The underrated link between vitamin D and strength/recovery
Vitamin D is usually associated mainly with bone health, but its receptors are also found in muscle tissue, where it's involved in processes tied to muscle protein synthesis and muscle fiber function. Studies link vitamin D deficiency to reduced muscle strength, slower recovery after training, and a higher risk of certain injuries, including stress fractures in high-training-volume athletes.
Paradoxically, deficiency is also common among people who train regularly — especially those who mostly work out indoors, live at northern latitudes with limited sunlight for much of the year, or wear sunscreen consistently (which blocks vitamin D synthesis in the skin). Vitamin D is synthesized in the skin under UV exposure, and without enough sun time it's hard to make up the gap through diet alone — there simply aren't many food sources of it in a typical diet.
For someone who trains, especially through fall and winter or with mostly indoor/gym-based training, it's worth checking vitamin D levels with a blood test — a deficiency is easy to correct with a supplement, but it often goes unnoticed, masquerading as ordinary training fatigue.