Exercise may help maintain active vitamin D levels in winter, even without weight loss or supplements.

Two research studies back up this claim. One found that people who exercised for 12 weeks had higher active vitamin D levels than non-exercisers, suggesting exercise could stop the normal winter drop. Another study showed that indoor exercise during winter kept active vitamin D from falling, and this worked regardless of weight loss or vitamin D supplementation. However, other research contradicts this: a study that measured a vitamin D marker found that even people who exercised still had lower levels after winter, so the claim isn't universally supported. The conflicting results might stem from different ways of measuring vitamin D or different study designs.