Vitamin D supplementation and central adiposity in male shift workers: a randomized clinical trial on metabolic health.

Vitamin D deficiency is common among individuals with obesity; however, the association between low 25-hydroxyvitamin D [25(OH)D] concentrations and adiposity remains unclear. Shift workers are at increased risk for both vitamin D deficiency and metabolic disorders, making this population particularly relevant for investigating this relationship. This study aimed to evaluate the effect of cholecalciferol supplementation on central adiposity, assessed by waist circumference (WC) and waist-to-height ratio (WHtR), among male shift workers. Randomized, double-blind, placebo-controlled clinical trial involving 198 male shift workers employed by an iron ore extraction company in Minas Gerais, Brazil. Participants with 25(OH)D concentrations <30 ng/mL and at least one metabolic abnormality, including metabolic syndrome, impaired fasting glucose, dyslipidemia, elevated blood pressure, or increased WC, were randomly assigned to receive placebo, cholecalciferol 14,000 IU/week, or cholecalciferol 28,000 IU/week for 4 months. Plasma 25(OH)D concentrations, WC, and WHtR were evaluated at baseline and post-intervention. After 4 months, no significant differences in WC or WHtR were observed between the vitamin D supplementation groups (14,000 IU/week and 28,000 IU/week) and the placebo group. Nevertheless, serum 25(OH)D concentrations increased significantly in the supplementation groups: +6.23 ng/mL (p25 = 0.58; p75 = 10.61) in the 14,000 IU/week group and +15.25 ng/mL (p25 = 10.57; p75 = 19.68) in the 28,000 IU/week group. In contrast, the placebo group showed a decrease of -0.50 ng/mL (p25 = -1.03; p75 = 2.71). Despite these increases, supplementation produced no significant effect on central adiposity outcomes (p > 0.05). Although vitamin D supplementation effectively increased serum 25(OH)D concentrations, it did not reduce central adiposity among male shift workers. These findings indicate that vitamin D supplementation alone is unlikely to improve obesity-related outcomes in this high-risk occupational population and underscore the need for comprehensive strategies targeting metabolic health in shift workers. This study was registered in the Brazilian Registry of Clinical Trials (ReBEC) under identifier RBR-6CS352 on October 25, 2017.