Vitamin D deficiency in obesity: Epidemiological evidence, biological mechanisms, and clinical considerations

Background Obesity is a global public health crisis, contributing to substantial morbidity and mortality due to its strong association with chronic diseases such as type 2 diabetes, cardiovascular disorders, and malignancies. Concurrently, vitamin D deficiency has become widespread, affecting nearly one billion people worldwide. Epidemiological studies consistently demonstrate an inverse relationship between obesity and vitamin D levels, with emerging hypotheses suggesting a bidirectional link. Objectives This review examines the association between obesity and vitamin D status, the underlying biological mechanisms, the role of vitamin D in metabolic disease, and the implications of vitamin D supplementation in obese populations. Results Epidemiological studies confirm that obese individuals exhibit significantly lower circulating 25-hydroxyvitamin D [25(OH)D] levels than their normal-weight counterparts. Mendelian randomization analyses further establish a causal link, indicating that obesity leads to vitamin D deficiency, rather than vice versa. Proposed biological mechanisms include volumetric dilution due to increased adipose tissue, sequestration of vitamin D in fat stores, and obesity-related impairments in vitamin D metabolism. Vitamin D plays a crucial role in metabolic health by influencing insulin secretion, lipid metabolism, and blood pressure regulation. Deficiency has been associated with insulin resistance, dyslipidemia, and overactivation of the renin-angiotensin-aldosterone system (RAAS), exacerbating obesity-related metabolic complications. However, while vitamin D supplementation effectively raises serum 25(OH)D levels, its impact on improving metabolic outcomes remains inconsistent across interventional studies. In addition, the joint impact of obesity and vitamin D deficiency on bone health remains unexplored, despite distinct mechanisms suggesting additive or synergistic harm. Targeted studies are needed to assess their combined effects using robust designs and comprehensive skeletal outcomes. Conclusions The strong inverse relationship between obesity and vitamin D status highlights the need for targeted interventions. While weight loss modestly improves vitamin D levels, BMI-adjusted vitamin D supplementation may be the most effective strategy for correcting deficiency in obese populations. Current guidelines suggest that obese individuals require higher doses of vitamin D to achieve optimal serum levels. However, further research is necessary to refine dosing strategies and determine the long-term impact of supplementation on metabolic health outcomes. Future investigations should integrate personalized supplementation approaches with lifestyle and pharmacological interventions to mitigate obesity-related metabolic disturbances effectively. Furthermore, obesity and vitamin D deficiency may jointly exacerbate skeletal deterioration, warranting dedicated investigation.