Vitamin D, a fat-soluble nutrient, is sequestered in adipose tissue, limiting its circulation—an effect amplified in obesity. Obese adults, for example, exhibit a 57 % smaller rise in circulating vitamin D₃ after whole-body ultraviolet exposure and are more than half as likely to reach sufficient vitamin D levels than normal-weight adults. Against this backdrop, the VitaDEx randomized trial showed that ten weeks of indoor exercise during winter sharply attenuated seasonal vitamin D loss and fully preserved its active hormone, 1,25(OH)₂D₃—without weight loss or supplementation.

• Active hormone maintained: Exercise fully prevented the winter decline in 1,25(OH)₂D₃, while controls experienced a 15 % drop. (The precursor 25-hydroxy-vitamin D fell only 15 % in exercisers versus 25 % in sedentary controls.)
• Adipose sequestration puzzle: Contrary to expectations, adipose vitamin D concentrations remained largely unchanged. “Exercise did not drive a greater decrease in adipose tissue concentrations of vitamin D…there was no correlation between the change in serum 25(OH)D and changes in adipose vitamin D₃ concentrations.”
• Mechanistic pivot: Researchers suggested possible transient vitamin D mobilization, depot-specific effects, or direct metabolic adaptations improving vitamin D efficiency. They speculated that regular physical activity might enable “more ‘efficient’ vitamin D metabolism, making better use of the available substrate to generate the active metabolite without tipping the balance into a negative feedback loop.”

Taken together, the findings indicate exercise is not merely releasing vitamin D from fat stores; it is altering the flux and enzymatic handling of the hormone, offering a route to restore endocrine availability where supplementation often fails in obesity.

Press Release: Regular Exercise Helps Maintain Vitamin D Levels During Winter
Study: Exercise without Weight Loss Prevents Seasonal Decline in Vitamin D Metabolites: The VitaDEx Randomized Controlled Trial