Overview of Vitamin D
Vitamin D is a hot topic trending in nutrition right now-- especially for athletes and fitness enthusiasts. Over the last few years, emerging research has shown a possible correlation between vitamin D levels and performance. Some professionals have even speculated that vitamin D needs are higher in the athletic population than the general public in order to improve performance.
We currently know that vitamin D is involved in the process to strengthen bones, support immune system and has a role in skeletal muscle function. Additionally, vitamin D receptors (VDRs) have been found on organs and tissues in hundreds of different locations in the body, meaning there could be a myriad of other uses for the "sunshine vitamin" that are just beginning to be understood.
An emphasis has been placed on making sure the general public are meeting their needs for vitamin D through either diet or adequate sun exposure. Despite this however, the majority of athletes appear to be deficient, especially during winter months or those in northern climates. A meta-analysis conducted in 2015 on over 2000 athletes showed that 56% of participants were classified as having inadequate vitamin D levels. Since vitamin D plays a role in skeletal muscle, there may be a link between this vitamin and athletic performance.
Currently, the best indicator of vitamin D status in the body is measured as serum 25(OH)D. Clinical levels are currently accepted as:
Recommendations for adequate vitamin D levels are defined only in relation to adequate bone function and metabolism and do not take into account other roles vitamin D may have.
Vitamin D and Muscular Performance
The correlation between vitamin D and muscular performance is unclear at this time. While many studies have shown positive results, others have shown no effect. One study looked at the effects of giving 10 athletes 5000 IU/day of vitamin D3 supplementation for eight weeks on musculoskeletal performance. Results showed a significant improvement in 1-meter sprint times and vertical jump height for the participants, but no significant difference in 1-RM bench press or back squat performance. The vitamin D concentrations that had a positive influence on performance were elevated to well above the recommended levels for the general population.
Another study  looked at the relationship between serum vitamin D and muscle strength in 419 participants with researchers concluding that vitamin D was a positive predictor of arm strength but not significantly associated with leg strength.
While these studies showed different but positive correlations between vitamin D and certain performance measures, many others have shown no effect at all. Research  conducted on 30 UK based athletes looked at the effects of 20,000 IU and 40,000 IU vitamin D3 given weekly on performance compared to a placebo group, however there were no significant differences seen in performance testing between the experimental groups and the placebo.
Unfortunately, the current research regarding serum vitamin D and muscular performance remains conflicted. There is not enough supporting evidence to give specific recommendations for vitamin D supplementation to individuals looking to improve strength and performance.
Vitamin D and Muscular Recovery
Research on the link between vitamin D and recovery time of athletes is very promising with most studies showing a relationship between adequate vitamin D levels and reduced recovery time.
One study  showed that there was a negative correlation between circulating vitamin D and inflammatory markers cytokine IFN-γ , ALT and AST (biomarkers representative of muscle damage) following strength training. . The more circulating vitamin D in the blood, the less this pro-inflammatory marker was present. Additionally, serum vitamin D levels were significantly correlated with less muscular strength both immediately following exercise and for 48 and 72-hours after.
A similar study  looking at the effects of 4000 IU of vitamin D in strength recovery following intense exercise found that the participants taking the vitamin D supplement had significantly less circulating levels of ALT and AST levels and had improved recovery time of skeletal muscle strength compared to the group not receiving a vitamin D supplement. Interestingly enough, these participants had already been classified as having adequate vitamin D before the start of this study.
Vitamin D and VO2 Max
VO2 max is a measure of maximal oxygen uptake by the body and is one of the main ways to determine cardiorespiratory fitness and is used to measure aerobic fitness in individuals. Vitamin D receptors have been found not only in muscular tissue, but also in cardiovascular tissues as well, meaning there may be a relation between vitamin D and VO2 max. Unfortunately, much like most research, the results are not conclusive at this time.
While one study found that serum vitamin D levels positively predictor VO2 max , another found no significant relationship between the two .
While some of the research is promising, much is not. This is partially due to the fact that researchers don't fully understand the mechanism of action of how vitamin D would affect performance levels. The most promising studies at this time point towards a stronger relationship between vitamin D and muscular recovery, yet despite this there still isn't enough definitive research to be able to make vitamin D recommendations to athletes different from the general population. Furthermore, individuals supplementing should be careful not to exceed the upper limit of vitamin D which is set at 4000 IU/day as there are adverse affects related to over consumption.
Even though there are no athletic specific recommendations for vitamin D, the majority of athletes may be vitamin D deficient even according to the current recommendations that are set for bone function and metabolism. Vitamin D deficiency has been associated with rickets: poor bone mineralization in children, osteomalacia: soft bones in adults, insulin resistance, poor immune function and muscle weakness.
Current vitamin D recommendations for the general, healthy public are 600 IU/day and 800 IU/day for those >70 years old.
Vitamin D can be found in foods such as:
If you don't have a balanced diet that contains foods rich in vitamin D or don't get adequate sun exposure, then a supplement may be beneficial. While it is unknown if athletes have higher vitamin D requirements, a deficiency can certainly affect your performance. With over half of all athletes at risk of having inadequate vitamin D intake, it would be a good idea to get serum levels tested by a professional if a deficiency is suspected.