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Vitamin D: The Complete Guide

The "sunshine hormone" most indoor athletes are low in — with measurable effects on strength, bone health, immunity, and mood.

✍️ By Filip Mesec 🔄 Last updated 11 June 2026 ⏱ 7 min read ✅ Evidence-based
🔬 A hormone, not just a vitamin

What is Vitamin D?

Vitamin D is technically a misnomer — it's a hormone precursor. Your skin synthesises it from UVB sunlight, your liver converts it to 25(OH)D (the form measured in blood tests), and your kidneys activate it into calcitriol, a steroid hormone that regulates gene expression in muscle, bone, immune, and nervous tissue.

Deficiency is remarkably common: US NHANES survey data suggest roughly 40% of adults have 25(OH)D below 20 ng/mL, the Endocrine Society's deficiency threshold — and studies of indoor athletes routinely find even higher rates during winter.5

Deficiency symptoms: Fatigue, frequent illness, low mood (especially in winter), bone and joint aches, slow recovery, and stress fractures are common signs of low vitamin D status.

Who is most at risk?

  • Northern latitudes — above ~35–40° latitude, winter UVB is too weak for skin synthesis for several months of the year
  • Darker skin — melanin acts as natural sunscreen, requiring substantially more sun exposure for the same vitamin D production
  • Indoor lifestyles — office work plus gym training means many athletes get almost no midday sun; glass blocks UVB entirely
  • Winter months — blood levels typically hit their lowest point in late winter and early spring

How It Works

Vitamin D receptors (VDRs) are found in nearly every tissue in the body. For athletes, four mechanisms matter most:

1
Muscle function via VDR receptors. Skeletal muscle expresses vitamin D receptors, and activated vitamin D influences protein synthesis and the function of fast-twitch (type II) fibres. A 2015 meta-analysis in the Journal of Science and Medicine in Sport found small but significant strength improvements when deficient athletes supplemented — though benefits largely disappear once levels are already sufficient.
2
Bone health & calcium absorption. Vitamin D's most established role: it increases intestinal calcium absorption several-fold. Low status impairs bone mineralisation and is associated with higher stress-fracture risk in athletes and military recruits — a real concern for runners and anyone in high-impact sports.
3
Immune function. Immune cells both carry VDRs and activate vitamin D locally. A 2017 meta-analysis of 25 randomised trials in the BMJ found vitamin D supplementation reduced acute respiratory infection risk, with the largest benefit in those who started deficient.3 Fewer sick days means fewer missed training sessions.
4
Testosterone link — promising but mixed. A 2011 randomised trial in Hormone and Metabolic Research found ~3,300 IU/day for a year raised total testosterone in deficient, overweight men.4 However, later trials in men with normal vitamin D status found no effect. The honest summary: correcting a deficiency may help; megadosing on top of healthy levels won't.

Vitamin D and mood

Low vitamin D status is consistently associated with low mood and seasonal symptoms, and VDRs are present in brain regions involved in mood regulation. Supplementation trials are mixed — benefits appear most reliably in people who are deficient to begin with — but for a winter indoor athlete, restoring normal levels is a cheap, low-risk intervention.

Dosage, Testing & K2 Pairing

1,000–4,000 IU
Daily dose (D3)
Higher end if deficient
30–50
Target 25(OH)D (ng/mL)
Test to confirm
4,000 IU
Daily upper limit
Don't exceed without testing

D3 vs D2

Choose D3 (cholecalciferol) over D2 (ergocalciferol). A 2012 meta-analysis in the American Journal of Clinical Nutrition found D3 is more effective at raising and maintaining blood 25(OH)D.2 Vegans can use lichen-derived D3.

How to take it

  • Take with a meal containing fat — vitamin D is fat-soluble, and absorption improves meaningfully when taken with dietary fat
  • Daily beats weekly megadoses — consistent daily dosing tracks natural physiology better; very large infrequent boluses have underperformed in some trials
  • Typical maintenance: 1,000–2,000 IU/day; correcting deficiency: 2,000–4,000 IU/day, then retest

Why pair it with K2?

Vitamin D increases calcium absorption; vitamin K2 activates osteocalcin and matrix Gla protein — the proteins that direct that calcium into bone and away from arteries and soft tissue. The mechanism is sound and K2 is very safe, but head-to-head trial evidence that D3+K2 beats D3 alone is still limited. Treat K2 as a sensible, low-risk addition at higher vitamin D doses, not a proven requirement.

Testing: the only way to know

Ask for a 25(OH)D blood test (cheap, widely available). Common interpretation: below 20 ng/mL = deficient, 20–30 ng/mL = insufficient, 30–50 ng/mL = a sensible target range for athletes. Test in late winter when levels are lowest, supplement accordingly, and retest after ~3 months.

Practical advice: Take 1,000–2,000 IU of D3 daily with your largest meal year-round (2,000–4,000 IU in winter or if a test shows you're low). Add K2 (MK-7) if you're at the higher end of dosing.
Toxicity warning: Vitamin D is fat-soluble and accumulates. Sustained intakes above ~10,000 IU/day or blood levels above 150 ng/mL risk hypercalcaemia — nausea, kidney stones, and worse. Stay at or below the 4,000 IU/day upper limit unless supervised by a doctor.1

Best Vitamin D Supplements

We prioritise: D3 form, sensible dosing options, K2 pairing where it makes sense, no unnecessary additives, and third-party testing.

#1
Thorne Vitamin D/K2 Liquid
D3 plus K2 in an adjustable liquid drop format — easy to fine-tune your dose to your blood test. Thorne's testing standards are among the best in the industry.
D3 + K2Adjustable dropsRigorous testing
Check PriceView on Amazon
#2
Sports Research Vitamin D3 + K2
5,000 IU D3 with 100mcg K2 (MK-7) in a coconut-oil softgel for absorption. Best suited to correcting a confirmed deficiency.
D3 + K2 (MK-7)Coconut oil baseThird-party tested
Check PriceView on Amazon
#3
NOW Foods Vitamin D-3
Simple, clean D3 softgels in 1,000–5,000 IU strengths at an unbeatable price. NOW's in-house lab testing is well regarded for a budget brand.
Best valueMultiple strengthsIn-house tested
Check PriceView on Amazon
🔗 Disclosure: Some links above are affiliate links — if you buy through them, FitCalc may earn a small commission at no extra cost to you. Our rankings are based on research and ingredient quality, never commission rates. Learn more.

Frequently Asked Questions

Can I get enough vitamin D from sunlight alone?

Only under the right conditions. Your skin makes vitamin D when UVB rays hit it, but UVB is too weak for synthesis during winter above roughly 35–40° latitude, and sunscreen, glass, clothing, darker skin, and indoor lifestyles all reduce production dramatically. If you train indoors and live outside the tropics, supplementation during at least the winter months is usually necessary to maintain optimal blood levels.

Should I take vitamin D3 or D2?

D3 (cholecalciferol). Multiple comparison trials and a 2012 meta-analysis in the American Journal of Clinical Nutrition found D3 is more effective than D2 (ergocalciferol) at raising and maintaining blood 25(OH)D levels. D2 is mostly relevant for strict vegans — though lichen-derived vegan D3 is now widely available.

Will vitamin D make me stronger?

Only if you're deficient. Correcting low vitamin D status can improve muscle function and strength — a 2015 meta-analysis in the Journal of Science and Medicine in Sport found small but significant strength improvements with supplementation in athletes. Taking extra vitamin D on top of already-sufficient blood levels hasn't been shown to add further strength benefits.

Do I really need to take K2 with vitamin D?

It's not mandatory, but the pairing is mechanistically sensible. Vitamin D increases calcium absorption, while K2 activates proteins (osteocalcin, matrix Gla protein) that direct calcium into bone and away from soft tissue. Direct trial evidence that the combination outperforms vitamin D alone is still limited, so treat K2 as a reasonable, low-risk addition rather than a proven requirement — especially at higher vitamin D doses.

Can you take too much vitamin D?

Yes. Vitamin D is fat-soluble and accumulates in the body. The tolerable upper intake level for adults is 4,000 IU per day; toxicity (hypercalcaemia) is generally only seen with sustained intakes above roughly 10,000 IU per day or blood 25(OH)D levels above 150 ng/mL. Stay at or below 4,000 IU daily unless a blood test and a healthcare professional indicate otherwise.

References

This guide is built from peer-reviewed research and authoritative health guidelines. Key sources:

  1. National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. Updated July 12, 2024. Full text
  2. Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. American Journal of Clinical Nutrition. 2012;95(6):1357–1364. PubMed
  3. Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. PubMed
  4. Pilz S, Frisch S, Koertke H, et al. Effect of vitamin D supplementation on testosterone levels in men. Hormone and Metabolic Research. 2011;43(3):223–225. PubMed
  5. Forrest KYZ, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutrition Research. 2011;31(1):48–54. PubMed

About the Author

FM
Written by Filip Mesec

Founder of FitCalc. Filip researches and writes FitCalc's training and nutrition guides, building each one from the peer-reviewed literature cited above and flagging clearly where the evidence is limited or contested. FitCalc's guides are educational and are not a substitute for personalised advice from your doctor or a registered dietitian.