Vitamin D and athletic performance

Vitamin D, the sunshine vitamin, is well known for its role in bone health. Other roles include muscular and immune function, hence the interest for studying vitamin D and athletic performance.

What is vitamin D

Vitamin D is an essential fat-soluble vitamin (1). The active form of vitamin D is 1,25-dihydroxycholecaciferol (a.k.a. 1,25-dihydroxivitamin D) (2, 3), which interacts with vitamin D receptors (VDRs) (1).

Sources of vitamin D

Most of the vitamin D we get (80-90%, 4) is synthesised in our bodies from 7-dehydrocholesterol (2) after exposure to ultraviolet B (UVB) radiation from the sun (2, 4). The remaining 10-20% is obtained from the diet (4).

Sunshine

The actual amount of vitamin D we can get from sun exposure depends on multiple factors including skin colour, clothing, sun avoidance (e.g. being indoors) (5) and age.

Food

Food sources of vitamin D include fish, mushrooms that have been irradiated with UV light, liver, eggs and fortified foods (3, 5). As it happens with other micronutrients, vitamin D2 (ergocalciferol) obtained from plant sources is less bioavailable than vitamin D3 (cholecalciferol) from animal sources (1).

Supplements

Vitamin D3 supplementation during winter months can improve serum levels in athletes who are deficient at baseline. Effective doses seem to range between 2857 and 5000 IU per day (6). No benefit has been observed with supplements in athletes who have adequate levels of vitamin D (2).

Roles of vitamin D

There are VDRs in many tissues in the body and therefore multiple roles of the vitamin, including:

• Bone health (2, 3, 4) through the regulation of calcium and phosphate homeostasis (2, 5). This is without a doubt the best well-known area of study as it relates to this vitamin.
• Muscle function, repair and remodelling (4, 2). This is especially true for type II (fast twitch) muscle fibres (1, 2).
• Neurological function (2, 1), including reaction time, balance and coordination (2)
• Immune function, including the reduction of post-exercise inflammation (1, 2, 4)
• Homeostasis of steroid hormones, such as testosterone and oestrogen (2)
• Potential roles in cardiac function (1, 2, 4), tumour suppression (2, 3) and glucose metabolism (1)

Vitamin D and athletic performance

Adequate amounts of vitamin D are important for health and may affect athletic performance. Vitamin D deficiency has been associated with a higher incidence of fractures, muscular injuries and upper respiratory tract infections (3).

Although there are some reports of performance enhancement post-vitamin D supplementation (3), there is no consensus in the literature supporting vitamin D as an ergogenic aid (1, 4). While some studies have shown an improvement on muscle strength, mainly in the legs of athletes training indoors (7), others have not found any benefit (6).

Measuring vitamin D

Levels of vitamin D are measured using the serum 25-hydroxyvitamin D (25[OH]D) concentration (4, 3). However, it seems that some testing methods are not accurate enough (5). Moreover, the use of this metabolite is controversial because, as seen before, it is not the bioavailable form of the vitamin (4).

Healthy ranges vary depending on the country and organisation defining them. In Australia, the ranges are:

• Vitamin D adequacy: ≥ 50 nmol/L at the end of winter
• Mild vitamin D deficiency: 30–49 nmol/L
• Moderate vitamin deficiency: 12.5–29 nmol/L
• Severe vitamin D deficiency: < 12.5 nmol/L (5)

Other sources consider vitamin D insufficiency as 50 to 75 nmol/L (7).

Recommended daily intake

The adequate intake according to Australian guidelines is 5 micrograms (μg) per day for adults 19-50 years old, 10 μg per day for those 51-70 years old and 15 μg per day for those over 70 years old (8). This translates to 200, 400 and 600 international units (IU) respectively. However, it’s been found that it takes 2000 to 5000 IU/day from all sources to maintain a level of 75 to 80 nmol/L (4)

The recommended sun exposure for people with fair skin is walking with the arms exposed for 6-7 minutes mid-morning or mid-afternoon in summer or 7-40 minutes with as much skin exposed as possible in winter. The times increase for people with darker skin and/or who are wearing clothing that covers more skin (5).

Can vitamin D be toxic?

As other fat-soluble vitamins, vitamin D can be toxic when levels in the blood exceed 180-200 nmol/L. Note that this can only happen when taking large doses of supplements (1, 4). Vitamin D works synergistically with vitamin K, so it is possible that vitamin D toxicity occurs only when vitamin K is deficient (1).

Summary and recommendations

Vitamin D deficiency can have detrimental effects to several aspects of health, which can impact training and athletic performance. Adequate levels of serum vitamin D are commonly defined as above 50 to 75 nmol/L. The main source of vitamin D is endogenous production triggered by sun exposure of the skin, but it can also be obtained from some foods and supplements. Supplements may improve health and performance outcomes in athletes who are deficient. Therefore, supplementation should be considered on an individual basis.

References

1. Dahlquist DT, Dieter BP, Koehle MS. Plausible ergogenic effects of vitamin D on athletic performance and recovery. J Int Soc Sports Nutr. 2015;12:33.
2. Stachowicz M, Lebiedzińska A. The role of vitamin D in health preservation and exertional capacity of athletes. Postepy higieny i medycyny doswiadczalnej (Online). 2016;70(0):637-43.
3. Sikora-Klak J, Narvy SJ, Yang J, Makhni E, Kharrazi FD, Mehran N. The Effect of Abnormal Vitamin D Levels in Athletes. The Permanente journal. 2018;22:17-216.
4. Owens DJ, Allison R, Close GL. Vitamin D and the Athlete: Current Perspectives and New Challenges. Sports medicine (Auckland, NZ). 2018;48(Suppl 1):3-16
5. Nowson CA, McGrath JJ, Ebeling PR, Haikerwal A, Daly RM, Sanders KM, et al. Vitamin D and health in adults in Australia and New Zealand: a position statement. Medical Journal of Australia. 2012;196(1):686-7.
6. Han Q, Li X, Tan Q, Shao J, Yi M. Effects of vitamin D3 supplementation on serum 25(OH)D concentration and strength in athletes: a systematic review and meta-analysis of randomized controlled trials. J Int Soc Sports Nutr. 2019;16(1):55.
7. Zhang L, Quan M, Cao ZB. Effect of vitamin D supplementation on upper and lower limb muscle strength and muscle power in athletes: A meta-analysis. PLoS One. 2019;14(4):e0215826.

[Photo by Fitsum Admasu on Unsplash]

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