Now, new research from the University of Bath, University of Birmingham, University of Cambridge and others, published in Advanced Science has revealed that regular, moderate-intensity exercise helps maintain crucial vitamin D levels during the darker months — even without weight loss or supplements.

Researchers found that people with overweight and obesity who completed a 10-week indoor exercise programme over winter experienced significantly smaller drops in vitamin D levels compared to those who didn’t exercise — even though their body weight was deliberately maintained.

The programme involved four sessions per week: two treadmill walks, one longer steady-state bike ride, and one high-intensity interval bike session.

Notably, exercise completely preserved the body’s active form of vitamin D (1,25(OH)₂D₃), which plays a key role in supporting bone health, the immune system, and various organs. Previous studies suggest that vitamin D supplements alone do not help maintain this active form.

Lead author Dr Oly Perkin from the Centre for Nutrition, Exercise and Metabolism, Department for Health at the University of Bath said:

This is the first study to show that exercise alone can protect against the winter dip in vitamin D. It’s a powerful reminder that we still have lots to learn about how exercise benefits our health.

Key findings:

• People who exercised saw a smaller drop in their overall vitamin D levels over winter — about 15%, compared to a 25% drop in those who didn’t exercise.

• Importantly, those who exercised kept healthy levels of the active form of vitamin D, which helps support the immune system and bone health.

• In the non-exercising group, levels of this active form fell by 15%.

• Weight was kept stable throughout the study, proving the effect was due to exercise — not weight loss.

To ensure sunlight didn’t skew results, the study ran between October and April — when natural vitamin D production from sunlight is minimal in this part of the UK — and participants were asked to avoid supplements.

A previous study from the team showed that a single workout can briefly boost vitamin D levels, this is the first to show that regular cardio exercise can maintain basal circulating vitamin D levels and offer lasting protection during the winter months.