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Alex Cook — Registered Clinical Dietician 

Vitamin D, the Sunshine Vitamin

Sometimes called the “Sunshine vitamin”, vitamin D is involved in so many physiological functions, when looking at the detail you almost wonder if there is anything this vitamin doesn’t do! It has received increased attention in last 10 years, with many people taking daily supplements as part of their everyday routine.

What is Vitamin D?

Vitamin D is normally obtained through the exposure of the skin to UVB through sunlight. This is great in the summer months, but during the winter, when days are shorter and exposure to the sun is less, we run the risk of developing vitamin D deficiency. This is quite widespread in those that live in northern latitudes where sunlight levels are lower. Variations of vitamin D status between individuals can also be seen as a result of dietary intake, the clothing worn during exercise and overall lifestyle. Whatever the reason for vitamin D deficiency, it can have a significant effect not only sporting performance but more importantly overall health.

Why Do We Need Vitamin D?

Vitamin D plays many vital roles in our body. The most renowned is its role in aiding the absorption of dietary calcium and subsequent role in maintaining bone health. It also plays a role in muscle function and repair.

A study in the American Journal of Physiology showed that supplementing with 4000IU/day of vitamin D had a positive effect on recovery following a bout of damaging eccentric exercise (1). Vitamin D also plays an important role in immune function. A study in 2011 (2) looked at vitamin D levels in college athletes over the winter and spring. It showed that those athletes with vitamins D levels less than 95 nmol/l experienced one or more episodes of upper respiratory tract infection compared to those with higher concentrations of measured vitamin D.

A recent study published in the European Journal of Preventative cardiology (3) looked into a link between vitamin D and cardiorespiratory fitness (CRF). They tested the blood of 2000 subjects to determine their vitamin D levels compared to their VO2 max (a marker of cardiorespiratory fitness showing how efficiently your body utilises oxygen). They found that those in the higher quartile of vitamin D levels had significantly higher VO2 max levels compared to those in the lower quartile. This suggested an association between CRP fitness and vitamin D levels.

However, it is a bit of a chicken and egg scenario as the researchers are still yet to conclude from this if CRF is better because of higher vitamin D levels or if vitamin D levels are better because of higher CRF.The general consensus is that vitamin D deficiency can impact athletic performance, but it is still unknown if vitamin D supplementation in those that are not deficient will have the same effects on performance and recovery as those that have a proven Vitamin D deficiency. More studies on this are needed.

Sources of Vitamin D

Vitamin D exists in very few foods (oily fish, red meat, liver, eggs yolks and fortified foods such as cereal and spreads). As a result, it’s very hard to meet daily requirements through food consumption alone. The Department of Health recommends a daily supplement of 10 micrograms / 400iu through the winter months and all year round if you are not outside very often or if you wear clothes that cover you up when outdoors.

Signs of Vitamin D Deficiency

If you are deficient you may need to supplement with more but as there are no clear signs to look out for it is hard to judge. General signs such as recurrent injury, fatigue and muscle soreness can be warning signals but hard to identify as a lot of people can feel like this simply as a result of training.

Rather than guessing, the best way to find out is having a blood test. This can give you a clear answer whether you need to supplement or not and will be advised by your doctor or dietitian as to what level to take. Although rare, as vitamin D is a fat-soluble vitamin, there is always a chance that you can obtain too much vitamin D if you supplement with high levels without a clear reason to.

Although we know maintaining vitamin D levels within the recommended amount is beneficial for our health and athletic performance, it is still unproven that vitamin D supplementation is a direct performance enhancer.


1. Owens DJ, Sharples AP, Polydorou I, et al. A systems based investigation into vitamin D and skeletal muscle repair, regeneration and hypertrophy. Am J Physiol. 2015;309:E1019–E1031

2. Halliday TM, Peterson NJ, Thomas JJ, et al. Vitamin D status relative to diet, lifestyle, injury, and illness in college athletes. Med Sci Sports Exerc. 2011;43:335–43.

3. Marawan A, Kurbanova N, Qayyum R. Association between serum vitamin D levels and cardiorespiratory fitness in the adult population of the USA. Eur J Prev Cardiol. 2019 May;26(7):750-755

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