Vitamin D: The Sunshine Vitamin’s Role in Health and Performance
Vitamin D, often called the “sunshine vitamin,” is a unique fat-soluble vitamin that our skin can produce with sufficient sunlight exposure. As it can be made in the body, it is only conditionally essential in our diets. However, limited sunshine exposure due to modern lifestyles or geographical location (à la the Vikings) necessitates dietary sources and supplements of vitamin D.
In this post, we’ll explore what vitamin D is, how we get it, and why it’s essential for daily health and function. This post will begin with a beginner’s overview, which I hope is accessible to all readers, before getting a bit more technical towards the end (I am a scientist, after all).
This is a safe space for nerds, so I’ll provide a list of further reading materials for any of you who wish to delve deeper into this not-technically-a-vitamin vitamin.
What is Vitamin D?
The term “vitamin D” doesn’t refer to a single chemical compound, but rather a family of related hormones, mainly vitamin D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D3 is the form made in human skin that is radiated with UVB rays, but can also come from the diet through fatty fish and egg yolks. D2, on the other hand, comes from plant/fungal sources (Martens et al., 2020).
Vitamin D is typically inert in the body (i.e. it doesn’t too much) until it undergoes a chemical process called hydroxylation by the liver (1st hydroxylation) and kidneys (2nd hydroxylation), which then “activates” the vitamin D. (NIH, 2025)
Once activated, vitamin D promotes calcium absorption and is necessary for bone and teeth development and remodelling (McArdle, Katch and Katch, 2015). This is why low levels of vitamin D cause defects of the bones (e.g. rickets in children or osteomalacia in adults; NIH, 2025). Vitamin D is also involved in reducing inflammation and modulation immune function and glucose metabolism.
Around a billion people globally are estimated to be deficient in vitamin D levels (Ribbans et al., 2021), leaving them susceptible to various conditions detailed in this post.
Sources of Vitamin D
Note: RDA = Recommended Daily Allowance, µg = micrograms
As already discussed, there are a few common sources of vitamin D, as outlined below:
Vitamin D2 sources
UV-exposed mushrooms (~100% RDA per 100g, Cardwell et al., 2018)
Fortified milk (~20% RDA per 100ml)
Fortified cereals (35% RDA per 100g)
Vitamin D3 sources:
Produced in human skin with sufficient sunlight exposure. 10-15 minutes of sunlight on the arms and legs a few times a week can generate sufficient vitamin D (Harvard Medical School, 2008)
Fatty fish
Cod (muscle, 100g) - 0.7-1.2µg (7-12% RDA)
Cod (liver, 10g) - 12.5µg (125% RDA Dosti and Marku, 2023)
Salmon (100g) - 13.15µg (131.5% RDA)
Tuna (100g) - 6.73µg (67% RDA)
Mackerel (100g) - 16.1µg (161% RDA; US Department of Agriculture, 2019)
Egg Yolks (a single 50g egg yolk contains around 5% RDA)
Why do we need vitamin D?
Sufficient vitamin D is essential for human health, playing several vital roles in the body. Some of these functions include:
Bone health: This is vitamin D’s best known role. It promotes calcium and phosphate absorption from the gut, both crucial for building, remodelling and mineralizing bone (NIH, 2025).
Muscle function: Vitamin D is necessary for normal muscle development and function. Muscles have vitamin D receptors, and vitamin D aids the growth and maintenance of muscle fibres. When vitamin D levels are low, muscle strength can decline, potentially leading to weakness (vitamin D deficiency myopathy), aches, and, especially in older people, an increased risk of falls and injuries.
Immune support: Vitamin D plays some important roles in the immune system too. It modulates both innate and adaptive immunity (if you want to learn more about these two branches of the immune system, watch this video by Science ABC). It can enhance the ability of white blood cells to fight infections and reduce excessive inflammation (Aranow, 2011). For a specific example, a 2017 meta-analysis (a study of studies) of 25 randomised controlled trials, with 11,321 total participants between them, found that Vitamin D supplementation reduced the risk of acute (short term) respiratory tract infections (Martineau et al., 2017). Vitamin D deficiency has also been linked to a predisposition to autoimmune diseases (Lang et al., 2017).
As you can see, vitamin D is quite the multi-tasking micronutrient. It is vital for skeletal health, but also contributes to muscle strength, immune defence, and potentially much more.
Too much of a good thing?
It is important to note that taking massive amounts of vitamin D (e.g. eating the high-vitamin-D foods listed above and taking vitamin D supplements) is not the shortcut to better health you might think.
Vitamin D is a fat-soluble vitamin and can become toxic in excessive amounts, leading to elevated levels of calcium in the blood and thus causing kidney stones.
The tolerable upper limit is currently defined as around 100µg per day (remember that the recommended daily intake is only 10µg), so this is pretty hard to do by accident.
The goal of vitamin D supplementation is simply to avoid deficiency; going beyond that serves little purpose at disproportionate risk.
Vitamin D and Exercise Performance
This is why you’re really here. You want to know what vitamin D supplementation might offer to your sports performance.
The interest in vitamin D supplementation for potential sports performance benefits stemmed from the discovery that muscle cells have vitamin D receptors (Simpson, 1985) and that vitamin D influences protein synthesis (Birge & Haddad, 1975).
Prevalence of vitamin D deficiency in athletes: One might think that vitamin D deficiency would be lower in athletes, who are likely to (A.) spend more time outdoors, depending on their sport, and (B.) look after their nutrition more carefully. However, this is not the case, and vitamin D deficiency in athletes is approximately as common as it is in the general population of us mortals (Wyatt et al., 2024).
Muscle strength and anaerobic power: Vitamin D, as already mentioned, affects muscular function and maintenance by influencing protein synthesis. In the quest to boost athletic performance, however, we want to know if supplementation of vitamin D can improve muscle strength and power. The short answer, it seems, is yes. A 2024 systematic review of 14 randomised controlled studies found that, in five of the seven studies that investigated anaerobic performance, athletes who took vitamin D supplementation showed greater improvements in measures such as jump height, sprint times and weightlifting gains than those who did not take the vitamin D supplement (Wyatt et al., 2024).
Endurance and aerobic performance: The same review by Wyatt et al. (2024) also identified some potential benefits for aerobic endurance with vitamin D supplementation, noting that in two of the three studies that investigated aerobic performance, athletes using vitamin D supplementation had significantly greater improvements in VO2 max (maximal oxygen uptake) scores. Vitamin D may aid endurance by improving cardiovascular function or oxygen utilisation in muscles.
It is worth noting, however, that not every study finds a significant effect. A few trials show no significant difference in performance tests.
Want to learn more about the aerobic and anaerobic energy systems? Read my blog post on them.
The consensus from this systematic review was that the largest gains from vitamin D supplementation in elite athletes might be in aerobic endurance, anaerobic power, and strength. The review states that more research is needed on the influence of vitamin D on bone health and injury risk in this population.
It is worth noting, also, that the populations in this systematic review were trained athletes, already at the upper limits of performance, so observing any significant gains in this population is particularly impressive!
In Conclusion
You should now be a bit more informed about vitamin D and its array of functions in our bodies, both for general health and sports performance. Carry this knowledge forward into your diet and lifestyle to reap the mentioned benefits.
