What is Vitamin D?

What is Vitamin D?

Vitamin D is a fat-soluble vitamin that behaves like a hormone once your body activates it. You get it two ways: your skin makes vitamin D3 (cholecalciferol) when UVB rays hit 7-dehydrocholesterol in skin cells, and you absorb smaller amounts of D3 and D2 (ergocalciferol) from fatty fish, fortified milk, egg yolks, and supplements. Neither form does anything on its own.

The liver converts it to 25-hydroxyvitamin D, or 25(OH)D, the storage form doctors measure on a blood test, and the kidney converts a small fraction of that into 1,25-dihydroxyvitamin D (calcitriol), the active hormone that binds receptors throughout the body. For lifters and athletes, two of those receptor sites matter most: the gut, where calcitriol raises calcium absorption to build and protect bone, and skeletal muscle, where vitamin D receptors on muscle satellite cells support repair and regeneration after training.

Deficiency is common, not rare, in people training indoors, training through winter, living above roughly 35 degrees latitude, or carrying more melanin in their skin, all of which cut UVB synthesis. Surveys of professional basketball, hockey, and soccer rosters routinely find that half or more of players carry insufficient or deficient levels, even training year-round. Because bone stress and recovery are constant concerns for anyone lifting heavy or running high mileage, vitamin D status is one of the few blood markers worth checking on a normal physical.

How it works

Vitamin D works as a slow, two-organ conversion chain, not a direct-acting nutrient. Sunlight's UVB rays convert 7-dehydrocholesterol in your skin to previtamin D3, which isomerizes to D3 over the following hours. The liver then hydroxylates D3 (and dietary D2) into 25(OH)D, the inactive but stable form that reflects weeks of vitamin D status, which is why blood tests measure it instead of the active hormone.

When calcium intake drops or parathyroid hormone rises, the kidney's 1-alpha-hydroxylase enzyme converts a fraction of that 25(OH)D into 1,25-dihydroxyvitamin D, the active hormone. Active vitamin D binds the vitamin D receptor (VDR) in the small intestine and increases calcium-transport protein production, lifting fractional calcium absorption from roughly 10 to 15 percent in deficient people toward 30 to 40 percent once levels are adequate — the mechanism behind vitamin D's bone-protective effect and its link to stress-fracture risk in athletes running high weekly mileage.

VDR is also expressed on skeletal muscle satellite cells, the stem cells that repair muscle fibers after training damage; deficient muscle biopsies show atrophy concentrated in type II (fast-twitch) fibers, which improves after repletion. That satellite-cell role, more than any direct effect on force production in already-adequate lifters, is why research on vitamin D and measured strength is inconsistent: it helps most when you start deficient, and adds little once you're already sufficient.

The scale

Practical guideline for a confirmed deficiency: 50,000 IU of vitamin D2 or D3 once weekly for 8 weeks (or roughly 6,000 IU/day), then maintain with 1,500-2,000 IU/day once levels exceed 30 ng/mL. Get a blood test before dosing; never guess your way to a high dose.

How to apply it

• Get 10-30 minutes of midday sun: Expose roughly 25-40% of your skin (arms, legs, or torso) to midday sun several times a week without sunscreen. Darker skin, higher latitude, and winter months all slow synthesis substantially, so this alone often isn't enough for indoor or northern-latitude athletes.
• Eat vitamin D-rich foods: Fatty fish like salmon or mackerel supply roughly 400-1,000 IU per 3.5-ounce serving; fortified milk and cereal add about 100 IU per cup; egg yolks and UV-exposed mushrooms contribute smaller amounts. Diet alone rarely closes a real deficiency.
• Choose D3 over D2 when supplementing: Cholecalciferol (D3) raises and sustains blood 25(OH)D more effectively than ergocalciferol (D2) at an equivalent dose. Most over-the-counter athlete supplements use D3; reach for D2 only if that's what's available or prescribed.
• Test before you dose: Get a 25(OH)D blood test before starting a high dose. Correcting a confirmed deficiency looks different from simply maintaining an already-adequate level, and guessing risks either under-treating it or pushing toward toxicity.
• Use a loading dose for confirmed deficiency: Under medical guidance, 50,000 IU of D2 or D3 once weekly for 8 weeks (or roughly 6,000 IU daily) is the standard protocol to correct levels below 20 ng/mL before switching to a maintenance dose.
• Maintain, then retest: Once you're above 30 ng/mL, 1,500-2,000 IU per day typically holds levels stable through the year, more in winter or if you carry higher body fat, which sequesters this fat-soluble vitamin. Retest at 3 to 6 months.

Worked example

Here is a realistic correction timeline for a training athlete flagged as deficient on a routine blood panel, using the loading-dose protocol sports-medicine clinicians commonly use rather than guessing at a supplement dose.

Sixteen weeks from deficient to a stable, athlete-appropriate level is typical. Body weight, body-fat percentage, latitude, and season all shift the exact numbers, which is why retesting, not guessing, drives the maintenance dose.

Vitamin D2 vs vitamin D3

Both forms raise blood 25(OH)D, but D3 does it more efficiently and for longer, which is why it's the default choice for correcting a deficiency or maintaining levels through an indoor training season.

By goal

• Indoor and winter-sport athletes (basketball, hockey, gymnastics, indoor swimming): Test twice a year. Indoor training combined with low winter sun exposure pushes deficiency rates above 50% in these sports. Supplement proactively through fall and winter rather than waiting for symptoms or a stress fracture to show up.
• Endurance and outdoor-training athletes: Regular outdoor training helps, but doesn't guarantee adequate levels once sunscreen, clothing coverage, and short winter days are factored in. High weekly mileage already raises stress-fracture risk, so a deficient runner has more to lose from reduced bone-building capacity.
• Strength and physique athletes, especially while cutting: Vitamin D is fat-soluble and stored in adipose tissue, so a higher body-fat percentage can sequester more of it away from circulation. Female athletes with low energy availability face compounded bone-health risk and should treat testing as routine, not optional.

Common misconceptions

• "Taking more vitamin D always builds more muscle and strength." A 2024 meta-analysis of randomized trials found no significant pooled strength benefit across handgrip, bench press, and vertical jump, with only quadriceps strength improving significantly. Supplementation helps most when you start deficient; topping up an already-sufficient athlete adds little measurable strength.
• "Regular sun exposure guarantees you'll never be deficient." Sunscreen, window glass (which blocks UVB entirely), higher latitudes in winter, and more melanin in the skin all cut synthesis substantially. Indoor athletes and anyone training through northern winters commonly test deficient despite feeling like they get outside enough.
• "Since it's natural, you can't take too much." Vitamin D toxicity is real: sustained intakes above roughly 50,000 IU/day, or blood levels above 150 ng/mL, cause hypercalcemia — nausea, weakness, kidney stones, and in severe cases cardiac arrhythmia. The tolerable upper limit is 4,000 IU/day for most adults without medical supervision.
• "Low vitamin D is only a bone-health issue." Deficiency is tied to type II muscle-fiber atrophy, impaired satellite-cell repair, and immune dysfunction, alongside the well-documented doubling of stress-fracture risk below 20 ng/mL. Bone is the best-studied outcome of deficiency, not the only one.

Related terms

Vitamin D FAQ

What is vitamin D good for?

Vitamin D raises calcium absorption in the gut to build and maintain bone density, and it supports skeletal-muscle repair through receptors on muscle satellite cells. It also plays a documented role in immune function. Deficiency raises the risk of stress fractures, muscle weakness, and slower recovery from training.

How much vitamin D should an athlete take daily?

Most adults need 600-800 IU/day to prevent deficiency, but athletes training indoors or through winter often need 1,000-2,000 IU/day to reach the 30-50 ng/mL range linked to better bone and muscle outcomes. Get a blood test before choosing a dose.

What are the signs of vitamin D deficiency?

Common signs include persistent fatigue, dull muscle or bone aches, slower recovery from training, and, in athletes, unexplained stress fractures or stalled strength gains. Many deficient people have no obvious symptoms at all, which is why a blood test, not symptoms, confirms it.

What blood level of vitamin D is considered low?

A 25(OH)D level below 20 ng/mL (50 nmol/L) is deficient, 20-29 ng/mL is insufficient, and 30 ng/mL and above is sufficient. Sports-medicine research suggests athletes recover and perform best in the higher 30-50 ng/mL range, not just above the minimum cutoff.

Can too much vitamin D be harmful?

Yes. Sustained intakes above roughly 50,000 IU/day, or blood levels above 150 ng/mL, cause hypercalcemia, with symptoms like nausea, weakness, confusion, and kidney stones. The tolerable upper limit is 4,000 IU/day for most adults unless a doctor supervises a higher, short-term dose.

Does vitamin D help build muscle or strength?

The evidence is mixed. A 2024 meta-analysis found no significant pooled effect on handgrip, bench press, or vertical jump, though quadriceps strength improved significantly. Correcting a real deficiency helps recovery and fiber health; supplementing an already-sufficient athlete for extra strength isn't well supported.

What foods are highest in vitamin D?

Fatty fish like salmon, mackerel, and sardines supply roughly 400-1,000 IU per 3.5-ounce serving. Fortified milk and cereal add about 100 IU per cup, egg yolks a small amount, and UV-treated mushrooms a moderate amount. Few whole foods provide enough to prevent deficiency on their own.

How long does it take to correct a vitamin D deficiency?

Using a standard loading dose of 50,000 IU weekly for 8 weeks, most deficient adults cross into the sufficient range (30+ ng/mL) by the 8-week retest, then hold there on a 1,500-2,000 IU/day maintenance dose. Severe deficiency can take longer to fully correct.

Is vitamin D3 better than vitamin D2?

Yes, for most purposes. Cholecalciferol (D3), made in skin and found in animal foods, raises and sustains blood 25(OH)D more effectively than ergocalciferol (D2), which comes from plants and fortified foods. Most athlete-focused supplements use D3 as the default form.

Which athletes are most at risk of vitamin D deficiency?

Indoor and winter-sport athletes (basketball, hockey, gymnastics, indoor swimming), athletes training at higher latitudes, those with darker skin, and anyone who trains mostly before sunrise or after sunset face the highest risk. Studies of professional basketball and hockey rosters often find over half insufficient or deficient.

References

1. Dietary Reference Intakes for Calcium and Vitamin D. Institute of Medicine (US), NCBI Bookshelf, 2011
2. Vitamin D Deficiency. StatPearls, NCBI Bookshelf
3. Vitamin D Toxicity. StatPearls, NCBI Bookshelf
4. Close GL, et al. Assessment of vitamin D concentration in non-supplemented professional athletes and healthy adults during the winter months in the UK: implications for skeletal muscle function. J Sports Sci, 2013. PubMed 23083379
5. Owens DJ, Allison R, Close GL. Vitamin D and the Athlete: Current Perspectives and New Challenges. Sports Med, 2018. PMC5790847
6. Effects of vitamin D3 supplementation on strength of lower and upper extremities in athletes: an updated systematic review and meta-analysis of randomized controlled trials. Front Nutr, 2024. PMC11163122
7. Knechtle B, et al. Vitamin D and Stress Fractures in Sport: Preventive and Therapeutic Measures — A Narrative Review. Medicina (Kaunas), 2021. PMC7999420