Short winter days could affect you more than you think. The lack of sunlight on your skin in the cold months can leave you with low levels of vitamin D, which may be limiting your strength gains in the gym.
In a systematic review by Chiang et. al, six studies involving athletic subjects between the ages of 18 and 45 were analyzed. The studies used varying doses of D2 and D3 (both precursors to active vitamin D in the body) and tested strength using various methods.
Two of the studies supplemented with D2, which showed no significant differences in strength between the control and experimental group.
In the four studies examining D3 supplementation, two of them showed significant increases in strength, and two showed no difference in strength. However, the two D3 studies that showed no statistically significant differences between the control and experimental group did still increase strength.
With the study noting that up to 80% of athletes could be deficient in vitamin D, supplementing – especially during the winter when sunlight exposure is low – is crucial.
The two types of vitamin D are not treated equally by the body. D2 won't increase levels of 25(OH)D – vitamin D in the body – while D3 will. The researchers recommend 2,000 IU of D3 daily for 8-12 weeks to increase levels.
The optimal levels of vitamin D in the body are disputed by researchers and the Institute of Medicine and the Endocrine Society. It seems levels close to 50 ng/ml of 25(OH)D are best for the athletic population and strength display, and the average subject values pooled from all the studies in the review had a value of 23.59 ng/ml.
With the high prevalence of suboptimal vitamin D levels, supplementation is most likely beneficial, if not necessary, in most of the population, both athletic and non-athletic. If you want to be certain, a simple blood test will allow you to see your serum levels and let you monitor it throughout the year.
- Chiang CM et al. Effects of vitamin D supplementation on muscle strength in athletes: A systematic review. J Strength Cond Res. 2017 Feb;31(2):566-574. PubMed.