Figure 5. Overview of changes in calcitropic hormones, calcium absorption, serum calcium, bone markers, and BMD during the summer and winter. The curves were fitted using the periodic function of the form, F(m) = A*sin(B + m*/6) + C, where m designates the month variable while A, B, and C are parameters to be estimated. The effect of the covariates, age, height, weight, BMI, calorie intake, fiber intake, dietary calcium intake, dietary caffeine intake, alcohol status, smoking status, and age at menopause were adjusted for in these analyses. A significant (P < 0.05) seasonal effect was noted for serum 25OHD, serum calcium, urine N-telopeptide/Cr, spine BMD, and total body BMD.