If you feel that you are performing better in summer than in winter, you will be happy to hear that more and more scientific studies are backing you up. There is mounting evidence that vitamin D is important for muscle strength and recovery from intense exercise, and as this vitamin is synthesized in your skin thanks to the sun, it is likely that your levels are at their best in summer.
In their latest study, Dr Tyler Barker and colleagues show that vitamin D supplementation enhances recovery immediately after intense exercise. They asked 28 volunteers to perform 10 sets of 10 single leg jumps with 20 sec rest between the sets, and tested their strength immediately before, just after, and 24h, 48h, 72h and 168 h after the effort. 15 of the volunteers took vitamin D supplements and the others a placebo. As you could expect, everybody lost strength and experienced delayed muscle soreness after the jumps. The group who took supplements however, lost less strength immediately after the effort. From 24 hours later onwards, the strength deficit was the same in both groups.
This is important, because a bit more strength could be just enough to maintain good running form -or any other proper technique- at the end of a hard workout, and therefore reduce your risk of injuries. There is an association indeed between low vitamin D levels and injury rates in athletes, and this could be one the reasons why.
This study follows an earlier one (April 2013) by the same group showing that people with higher vitamin D blood levels experience less muscle strength loss after intense exercise than people with low levels.
What is vitamin D?
Vitamin D comes in different forms; the best known are vitamin D2 and vitamin D3. Some scientists think that vitamin D3 is the more potent form, while others think that they are equally effective.
As a matter of fact it is a hormone, produced when the skin is exposed to ultraviolet B radiation. We can also obtain it from food, but as our Western diets usually provide very little of it, most of it comes from sun exposure.
Vitamin D is transformed into calcidiol or 25-hydroxyvitamin D by the liver and can be stored to be used at a later date, for example in winter when we are unable to produce any because the sun is too low.
When needed, calcidiol is transformed into calcitriol or 1, 25-hydroxy vitamin D, which is the biologically active form. Calcitriol is not only important for calcium absorption and bone strength, but it also alters the expression of genes affecting muscle protein synthesis, muscle size and strength, reaction time, coordination, balance, endurance, immunity, inflammation…. It is a very busy substance indeed, and crucial for general health as well as for athletic performances.
Vitamin D deficiency in our modern world
The benefits of vitamin D depend on how much you have: a blood level of less than 5 ng/ml leads to rickets in children and osteomalacia (“soft bones”) in adults. The elderly need 40ng/ml to reduce their risk of falls and fractures, and a low vitamin D status has been linked to infections and chronic illnesses such as diabetes, multiple sclerosis and cardiovascular diseases.
Scientists think that a blood level of 50 ng/ml is required for optimal health and sport performances. Higher levels do not seem to have additional benefits.
Vitamin D deficiency is defined as a calcidiol blood level of less than 30 ng/ml. Studies have shown that about 70% of the population is vitamin D deficient, and the problem is getting worse due to our modern lifestyle. Outdoor sports such as running should be an advantage, but research has shown that it does not make any difference, as most of us train in the mornings or evenings, when the sun is not strong enough to produce vitamin D.
Should I take supplements?
If you think you might be deficient, you should have your blood levels tested and decide with your doctor if and how much supplementation you need.
Experts are not sure what that they should advice for maintenance. If you are between 19 and 70 years old for example, the National Institutes of Health would recommend that you take 600 IU, but the Endocrine Society would recommend 1500- 2000 IU. We clearly need more research!
Vitamin D is a fat soluble substance your body cannot get rid of if you have too much of it. There is thus a risk of toxicity, even though that seems to be rare.
You could also try to make enough vitamin D in summer to get you through winter, as your body stores any excess produced.
However, even though you cannot get an overdose due to sun exposure, you can certainly get skin cancer! You therefore want to limit yourself to safe sunbathing, whereby you expose your skin (without sunscreen) to the midday sun for short periods avoiding reddening. As this is tricky (and dangerous if you get it wrong!) and might not be enough anyway, the best option is to take supplements, at least in winter.
Vitamin D is not a performance aid.
Even though vitamin D will help you to recover and protect you from fractures, it is unsure that it will make you faster. As yet, studies on athletes are inconclusive, even though many athletes report to perform better in summer and autumn than in winter.
You want an optimal level to be as healthy as possible, and excellent health will allow you to be the best runner you can be, which is totally different from doping indeed.
Disclaimer: this article is for general information only, and does not replace medical advice. It cannot be used to diagnose or guide treatment. If you have any concerns or questions, you should talk to a qualified health provider.
T Barker, V Henriksen, T Martins et all. Higher serum 25-hydroxyvitamin D concentrations associate with à faster recovery of skeletal muscle strength after muscular injury. Nutrients. 2013; 5(4): 1253-1275.
T Barker, E Schneider, B Dixon et all. Supplemental vitamin D enhances the recovery in peak isometric force shortly after intense exercise. Nutrition & Metabolism. 2013; 10: 69.
D Ogan and K Pritchett.Vitamin D and the athlete: risks, recommendations, and benefits. Nutrients. 2013; 5(6): 1856-1868.
F Shuler, M Wingate, G Hunter Moore et all. Sports health benefits of vitamin D. Sports Health. 2012; 4(6):496-501.