Vitamin D and Endurance: How the Sunshine Hormone Affects Bones, Muscles, and Immunity

Vitamin D Is Actually a Hormone

After being synthesized in the skin via sunlight exposure or obtained through diet, vitamin D is activated in the liver and kidneys into its active form, acting on tissues throughout the body that have vitamin D receptors. It affects calcium and phosphorus metabolism and bone mineralization, skeletal muscle function (protein synthesis, strength, Type II fibers), as well as innate and adaptive immunity. Its role goes far beyond “helping calcium absorption for bones.”

The Prevalence of Deficiency in Athletes Is Underestimated

It is often assumed that athletes who spend time outdoors in the sun won’t be deficient, but the actual deficiency rate is not low. Reasons include: insufficient sunlight angle at high latitudes/in winter, UVB blocked by air pollution, indoor disciplines (gyms, pools, indoor trainers), sunscreen use and long-sleeved clothing, lower synthesis efficiency in darker skin tones, and early morning or nighttime training that avoids sunlight. In subtropical regions like Taiwan, deficiency is not uncommon due to widespread sunscreen use and indoor lifestyles.

Associations with Endurance Performance and Health

• Bones: Vitamin D insufficiency combined with low energy availability significantly increases the risk of stress fractures (consistent findings across runners, race walkers, and military training studies).
• Muscles: Deficiency is associated with decreased strength, Type II fiber atrophy, and poorer recovery; correcting deficiency in deficient individuals can improve muscle function (but additional supplementation provides no benefit for those already sufficient).
• Immunity: Deficiency is associated with an increased risk of upper respiratory tract infections, which is especially important during the vulnerable window of high training volume.

Supplementation Principles

• Test 25(OH)D first, then supplement—don’t supplement blindly. Deficient individuals should supplement under medical guidance (common maintenance doses are about 1,000–2,000 IU daily; deficient individuals need higher loading doses and follow-up testing), with the target generally set at 30–50 ng/mL.
• Over-supplementation can cause hypercalcemia—more is not better—exceeding the sufficient range provides no additional performance benefit.
• Pair with calcium, vitamin K2, and adequate overall energy intake to fully support bone health.

Sunlight and Diet

Moderate sun exposure (depending on skin tone, latitude, and season; exposing forearms and legs for several tens of minutes, while avoiding excessive sunburn risk) is a natural source; dietary sources are limited (oily fish, egg yolks, fortified foods). Athletes in winter and indoor disciplines especially need testing and supplementation.

Not a Panacea

Vitamin D supplementation is meaningful for “repleting those who are deficient”; for those already sufficient in the blood, adding more will not further enhance endurance or strength. Invest resources in testing and correcting deficiency rather than indiscriminately loading up.

Vitamin D is the nutrient endurance athletes most often “assume they aren’t deficient in.” If you train on indoor trainers, ride early in the morning, or are diligent about sunscreen, your stress fractures and recurrent colds may stem from exactly this—get your blood tested first, then decide how much to supplement. Don’t guess.

Related Reading

• Vitamin D for Athletes: The Relationship Between Sunlight, Supplements, and Athletic Performance
• Vitamin D and Endurance Performance: Why Taiwanese Cyclists May Be Deficient Even in Summer
• An In-Depth Analysis of Vitamin D, Athletic Performance, and Health: Practical Notes from a Sports Science Consultant
• A Complete Guide to Vitamin D Supplementation Strategies for Athletes: Prevalence of Deficiency, Dosing, and Seasonal Adjustments