Vitamin D for athletes

Vitamin D’s reach across the body is far and wide. Its receptor exists in pretty much all tissues^[1], and it’s probably there for a reason. But, that reason differs depending on the tissue in question. This review focuses on muscle.

When vitamin D binds its receptor, the complex goes to the nucleus and affects the transcription of genes. In muscle cells, this process^[2] affects genes related to calcium handling (which is essential for muscle contraction) as well as genes involved in muscle cell development and proliferation. Also, some mutations^[3][4] in the vitamin D receptor are correlated with changes in muscle strength. Finally, mouse experiments^[5] that delete the vitamin D receptor show direct effects on muscle. So, mechanistically, these suggest that vitamin D and its receptor impact muscle. Some of the physiological effects of vitamin D and its receptor on muscle cells are summarized in Figure 1.

While muscle function is important for everyone, it’s of particular concern to athletes. In fact, it may be doubly concerning, since vitamin D deficiency is prominent^[6] even in these paragons of health. Deficiency is especially common for athletes at higher latitudes who participate in indoor sports during the winter months. These conditions lead to limited sun exposure and thus and a paucity of vitamin D synthesis. While there’s no universal agreement on what “deficiency” means, the Endocrine Society recommends^[7] that 25(OH)D levels should be above 30 ng/mL (75 nmol/L), although worldwide recommendations^[8] for preventing rickets provide lower cutoffs. Some researchers^[9] even argue that optimal levels for athletes should be 40 ng/mL (100 nmol/L). Regardless of the specifics, it seems that athletes living further away from the earth’s equator could use a little more vitamin D, at least in the winter months.

The question is: how much do they need? And would supplementation actually have an effect on athletic performance? After all, just because vitamin D influences muscle cells, dietary supplementation may not necessarily translate to improved performance. While there have been several small trials looking at this issue, they haven’t led to firm conclusions, in part due to their small sample sizes. The study under review is the first meta-analysis attempting to synthesize them to answer these questions.

Vitamin D affects practically every tissue in the body. In muscle, it leads to the transcription of genes that affect muscle development and function. Since many athletes are vitamin D-deficient, this raises the question of how much supplementation athletes need to get their levels up and whether this would have any effect on athletic performance.