Vitamin D and sunlight

Carlos Soler

Graduado en Nutrición Humana y Dietética, Universidad de Valencia. Máster en Fisiología del Deporte, Universidad de Barcelona. Certificación ISAK nivel 1, The International Society for the Advancement of Kinanthropometry.

Vitamin D: more than we once thought

Vitamin D was once discussed mainly for its role in bone health. Advances in research have improved our understanding of its endocrine system and wider biological importance.

Vitamin D receptors and molecular targets have been identified in bone, immune cells, the cardiovascular system and skeletal muscle, showing that its functions extend beyond the skeleton.

Several of these functions matter for sporting performance, including normal muscle function, bone health and immune function.

Vitamin D should be viewed as a biological requirement rather than an automatic performance enhancer. Individual circumstances should be assessed before supplementation.

What is vitamin D deficiency?

Low vitamin D status occurs in the general population and among athletes. Spending much of the day indoors can contribute, as can season, latitude, skin pigmentation and other factors.

Panel C of the historical image below uses 100–150 nmol/L as a proposed athlete range. This is not a universal target: NIH guidance considers 50 nmol/L or more adequate for most people, and levels above 125 nmol/L may have adverse effects.

The image’s proposed 40–60 ng/mL range should therefore not be treated as a routine supplementation goal.

For context, NIH categorises less than 12 ng/mL as a risk of deficiency, 12–20 as potentially inadequate, and 20 or more as adequate for most people. Higher is not always better. The old thresholds of 5 and 72 in the source are not universal diagnostic or toxicity cut-offs.

Blood concentrations must not be converted into international units (IU), which describe an amount of vitamin D, not a serum concentration.

The source listed 500, 500–1200, 1200–2000, above 2000 and above 7200 as if they were equivalent blood-status ranges in IU. That conversion is invalid and should not guide dosing.

Why does sunlight matter?

The skin can synthesise vitamin D following exposure to ultraviolet B radiation (UVB). UVB acts on 7-dehydrocholesterol in the skin to produce previtamin D3, which becomes cholecalciferol, or vitamin D3.

Food and supplements can supply vitamin D3 or vitamin D2, depending on the source.

Vitamin D undergoes two hydroxylation steps to form its active hormone: mainly in the liver and then the kidneys, with some local activation in other tissues.

Active 1,25(OH)2D3 binds to its receptor and influences gene expression and proteins involved in several regulatory processes.

Dietary vitamin D absorption varies with the food, supplement and individual; the original article’s figure of around 50% is not a universal rule. Sun exposure can contribute, but it is not a risk-free or precisely controllable vitamin D dose. The amount produced depends on skin type, latitude, season and time of day. Avoid sunburn and follow sun-protection advice.

Foods that provide vitamin D include:

• Oily fish such as salmon, tuna and mackerel
• Beef liver
• Cheese
• Egg yolk
• Milk or plant-based drinks fortified with vitamin D

Low vitamin D levels have been associated with the following conditions in research. Association does not establish that deficiency causes each condition or that supplements prevent it:

• Cancer risk
• Cardiovascular risk
• Insulin resistance
• Neurodegeneration
• Depression
• Inflammation
• Muscle weakness

Muscle health is also relevant to athletes. Deficiency can be associated with weakness and altered function, but proposed pathways involving oxidative stress and mitochondria should not be presented as inevitable effects in every athlete.

What does vitamin D do for athletes?

Muscle repair and remodelling

Adequate vitamin D supports normal muscle function. Research has explored its relationship with strength recovery and satellite cells after exercise, without establishing a universal recovery benefit from supplementation.

Muscle function

Vitamin D regulates calcium metabolism, which is important for muscle contraction. Correcting deficiency can be relevant to muscle health, particularly in older adults, but this does not mean extra vitamin D improves strength in athletes who already have adequate levels.

Innate and adaptive immunity

Several immune cells express vitamin D receptors and enzymes involved in local vitamin D activation. Antibodies themselves do not contain the hydroxylase enzyme, as the original wording suggested.

Heart structure and function

Cardiovascular tissues contain vitamin D receptors. Research has examined links with cardiac and vascular function, but observational associations do not establish that supplements prevent cardiovascular disease.

Bone health and fracture risk

Vitamin D supports calcium absorption and bone mineralisation. Deficiency, together with other factors, can contribute to poor bone health and fracture risk.

It also participates in regulating the cells responsible for bone formation and resorption during remodelling.

How do you know whether you need a supplement?

Serum 25(OH)D is the main marker used to assess vitamin D status because of its relatively long half-life and relationship with supply from skin synthesis and diet. Testing and interpretation should be guided by clinical need.

Advice from the lockdown period

• Ask a qualified nutrition professional to help plan an appropriate diet.
• Spend time outdoors when possible while following sun-protection advice. Deliberate unprotected UV exposure is not a standard treatment for deficiency.
• Window glass blocks most UVB needed for vitamin D synthesis while allowing some UVA through. Sitting behind glass is not a reliable way to make vitamin D and does not remove the need for sun protection.
• Include dietary sources of vitamin D, including fortified foods when suitable. Their contribution remains useful even though absorption varies.
• Ask your healthcare professional whether testing or supplementation is appropriate for your circumstances, rather than arranging routine testing without an indication.

Conclusions

• Vitamin D deficiency can affect health and potentially performance. Assessment should be individualised.
• Avoid megadoses. Where deficiency is suspected or treatment is planned, clinical assessment and appropriate blood testing can guide the decision and dose.
• There is no established consistent performance benefit from extra vitamin D in athletes who already have adequate status.
• For help planning your diet, explore our online sports nutritionist service.

Owens DJ, Allison R, Close GL. Vitamin D and the Athlete: Current Perspectives and New Challenges. Sports Med. 2018 Mar;48(Suppl 1):3–16. doi: 10.1007/s40279-017-0841-9.