Sports Science Monthly – November 2016


Welcome back to another monthly round up of all that is sports science. In this edition, we take an extended look at vitamin D, which for the last few years has been getting a lot of attention for it’s effects on muscular performance. We also have a conceptual piece on the reproducibility of training improvements, Kenyan runners, recovery methods, coaching stress, and jumping as a monitoring tool.

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This Month’s Topics

Vitamin D

→ Quick Summary: Vitamin D levels decrease in athletes in winter months; both sunlight and supplementation are useful for increasing vitamin D levels, although very high doses perhaps aren’t ideal.

Vitamin D gets a lot of attention, especially within the sporting world. It’s the only nutrient that I was ever tested for as a sports person, and my levels were monitored over a long time period. Recently, vitamin D has been getting even more attention, with news reports in the UK suggesting more people should consider supplementing. I’ve written previously about vitamin D, its function, and how much an athlete might need. This month, no fewer than four papers of vitamin D caught my attention, so let’s consider this a mini-review and update on the science.

The first paper was a longitudinal study, done on athletes, over the course of a year, monitoring vitamin D levels and seeing if and how they vary. Athletes were recruited into this study if their vitamin D status was determined to be sufficient – that is above 75 nmol/L. Of the originals 100 subject, 52 were determined to be at a sufficient level. These 52 athletes were then studied further. The athletes were all living in the Netherlands, which means they likely have similar sun exposure to people living in the UK and the northern US states. The majority of these athletes were white; this is important to consider because skin tone and colour can have an impact on how well someone produces vitamin D from the sun – typically, we would expect that the darker the skin colour, the less vitamin D is produced. The original vitamin D testing was done in June, and then repeated at three-month intervals. Sun exposure was calculated via questionnaires – this is obviously something that is important to control for, given that the sun represents the biggest non-supplemental source of vitamin D in our daily lives.

As you might expect, vitamin D levels were highest at the end of summer, when sun exposure had been high for an extended period of time. Conversely, and again unsurprisingly, vitamin D levels were lowest at the end of winter, when sun exposure had been lowest. Remember as well that in June, all of these athletes had vitamin D levels in excess of 75 nmol/L; by December 46% of them were under this threshold. On average, vitamin D levels were around 35% lower at the end of winter compared to the end of summer. One thing this study found, which was new to me, was that females typically had higher levels of vitamin D in the summer compared to males, and also saw a smaller reduction than men in the winter months. This might have a biological mechanism, or it could be down to other factors such as sun seeking behaviours, but it’s certainly interesting to note.

A similar, but perhaps more substantial, study was published in PLoS One; this time looking at elite Polish athletes over a 4-year time period. The trends in the Polish athletes were similar to the Dutch athletes from the previous study: vitamin D levels were highest in the summer months and lowest in the winter months. The Polish researchers took this a step further, and compared athletes who spend most of their time training outdoors with those training indoors. Perhaps unsurprisingly, the indoor training group tended to have significantly lower vitamin D levels that the outdoor training group. There was an additional sub-group of subjects who went to sunnier climates during the European winters for training camps – again, completely unsurprisingly, these subjects had higher winter vitamin D levels than those who stayed in Poland.

A further subgroup in this study was comprised of track and field athletes who were deemed to have too low levels of vitamin D, and received vitamin D supplementation. The athletes were prescribed an individualised dose of vitamin D depending on their insufficiency, but it was between 2,000-8,000IU per day (2,000IU in those with insufficiency, 4,000-8,000IU in those who were deficient). The use of vitamin D supplementation was effective, increasing the vitamin D levels of the athletes.

What I find to be perhaps the most interesting aspect of this study, however, is that sun exposure was more effective at increasing vitamin D levels that supplementation. This is something I wasn’t aware of before, although it has cropped up in other studies that are referenced in this paper. The authors write that around 15 minutes of summer sun exposure on 20% of the bodies surface is equivalent to around 1,400-2,000IU, whilst 30 minutes of sun exposure across the whole body (so being more or less naked) can give between 10,000-20,000IU of vitamin D.

So let’s take a step back and recap the main findings so far:

  • In the summer, vitamin D levels are highest; this is because of increased sun exposure.
  • In the winter, vitamin D levels are lowest; this is because of a decrease in sun exposure.
  • Winter sun exposure is very effective at boosting vitamin D levels – which might encourage warm weather training habits.
  • Sun exposure is more effective than vitamin D supplementation, although both represent a good way to increase vitamin D.

I came across another paper this month looking at how effective vitamin D supplementation in athletes was. These authors recruited almost 50 elite European athletes, and randomly assigned them to a supplemental regime of either 35,000 or 70,000IU per week. The slight difference here is that whilst most vitamin D supplements are consumed daily, this was a bolus dose taken just once per week. There is some evidence that this might be more effective at increasing vitamin D levels, although the jury is still out – it is, however, a good way to increase adherence, as its much easier to take one tablet per week than one per day. The supplementation period was 12 weeks, and it took place during the winter months, so sunlight exposure was minimal. Both supplement regimes significantly increased vitamin D levels, and this happened more rapidly in the 70,000IU dose group. In this group, there was also an increase in a vitamin D metabolite, which lead to an increased breakdown of the active form of vitamin D to a greater extent. Therefore, when high-dose supplementation was ceased, there was quite a rapid return to pre-supplemental levels of vitamin D, taking about 6 weeks. This suggests that very high weekly doses of vitamin D might not be a great idea in athletes.

Vitamin D decreases in winter. Sunlight and supplementation help, but avoid high doses. @craig100m Click To Tweet

As a very brief aside into the world of medicine, this is something that has been reported for a few years. There are a number of studies where large doses of vitamin D are given to elderly subjects, who tend to be those that are studied, because vitamin D can increase muscle strength and therefore reduce the risk of falling. It was expected that these high doses of vitamin D would lead to fewer falls, but in fact the opposite happened; those that had higher doses of vitamin D had significantly more falls when compared to placebo. The authors of this study suggest it could be down to the increase in the vitamin D metabolite, which might reduce muscle function and strength. However, it has so far been hard to tease out the true cause of these falls; it may well be that the elderly subjects who receive the large dose feel much more confident, and so as a result move around the house more, increasing their fall risk. We just don’t really know yet – but it’s a great example of a confounder that could be important (see, I’m also increasing your scientific literacy as well as knowledge!).

Based on their findings, the authors of this study made the following suggestions:

  1. It’s probably best to avoid large bolus doses of vitamin D in athletes (equivalent to >5,000IU per day, so >35,000IU/week).
  2. Rapid withdrawal from higher doses of vitamin D supplements should be avoided, instead, a gentler taper should be used.

In fact, building on that study, we come full circle; the authors of Dutch Athlete study we began with published a separate paper back in September. Here, they found that supplementation with 2,200IU per day was enough to take athletes from deficient or insufficient vitamin D level to one that was sufficient within three months.

If we put all that together, we can possibly get some best practice guidelines that we can use. Firstly, when possible, it’s probably a good idea to spend at least some training time outdoors. If you are able to go warm weather training in the winter months, again this is probably a good idea. If you can’t, a vitamin D supplement is a good substitute, although try to avoid very high doses. Finally, testing of vitamin D levels would be a good idea, as it can allow for more targeted supplementation/sun-seeking behaviour.

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