New Insights into the Role of Vitamin D in Athletes: Soccer Players Offer Ideal Study Subjects - Vitamin D (25-OHD) Predicts Performance, Training Season Predicts Vitamin D

Not the ideal soccer weather. If you put faith in the authors' interpretation of the results, soccer is a game that must be played in the sun. Why? Well, to maximize the vitamin D production ;-)
It has been a while since I have seen a study on vitamin D that provides new-worthy results beyond the obvious "vitamin D supplements are useless in men and women with normal 25-OHD levels". The study I am about to talk about today was conducted by researchers from the University of Crete. The subjects were sixty seven Caucasian professional male soccer players, members of two Greek Super League Teams (n = 45) and one Football League Team (n = 22). The goal was to (a) examine the potential relationship between vitamin D levels and muscle strength, maximal oxygen consumption (VO2max), 10 and, 20 meters sprint performance in two different occasions, prior to the beginning and at the end of the off-season soccer period; and (b) to examine the vitamin D response to the reduced exercise training during the six-week off-season transition period.

The authors speculated that in both experimental sessions vitamin D levels would correlate with soccer players’ jumping, sprinting, and aerobic capacity, and that the off-season transition period, of reduced training stress, would favorably affect vitamin D concentration.
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As we are about to see a few lines below, this is exactly what Nikolaos E. Koundourakis and his colleagues found in what is "to the best of [the researchers] knowledge" the first study to examine the relationship "between vitamin D levels and muscle strength, VO2max and speed in professional soccer players, and/or the effects off-season detraining soccer period on its levels in any kind of athletic population." (Koundourakis. 2014)
Table 1: Correlations (correlation coefficients and p-values) between Vitamin D levels and exercise performance parameters; pre = acute pre-season training vs. post = after six-week off-season transition period (Koundourakis. 2014)
As you can see in Table 1, there was a significant correlation between the strength measures squat jump (SJ) and countermovement jump (CMJ), as well as with VO2max and the 10m and 20m sprint times. There was jet also a significant increase in vitamin D from pre to post season, i.e from 34.41 to 47.21ng/ml (37%) of which Koundourakis et al. write the following:
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"In our study the six-week transition period had a boosting effect on vitamin D levels. Indeed, at the first experimental period,although none of our participants was vitamin D deficient (<20 ng/ml) or severe deficient (<10 ng/ml), 55,22% of our players had insufficient vitamin D levels (<30 ng/ml), whereas at the second one only 4,47% were found to be below 30 ng/ml. [...] The most plausible explanation for the elevation of vitamin D levels at the second experimental session could be the consequence of an increased exposure to UVB during the off-season period.

Indeed, this transition period in Greek Superleague takes place during June and at the beginning of July at a favorable latitude (35,9°N). During this period UVB reaches its peak, resulting in increased vitamin D production." (Koundourakis. 2014)
These observations highlight the importance of adequate sun exposure and raise the question, whether it wouldn't be prudent to supplement with 2.000IU of vitamin D per day during the winter months to avoid the reduction in vitamin D levels in the first place.
Tanning beds are neither suitable to get your vitamin D levels up (many don't even radiate UV-B light / it's also blocked by glass), tanning on a sunbed is also not safer than doing the same in the sun | more
There is a different explanation: Next to the sun-exposure hypothesis, there is another possible reason for the increase in vitamin D, which would be related to the (albeit beneficial) stress the soccer players are exposed to during each and every of their training sessions. If vitamin D was the negative acute phase reactant, Waldron et al. say it was (2013), it would be logical that the levels increase in the absence of acute stressors.

Needless to say that the latter wouldn't make getting enough UV exposure obsolete - an exposure you better get in the real sun than on one of the allegedly less problematic tanning beds, which lack the vitamin D production kickstarting UV-B radiation.
  • Koundourakis, Nikolaos E., et al. "Vitamin D and Exercise Performance in Professional Soccer Players." PLOS ONE 9.7 (2014): e101659.
  • Waldron, Jenna Louise, et al. "Vitamin D: a negative acute phase reactant." Journal of clinical pathology (2013): jclinpath-2012.
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