Exercise May Protect Your Vitamin D All Winter, With No Supplements Required.
Recent trial finds regular cardio preserves the active form of vitamin D that supplements can't touch.
Every winter, vitamin D levels in people living at northerly latitudes drop sharply. The sun simply does not get high enough in the sky to trigger the skin to make it. For people carrying excess weight, the problem is even worse. Fat tissue traps vitamin D, pulling it out of circulation and leaving less available for the body to use.
Researchers from the University of Bath wanted to know: could regular exercise protect vitamin D levels over winter, even if people did not lose any weight? The answer, published in Advanced Science in 2025, was yes — and in a way that a vitamin D supplement cannot replicate.
Aim
The VitaDEx randomised controlled trial recruited 41 sedentary, overweight or obese adults (average BMI 34, average age 49) and ran entirely across the UK winter (October through April) when there is no meaningful UVB sunlight at northerly latitudes. The goal was to isolate the effect of exercise on vitamin D metabolism, free from the confounding effects of sunlight, weight loss, or supplementation. Participants were banned from taking vitamin D supplements and were required to maintain their body weight throughout. Those in the exercise group were instructed to eat more to offset the extra calories burned.
Methods
Participants were randomly assigned to either an exercise group or a control group. The control group maintained their usual lifestyle. The exercise group completed four indoor cardiovascular sessions per week for 10 weeks — roughly 166 minutes of exercise per week. Sessions included treadmill walking, steady-state cycling, and low-volume high-intensity interval training on a bike. All exercise took place indoors to prevent any sun exposure.
Adherence was high, with participants completing 88% of sessions on average. Body weight was successfully maintained in both groups — no one lost meaningful body fat.
Vitamin D metabolites in blood and fat tissue were measured using gold-standard liquid chromatography-tandem mass spectrometry (LC-MS/MS), a more accurate method than the immunoassays typically used in earlier research. Researchers measured not just the standard marker (25(OH)D), but also the biologically active form, 1,25(OH)2D3, as well as a catabolic breakdown product and fat tissue concentrations.
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Results
Both groups lost some vitamin D over winter — that was expected. But the two groups diverged in important ways.
The control group saw a total of 25(OH)D (the standard marker) drop by about 25% from baseline. The exercise group dropped only 15%. The difference was moderate in size, though it did not reach conventional statistical significance.
The more striking finding was in the active form of vitamin D — 1,25(OH)2D3. This is the form that actually binds to the vitamin D receptor, which is found in almost every cell in the body. In the control group, 1,25(OH)2D3 fell by 15% over the winter. In the exercise group, it was completely preserved — no decline at all. This difference was statistically significant.
This matters because vitamin D supplementation, according to the authors and existing literature, does not raise circulating 1,25(OH)2D3. Supplements raise the precursor (25(OH)D), but not the active metabolite. Exercise, in this study, did something supplements do not.
Vitamin D stored in fat tissue declined in both groups over winter, with no difference between them. The half-life of vitamin D in circulation also did not differ between groups. The mechanism behind the preservation of active vitamin D with exercise remains unclear.
Practical Takeaways
Regular exercise over winter — roughly four sessions per week — appears to slow the seasonal decline in circulating vitamin D and may fully protect the biologically active form, 1,25(OH)2D3, even without weight loss or supplementation.
This effect on active vitamin D is something oral supplementation does not appear to achieve, according to the authors.
The exercise prescribed was moderate and multimodal: treadmill walking, steady-state cycling, and short interval sessions. It was personalised but not extreme.
The participants were overweight or obese adults, a group at higher risk of vitamin D deficiency. Whether these results apply to leaner people is not known from this study.
Extrapolating the rates of decline observed, the researchers estimate the control group would have ended the full winter vitamin D deficient, while the exercise group would have ended insufficient, a worse outcome than optimal, but meaningfully better.
Supplementation may still have a role, but this study suggests exercise works through a different pathway, and the two strategies may be complementary.
Related
Perkin, O. J., Davies, S. E., Hewison, M., Jones, K. S., Gonzalez, J. T., Betts, J. A., Jenkinson, C., Lindsay, M. A., Meadows, S. R., Parkington, D. A., Koulman, A., & Thompson, D. Exercise without Weight Loss Prevents Seasonal Decline in Vitamin D Metabolites: The VitaDEx Randomized Controlled Trial. Advanced Science, 2416312. https://doi.org/10.1002/advs.202416312