Four years ago I wrote a blog post on the potential benefits for endurance athletes of maintaining optimal vitamin D, as shown by the research up to that point. You can read my 2012 post here. This area has continued to be of interest to sports scientists, which has led to further relevant studies, and vitamin D is now seen as a micronutrient of key interest when assessing the nutritional needs of athletes. For example, it is one of only four micronutrients given this status, alongside iron, calcium and certain anti-oxidants, in the recently published position stand on Nutrition and Athletic Performance from the American College of Sports Medicine. 1 So I consider it is time for an update and have taken a look at the more recent research to see what it holds for the endurance athlete.
Vitamin D status in athletes
A systematic review published in early 2015 looked at the prevalence of vitamin D inadequacy in athletes 2. Across 23 studies with 2313 subjects, 56% had inadequate levels of the active form of vitamin D in their blood, 25-hydroxyvitamin D – also known as 25(OH)D – which was defined as less than 80 nmol/L (32ng/mL). Inadequacy was more common in the winter and spring seasons, in those training indoors, and for those living above latitude 40°N. This makes sense since exposure to sunlight is required to convert cholesterol found in the skin to vitamin D. There is only a small amount of vitamin D in food; the best sources are sundried shitake mushrooms and wild salmon, followed by canned oily fish like salmon, mackerel and tuna, and egg yolks. It is also poorly absorbed from food. This means that a combination of sunshine and supplements are generally needed to correct vitamin D deficiency or inadequacy. Deficiency is usually defined as serum 25(OH)D of less than 50 nmol/L and inadequacy as between 50 and 80 nmol/L. Vitamin D can be measured with a relatively inexpensive blood spot test, which athletes can carry out at home, making it one of the easier nutrients to assess regularly. See the website if you live in the UK.
Vitamin D and athletic performance
Probably the best known function of vitamin D is its role in supporting calcium absorption and consequently bone density, bone growth and bone remodelling. Without adequate amounts of vitamin D, bone loss occurs, increasing the risk of injuries such as stress fractures. This clearly has implications for performance. But it is the other functions of vitamin D that have received more research attention in recent years. This is known as the autocrine pathway, which is involved in essential body processes like synthesising proteins and hormones, the immune/inflammatory response and the ability of cells to respond to signals. Vitamin D receptor (VDR) sites have been identified in virtually every tissue in the body, where they regulate expression in hundreds of genes that perform essential functions, which gives you some idea of vitamin D’s importance. This includes vitamin D being a regulator of skeletal muscle function. A research review published in 2013 discussed how poor vitamin D status adversely affects muscle strength, the link between vitamin D and muscle protein synthesis through the action of the VDR in muscle tissue and the role of vitamin D in protecting against stress fracture, as well as giving examples of how vitamin D supports the immune and cardiovascular systems. 3
However, recognition of these functions has not led to many new studies using vitamin D interventions in athletic populations, so we do not yet have very much information on how increasing vitamin D levels might impact on performance. Two studies which may have relevance for endurance athletes are as follows. A 2014 study looked at the effects of vitamin D supplementation on VO2 max in 14 elite lightweight rowers, who already had adequate vitamin D levels.4 They were given either 6000 IU of vitamin D (a fairly high dose) or a placebo daily during an 8 week training cycle. Those given the supplement saw significant improvements in their aerobic metabolism. But would rowers with inadequate vitamin D see similar results or require a higher level of supplementation? What’s the mechanism involved? More research is required.
The second study 5 was carried out in healthy and moderately active adults and shows benefits for recovery and power output which might translate to endurance athletes. In a randomized, double blind, placebo controlled trial, where 4000 IU of vitamin D was given for 35 days, it was shown that the subjects receiving the supplement showed reduced levels of the inflammatory bio markers ALT and AST after a jumping exercise. Peak power output in the supplemented group also showed a far smaller decline (6% vs 32%) immediately post exercise and this difference between the groups persisted 48 hours later. Again, the subjects already had adequate vitamin D levels.
Vitamin D and immune status
The immune system is one area that has seen specific research done on the influence of vitamin D levels in endurance athletes. This is relevant as illness means lost training days, so having a robust immune system has indirect benefits for performance. A study published in 2013 measured 25(OH)D levels in 225 endurance athletes at the start and finish of a 16 week winter training block. 6 During the period of the study, various markers associated with immune status were measured such as levels of antibodies in the saliva (SIgA) and production of pro-inflammatory cytokines by white blood cells. At the start of the 16 week study period, 38% of the athletes had inadequate or deficient levels of vitamin D, and at the end of the period this had increased to 55%, which is not surprising given the lack of sunshine at that time of year. The subjects did not take vitamin D supplements during the study. The results showed that a far higher proportion of the athletes who were deficient in vitamin D presented with the symptoms of upper respiratory tract infections (URTIs), such as the common cold, than the athletes who had optimal vitamin D levels. Additionally those with optimal vitamin D had higher levels of SIgA in their saliva. The researchers concluded that low vitamin D status could be an important determinant of URTI risk in endurance athletes. For this reason alone, it is probably worth taking a vitamin D supplement during the winter months. I will consider the dose required in the next section.
What is the optimal vitamin D level for athletes?
Recommendations on the required intake of vitamin D vary. For the general population, the Institute of Medicine in the US recommends 400-800 IU of vitamin D3 daily to maintain levels > 50 nmol/L. This is intended to prevent deficiency. The Endocrine Society, on the other hand, recommends 1500-2000 IU daily for adults to maintain adequate levels >75 nmol/L. Other recommendations have suggested optimal levels may be between 90 and 120 nmol/L. The optimal levels needed for athletic performance have not yet been determined. Levels >100 nmol/L have been proposed to be optimal for lower body skeletal muscle function. It has also been shown that it takes between 2000 and 5000 IU to optimise bone health by maintaining vitamin D levels at 75-80 nmol/L, which could not be obtained without supplementation in the winter months for those living above latitude 42.2°N 7
No study has yet looked at the effects of vitamin D supplementation and skeletal muscle function in an athletic population with vitamin D levels > 100 nmol/L. A 2013 study 8 did examine the effects that vitamin D supplementation of 20,000 and 40,000 IU per week (equivalent to 2800-5600 IU per day) versus placebo had on muscle performance in 30 athletes in the UK over a 12 week period. 57% of the subjects were vitamin D deficient (<50nmol/L) at the start of the study. This showed that 6 weeks of supplementation was enough time to correct any deficiency but it was not enough to obtain potentially optimal levels of >100 nmol/L. No significant improvements in muscle performance were observed over the 12 weeks, although vitamin D levels did increase in those taking the supplements, leading the researchers to conclude that higher levels of supplementation may be required to induce a physiological response within skeletal muscle.
So what does this mean for you, the endurance athlete? A 2013 review of the research 9 comes to the following conclusions:
- A goal of 25(OH)D levels of >100 nmol/L is recommended for athletes as this is the level at which vitamin D begins to be stored in the muscle and fat for future use.
- To maintain 25(OH)D levels of >100 nmol/L, vitamin D supplementation is warranted, especially during the winter months
- Maintaining 25(OH)D at levels under 80 nmol/L but over 50 nmol/L, which would prevent deficiency but not reach adequate levels, would likely result in insufficient vitamin D being available to support the autocrine pathways that might influence performance, eg muscle protein synthesis.
- Since vitamin D is used in so many metabolic pathways, it is possible that athletes may require increased intake of vitamin D to assure adequate availability and storage for optimal performance.
- The appropriate supplementation levels depend on an athlete’s current 25(OH)D levels, the time of the year, and exposure to sunlight, with a goal of maintaining levels >100 nmol/L year round.
- This might require short term supplementation on 5000 IU daily (eg for 8 weeks) followed by maintenance supplementation of 1000-2000 IU daily.
- While there is not yet sufficient evidence that this level will improve performance directly, it should help to prevent stress fractures and URTIs and maintain muscle function.
Summary and recommendations
Research has now shown a far broader role for vitamin D in the body than support for bone health. This includes muscle function, immunity and cardiovascular health, all of which are very relevant for the endurance athlete. A high proportion of athletes have been shown to have deficient or inadequate levels of vitamin D, often due to lack of sunlight, which should be identified and corrected through supplementation. There is a small but growing body of evidence suggesting that athletes may benefit from a higher level of vitamin D than the general population in order to promote performance benefits but more research is required in this area.
My suggestion is to get your vitamin D status tested, and follow these recommendations.
- If you already reach the potentially optimal level of >100 nmol/L, there is no need to take any action.
- If you have levels between 80 and100 nmol/L, supplement 1000 IU in the winter months if you live above latitude 40°N or train mostly indoors.
- If you have levels between 50 and 80 nmol/L (inadequate), supplement 5000 IU for 8 weeks then retest. If you have reached 100 nmol, take 2000 IU as a maintenance dose. If not, take 5000 IU for a further 8 weeks and test again
- If you have levels less than 50 nmol/L (deficient), consult a health professional such as your doctor or a nutritionist for advice on supplementation.
Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sport. Based in London, she also works as a Registered Nutritional Therapist, conducting one–to–one consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit www.nutritionforendurancesports.co.uk
1 Position of the Academy of the Nutrition & Dietetics, Dieticians of Canada and the American College of Sports Medicine. Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise: March 2016, 48(3): 543-568 http://www.ncbi.nlm.nih.gov/pubmed/26891166
2 Farrokhyer et al. Prevalence of Vitamin D Inadequacy in Athletes: A Systematic Review and Meta-Analysis. Sports Med. 2015;45 (3):365-78 http://www.ncbi.nlm.nih.gov/pubmed/25277808
3 Moran et al. Vitamin D and physical performance. Sports Med. 2013; 43(7):601-11 http://www.ncbi.nlm.nih.gov/pubmed/23657931
4 Jastrzebski Z. Effect of vitamin D supplementation on the level of physical fitness and blood parameters of rowers during the 8-week high intensity training. Facicula Educ Fiz si Sport. 2014; 2:57-67 http://www.fefsoradea.ro/Fascicula_Educatie_Fizica_si_Sport/2014/9.FEFS_2014_Zbigniew.pdf
5 Barker et al. Supplemental vitamin D enhances the recovery in peak isometric force shortly after intense exercise. Nutr Metab. 2013; 10:69 http://www.ncbi.nlm.nih.gov/pubmed/24313936
6 He et al. Influence of vitamin D status on respiratory infection incidence and immune function during 4 months of winter training in endurance sports athletes. Exerc Immunol Rev. 2013; 19:86-101 http://www.ncbi.nlm.nih.gov/pubmed/23977722
7 Dahlquist DT, Dieter BP & Koehle MS. Plausible ergogenic effects of vitamin D on athletic performance and recovery. J Int Sports Soc Nutr. 2015; 12:33 http://www.ncbi.nlm.nih.gov/pubmed/26288575
8 Close et al. The effects of vitamin D3 supplementation on serum total 25(OH)D concentration and physical performance: a randomised dose response study. Br J Sports Med. 2013; 47(11): 692-6 http://www.ncbi.nlm.nih.gov/pubmed/23410885
9 Organ D & Pritchett K. Vitamin D and the athlete: risks, recommendations and benefits. Nutrients. 2013; 5(6): 1856-68 http://www.ncbi.nlm.nih.gov/pubmed/23760056