Serum antirachitic activity (SARA) has been determined in 116 women from Michigan and sixty-five Puerto Rican women, age forty-five and over, with particular emphasis on its relationship to age, season, climate, degree of vertebral osteoporosis and vitamin D therapy. SARA for all Puerto Rican subjects was relatively independent of season and averaged 4.4 units per ml., compared to 2.6 units for all Michigan women in whom the levels during summer were nearly twice those during winter. For Michigan women, SARA was significantly less in symptomatic osteoporotic subjects than in the skeletally normal; it increased significantly more in the latter during the summer, and for osteoporotic subjects in particular it was lower in the older age groups. Vitamin D given orally in conventional doses to osteoporotic subjects restored SARA to levels obtained naturally in normal subjects during the summer and in Puerto Rican residents. Unexpectedly, serum calcium and phosphorus levels were found to average significantly higher in Puerto Rican than in Michigan women, respective values being 10.4 versus 9.6 and 3.8 versus 3.1 mg. per cent. Although other explanations may be presented for the higher values in Puerto Rican women, the latter are consistent with the known actions of vitamin D and suggest a better calcification potential in the island residents. The over-all findings are considered relevant to the current efforts to define the respective roles of endocrine imbalance and disordered mineral metabolism in the genesis of involutional osteoporosis. A link between these two casualties is suggested by the fact that androgens promote sebaceous secretion, a dermal function essential to vitamin D formation.