Exercise supports the synthesis of active vitamin D

- even during winter

At northern latitudes, our body can only produce vitamin D during the summer months, when the sun sits sufficiently high in the sky. That’s why sun worshippers now get a much-needed boost, as blood levels of vitamin D tend to drop during the winter. However, regular exercise appears to help maintain normal levels of active vitamin D even through the darker months, according to a UK study published in Advanced Science.

Vitamin D is essential to health and exists in several forms, each playing a role in how the body utilizes the vitamin to maintain overall health.
The body produces the precursor, cholecalciferol, when the sun’s UVB rays react with a cholesterol-like compound in the skin. Cholecalciferol is also found in fatty animal-based foods such as cod liver, herring, salmon, and egg yolks, as well as in vitamin D supplements.
In the liver, cholecalciferol is converted into calcidiol, the form measured in blood as 25-hydroxyvitamin D3.

When the body needs vitamin D for various functions, calcidiol is further converted into calcitriol (1,25-(OH)₂D₃), the form that binds to vitamin D receptors in nearly all cells in the body.
Calcitriol, the active steroid form of vitamin D, regulates around 10% of our genes and plays a crucial role in the immune system, nervous system, mood, blood sugar regulation, cancer prevention, and many other functions.
Our genes and other factors also influence how efficiently the body converts vitamin D from sun, food, and supplements into this active form.

Exercise and vitamin D metabolism

The UK study shows that regular, moderate-to-intense exercise can help maintain blood levels of vitamin D during the dark winter months.
Researchers found that overweight and obese participants who completed a 10-week exercise program during the winter (October to April) experienced a smaller drop in their blood vitamin D levels compared to a matched control group that did not exercise. This was the case even though the participants who exercised did not lose weight.
The program involved four weekly workouts: two treadmill sessions, one longer cycling session, and one high-intensity interval session on a stationary bike.
The researchers found that the complete exercise program helped maintain the body’s level of active vitamin D (1,25-(OH)₂D₃).
The study was conducted specifically in winter to rule out the influence of sunlight, and participants were instructed not to take supplements.
More specifically, the study showed the following:

• Participants who exercised experienced a smaller overall drop in their vitamin D levels during winter - about 15%, compared to 25% in the control group.
• Importantly, those who exercised maintained healthy levels of the active form of vitamin D, which is vital for immune function, bone health, mood, cancer prevention, and many other functions.
• The non-exercising group experienced a 15% drop in the active form of vitamin D.
• The study confirmed that these effects were solely due to exercise - even in the absence of weight loss.

According to lead author Dr. Oly Perkin from the Centre for Nutrition, Exercise and Metabolism at the University of Bath’s Department of Health, this is the first study to demonstrate that exercise alone can help prevent the typical winter decline in vitamin D levels.
The results serve as a strong reminder that we still have much to learn about the importance of regular exercise.
It should also be noted that genetic differences may affect our ability to activate vitamin D, and most people will still need supplements during winter to stay on the safe side.

References:

Oliver J Perkin et al. Exercise without Weight Loss Prevents Seasonal Decline in Vitamin D Metabolites: The VitaDEx randomized Controlled Trial. Advances Science. 2025

University of Bath. Exercise the key to maintaining Vitamin D level in winter. ScienceDaily May 14, 2025

Helene Sandström. Den optimala D-vitamindosen i vinter? Det beror på dina genar. Nordic Nutrition Council. Sep. 3 2020