The Pathway of Vitamin D: From Sun and Food to Circulation
To understand how exercise influences vitamin D, it's crucial to grasp the basics of how our bodies process this vital nutrient. Vitamin D can be obtained from two primary sources: sunlight and dietary intake. Sun exposure on the skin is a major source, triggering a cholesterol derivative (7-dehydrocholesterol) to be converted into previtamin D3, which then becomes vitamin D3. Dietary sources, including fatty fish, eggs, and fortified foods, provide both vitamin D2 and D3, which are absorbed in the small intestine. Both forms are then hydroxylated in the liver and kidneys to become the active form (1,25-dihydroxyvitamin D), which plays a key role in calcium absorption and overall health.
The Indirect Influence: Exercise and Circulating Vitamin D Levels
Rather than increasing intestinal absorption, exercise appears to influence vitamin D levels through several other physiological mechanisms. Research suggests that physical activity stimulates the release of stored vitamin D, particularly in individuals with low baseline levels or those engaging in high-intensity exercise.
Mobilization from Fat Stores Vitamin D is a fat-soluble vitamin, meaning it is stored in the body's adipose (fat) tissue. During exercise, particularly of moderate to high intensity, the body mobilizes fat for energy through a process called lipolysis. This process releases not only fatty acids but also the fat-soluble vitamin D stored within those cells, transiently increasing its concentration in the bloodstream. This effect helps explain why studies, including a 2025 finding from the University of Bath, have shown that overweight and obese individuals who exercise regularly can mitigate the seasonal dip in vitamin D levels.
Release from Muscle Tissue Emerging research indicates that skeletal muscle also stores vitamin D and can release it into the bloodstream during exercise. A 2022 study in young soccer players found that a single bout of intense exercise led to a significant, though temporary, increase in circulating 25-hydroxyvitamin D3. This suggests that muscles act as both a store and a regulator of vitamin D, releasing it when stimulated by physical activity.
The Outdoor Advantage: Sunlight and Exercise For many, the most significant benefit of exercising on vitamin D status comes from doing it outside. Combining physical activity with sun exposure naturally triggers vitamin D synthesis in the skin, a powerful double-benefit that indoor exercise lacks. Studies have demonstrated that outdoor activity leads to a far greater increase in vitamin D levels than indoor exercise alone.
- Aerobic Exercise (e.g., Running, Cycling): Performed outdoors, these activities maximize sun exposure. Studies have shown they can increase serum vitamin D levels, especially in those who are deficient. Indoor cycling, while offering metabolic benefits, lacks the sun exposure effect.
- Resistance Training (Weight Training): Research on resistance exercise and vitamin D levels is more mixed, with some studies showing increases and others showing minimal effect unless combined with supplementation. This may relate to factors like exercise intensity and training status.
Synergistic Benefits with Nutrition To maximize the benefits of exercise, it's essential to combine it with a balanced diet. Since vitamin D is fat-soluble, consuming dietary sources with healthy fats, like those found in salmon, nuts, and avocados, optimizes its absorption in the gut. This provides a steady supply of the nutrient, which exercise can then help mobilize and circulate more efficiently. For those with medical conditions that inhibit fat absorption, like Crohn's or Celiac disease, this dietary synergy is particularly important.
Comparing Different Exercise Approaches
| Approach | Mechanism | Effect on Vitamin D Levels | Key Takeaway |
|---|---|---|---|
| Outdoor Exercise (Running, Sports) | Sun exposure and fat/muscle mobilization | Significantly increases levels, especially in summer. May also prevent seasonal dips. | Highly effective due to combined benefits of sun and movement. |
| Indoor Exercise (Treadmill, Weights) | Primarily fat/muscle mobilization | Can help maintain or prevent a decline in levels, particularly for those with low baseline status. | Effective for boosting circulation, but lacks the primary sun-driven synthesis. |
| High-Intensity Interval Training (HIIT) | Intense lipolysis and muscle contractions | Can cause acute, transient spikes in circulating vitamin D levels. | Potent stimulus for mobilizing vitamin D from fat stores. |
| Endurance Training (Long-distance Running) | Mobilization from fat stores over a prolonged period | Increases circulating levels, with evidence suggesting a stronger effect in deficient individuals. | Sustained fat metabolism helps release stored vitamin D. |
Conclusion
While exercise does not directly enhance the absorption of vitamin D from your digestive tract, it undeniably plays a significant role in improving your overall vitamin D status. The key lies in its metabolic influence: regular physical activity helps mobilize stored vitamin D from fat and muscle tissue into the bloodstream, making it more available for the body's use. The most powerful effect is seen when exercise is performed outdoors, where the combination of sun exposure and metabolic activity provides the greatest boost. For optimal results, a comprehensive strategy incorporating outdoor exercise, a vitamin D-rich diet with healthy fats, and potentially targeted supplementation (especially during winter) is most effective.
