Series: Micronutrients: The Small Things That Keep the Engine Alive - Episode 8: Vitamin D and the Athlete's Body

Vitamin D is one of the most unusual vitamins. The body can produce it in the skin under ultraviolet B radiation, then transform it in the liver and kidneys into its biologically active form. In this sense, vitamin D behaves in many ways more like a hormone than a simple dietary micronutrient.

For strength athletes, its relevance goes beyond bone. Vitamin D participates in calcium and phosphate regulation, influences muscle function, gene expression, immunity and several adaptation processes. Deficiency can affect health and, indirectly, training capacity and recovery.

This episode follows vitamin D from sunlight and food to cellular receptors. We will discuss testing, risk factors for deficiency, its relationship with bone and muscle, and the difference between correcting a deficiency and the mistaken idea that very high doses automatically improve performance.

1. Vitamin D is a hormonal precursor

The term vitamin D includes several compounds, the best known being vitamin D3, cholecalciferol, and vitamin D2, ergocalciferol. The active form, calcitriol, binds to the vitamin D receptor and influences the expression of many genes.

2. The skin can produce vitamin D

In the skin, UVB radiation converts 7-dehydrocholesterol into a precursor that is subsequently converted into vitamin D3. Production depends on season, latitude, time of day, skin pigmentation, age, clothing and actual sun exposure.

3. The liver and kidneys activate vitamin D

Vitamin D is first hydroxylated in the liver to 25-hydroxyvitamin D, the form usually measured in blood. It is then converted mainly in the kidneys to 1,25-dihydroxyvitamin D, or calcitriol, the hormonally active form.

4. 25(OH)D is the standard test for vitamin D status

To evaluate vitamin D stores, clinicians usually measure 25-hydroxyvitamin D. Circulating calcitriol is not a good marker of total stores because it can remain normal or even rise in some stages of deficiency due to compensatory hormonal regulation.

5. Vitamin D and calcium work together

Calcitriol improves intestinal absorption of calcium and phosphate. When vitamin D status is inadequate, the body may absorb less calcium from the same diet, increasing reliance on hormonal mechanisms that defend blood calcium concentration.

6. Bone depends on a system, not a single nutrient

Bone health depends on vitamin D, calcium, phosphate, protein, hormones, energy intake and mechanical loading. Vitamin D does not build bone by itself, but severe deficiency can impair mineralization and increase skeletal risk.

7. For strongman, bone is performance tissue

Yoke, farmers walks, deadlifts, squats and log presses generate very large mechanical forces. A skeleton that is progressively loaded, adequately fueled and nutritionally supported is essential. Vitamin D deficiency is not the only cause of poor bone health, but it is one correctable piece when present.

8. Vitamin D receptors are also present in muscle

Skeletal muscle expresses vitamin D receptors, and signaling through these receptors is involved in muscle function and homeostasis. Severe deficiency is associated with muscle weakness, particularly proximal weakness, although the relationship between normal levels and elite performance is much more complex.

9. Correcting deficiency may help, but megadosing does not create strength

If an athlete is deficient, restoring vitamin D status can remove a physiological limitation. In an athlete who already has adequate levels, however, high-dose supplementation has not been shown to turn vitamin D into a universal ergogenic aid. More is not automatically better.

10. Vitamin D also influences the immune system

The vitamin D receptor is present in many immune cells. Vitamin D modulates both innate and adaptive immune responses. For athletes, the relevance is indirect but important because repeated illness can interrupt training and recovery.

11. Indoor athletes may face a higher risk of deficiency

Athletes who train mostly indoors, especially in winter or at latitudes where seasonal UVB exposure is limited, may receive insufficient sunlight. Work schedules, clothing and time spent indoors can increase that risk.

12. Large body mass can influence vitamin D status

People with a larger amount of adipose tissue often have lower circulating 25(OH)D concentrations. Proposed mechanisms include distribution of vitamin D into larger body compartments and lifestyle differences, but interpretation should remain individualized.

13. Dietary sources are relatively limited

Fatty fish, fish liver oils, egg yolks and fortified foods can provide vitamin D. Compared with many other vitamins, however, ordinary diets may provide modest amounts unless fortified foods or specific rich sources are consumed regularly.

14. Deficiency cannot be diagnosed from vague symptoms

Fatigue, muscle aches and poor recovery are nonspecific symptoms with many possible causes. Vitamin D status should be assessed using context and 25(OH)D testing, not by assuming every poor training phase reflects deficiency.

15. Target ranges must be interpreted clinically

Guidelines differ somewhat in the exact thresholds used for insufficiency and deficiency. Results therefore need to be interpreted according to units, laboratory methods, clinical context, dietary intake, sun exposure and individual risk factors.

16. Supplementation makes sense when there is an indication

Vitamin D supplements can be very useful for preventing or correcting deficiency, especially during periods of low sun exposure. The appropriate dose depends on baseline level, diet, body mass, season and medical context.

17. Vitamin D is fat-soluble, so it can accumulate

Because vitamin D is fat-soluble, the body does not simply eliminate unlimited amounts regardless of dose. Chronic intake of very high amounts can cause toxicity, primarily through hypercalcemia and its downstream effects.

18. Toxicity can affect the kidneys and heart

Vitamin D-induced hypercalcemia can cause nausea, weakness, excessive thirst, frequent urination, kidney stones and, in severe cases, kidney injury or rhythm disturbances. Unmonitored megadoses are not a performance strategy.

19. Vitamin D and testosterone: association is not a guarantee

Some observational studies have reported associations between vitamin D status and testosterone, but this does not prove that supplementation automatically raises testosterone in every man. Effects depend on baseline status and many metabolic and hormonal factors.

20. Athletic performance is a much larger equation

Maximal strength depends on muscle mass, architecture, nervous-system function, technique, recovery, energy availability and training design. Vitamin D supports normal physiology but cannot compensate for poor programming or inadequate nutrition.

21. Conclusion: vitamin D is part of the foundation, not a turbocharger

For athletes, vitamin D matters through bone, muscle, mineral metabolism, immunity and cellular regulation. Deficiency should be identified and corrected. Turning vitamin D into a supposed strength drug and taking megadoses without a reason ignores the underlying physiology.

Selected bibliography

National Institutes of Health, Office of Dietary Supplements. Vitamin D Fact Sheet for Health Professionals. Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Holick MF. Vitamin D deficiency. New England Journal of Medicine. Owens DJ et al. Vitamin D and the athlete: current perspectives and new challenges. Close GL et al. Assessment of vitamin D concentration in non-supplemented professional athletes and healthy adults during the winter months.