Sedentary behavior, but not daily step count, is associated with vitamin D status in postmenopausal women: An accelerometer-based study.

To examine the associations of objectively measured sedentary behavior and ambulatory activity with circulating 25-hydroxyvitamin D (25(OH)D) concentrations, this cross-sectional study analyzed baseline data from physically inactive postmenopausal women within 10 years of menopause onset and with vitamin D deficiency (25(OH)D <30 ng/ml). A total of 120 postmenopausal women were assessed, and analyses were restricted to those with vitamin D deficiency (n = 88). Physical activity and sedentary behavior were evaluated under free-living conditions using a wrist-worn triaxial accelerometer (Fitbit Versa 3) worn continuously for 7 consecutive days. Participants were stratified into tertiles of daily step count and sedentary time within the vitamin D-deficient subgroup. Differences in serum 25(OH)D concentrations across tertiles were examined using Kruskal-Wallis tests with Bonferroni-corrected Mann-Whitney post hoc comparisons when appropriate, and associations between continuous variables were assessed using Spearman correlation. No significant differences in serum 25(OH)D concentrations were observed across tertiles of daily step count (Kruskal-Wallis, p = 0.23), and daily step count was not correlated with 25(OH)D concentrations (Spearman's ρ ≈ 0, p = 0.52). In contrast, serum 25(OH)D concentrations differed significantly across sedentary-time tertiles (Kruskal-Wallis, p < 0.05), with lower levels observed in women in the highest compared with the lowest sedentary tertile (Bonferroni-corrected p < 0.05). Consistently, sedentary time showed a modest but statistically significant inverse association with circulating 25(OH)D concentrations (Spearman's ρ = -0.28, p = 0.010). In vitamin D-deficient postmenopausal women in early post menopause, objectively measured sedentary behavior, but not ambulatory activity volume, was associated with lower serum vitamin D concentrations. These findings suggest that sedentary behavior may be associated with lower vitamin D concentrations in this specific subgroup, highlighting the potential relevance of sedentary patterns as a behavioral correlate warranting further investigation.