Engaging in regular indoor physical activity during the winter (for example, 4 weekly sessions alternating between moderate endurance and high intensity) helps stabilize levels of the active form of vitamin D and mitigates the seasonal decline of its storage form.

noli's read

This very precise claim is based on a randomized clinical trial (the VitaDEx trial) published in 2025 in the journal *Advanced Science* by researchers at the University of Bath. This rigorous work confirms that a 10-week indoor exercise program, consisting of four weekly sessions, completely preserves the active form of vitamin D during winter. Indeed, while this active form drops by 15% in sedentary individuals, it remains stable in those who exercise, while the decrease in the circulating form is mitigated by 10%. However, an important nuance must be noted: the trial was conducted specifically on inactive adults who are overweight or obese. The exact impact of this protocol on individuals who are already at a healthy weight or have a very active lifestyle has not yet been tested. This remains a fascinating discovery showing that regular movement directly activates our metabolism, in a way that is quite distinct from simple supplementation.

the sources

1. 1NIH Office of Dietary Supplements 2026
2. 2Manson JE et al. 2019
3. 3foundmyfitness.com/topics