Written by Rakesh Behra, Pharmacist · Reviewed against NIH ODS, MedlinePlus and PubMed sources · Last reviewed: 31 August 2026
Vitamin D is often called the “sunshine vitamin,” and few nutrients attract as much attention — or as much marketing. This guide explains, in plain language, what vitamin D actually does in the body, who is genuinely at risk of deficiency, how it is tested, where you can get it, and whether the popular belief that “D3 is better than D2” holds up to evidence. Throughout, we separate what is well established from what is still uncertain.
What vitamin D is — and what it does
Vitamin D is a fat-soluble vitamin found naturally in only a few foods, added to some others, and made in your skin when ultraviolet-B (UVB) rays hit it. [1] Its best-established job is managing calcium and phosphate: it helps your gut absorb calcium and keeps blood levels steady so bones can mineralise properly. [1] Without enough, bones can become soft or misshapen — rickets in children and osteomalacia in adults. [2]
How your body absorbs, activates, and regulates it
The vitamin D you swallow or make in skin is biologically inactive. It goes through two activation steps:
- In the liver, it becomes 25-hydroxyvitamin D, or 25(OH)D (calcidiol) — the form measured in blood tests.
- In the kidney, it becomes the active hormone 1,25-dihydroxyvitamin D (calcitriol). [1]
That active form is tightly controlled by parathyroid hormone, calcium, and phosphate, which is why the body regulates vitamin D rather than letting it run freely. [1] Because vitamin D is fat-soluble, it is absorbed in the small intestine, and eating some fat with it improves absorption. [1]
Vitamin D deficiency: risk factors, symptoms, testing
How status is measured. Vitamin D status is judged by the blood level of 25(OH)D (it has a half-life of about 15 days, so it reflects recent weeks). [1] A widely used reference frame:
| 25(OH)D level | Interpretation |
|---|---|
| < 30 nmol/L (< 12 ng/mL) | At risk of deficiency |
| 30–50 nmol/L (12–20 ng/mL) | Possibly inadequate for some people |
| ≥ 50 nmol/L (≥ 20 ng/mL) | Sufficient for most people |
| > 125 nmol/L (> 50 ng/mL) | Possible adverse effects |
An important caveat on testing: lab assays vary, so the same sample can read differently between laboratories. [1] Partly for this reason, major bodies (the Endocrine Society; the US Preventive Services Task Force) do not recommend routine vitamin D testing for healthy people without risk factors. [4] Testing is best reserved for those with risk factors or relevant symptoms, guided by a clinician.
Who is most at risk of deficiency?
- Exclusively breastfed infants (breast milk alone is low in vitamin D)
- Older adults (skin makes less with age)
- People with little sun exposure (homebound, covered clothing, indoor jobs)
- People with darker skin (more melanin lowers skin synthesis)
- People with fat-malabsorption conditions (coeliac disease, Crohn’s, cystic fibrosis)
- People with obesity or after gastric-bypass surgery
Symptoms. Mild deficiency often has no obvious symptoms. When effects appear, they relate mainly to bone and muscle — bone pain, and muscle weakness or aches (myopathy). [1] Severe deficiency causes rickets or osteomalacia. [2]
Sources: sunlight, food, and fortified products
Sunlight. UVB rays convert a cholesterol-like compound in skin into vitamin D3. But synthesis depends on season, time of day, latitude, skin pigmentation, sunscreen, and age — and UVB does not pass through glass, so sitting by a sunny window doesn’t count. [1] Some researchers suggest short exposures may help, but there is no universal “safe dose” of sun, and health agencies stress that UV radiation is a carcinogen and recommend sun protection (SPF 15+). [1] The takeaway: sunlight contributes, but it is not a controllable or risk-free source.
Food (natural). Few foods are naturally rich in vitamin D. The best are fatty fish and fish liver oils, with smaller amounts in egg yolks, liver, and cheese; UV-exposed mushrooms supply vitamin D2. [1]
| Food (per serving) | Vitamin D |
|---|---|
| Cod liver oil, 1 tbsp | 1,360 IU |
| Rainbow trout, 3 oz | 645 IU |
| Salmon, 3 oz | 570 IU |
| UV-treated mushrooms, ½ cup | 366 IU |
| Fortified milk, 1 cup | ~120 IU |
| Egg, 1 large | ~44 IU |
Fortified foods. Because natural sources are limited, fortified milk, plant milks, cereals, and juices supply much of the vitamin D in many diets. [1] Check the nutrition label for the actual amount.
Supplements: forms, and the “best form” question
Supplements come as D2 (ergocalciferol) — made by UV-irradiating yeast — or D3 (cholecalciferol) — usually from lanolin (sheep’s wool), with a vegan D3 from lichen also available. [1]
Is D3 better than D2? For the common goal of raising and maintaining blood 25(OH)D, the evidence favours D3: both forms are absorbed well and both can cure rickets, but most evidence shows D3 raises 25(OH)D more and sustains it longer. [1] A systematic review and meta-analysis reached the same conclusion — D3 was more efficacious at raising serum 25(OH)D than D2. [3] So D3 is a reasonable default for most people, while D2 remains effective and is often used in high-dose prescription regimens. This is an area where the practical difference is modest and individual needs vary — a pharmacist or doctor can advise.
How much — and safety guidance
For general reference, the FDA Daily Value for vitamin D is 20 mcg (800 IU) for ages 4 and older; exact recommended intakes vary by age and country, so follow local guidance (for readers in India, ICMR guidance and your doctor’s advice apply). [1]
On the safety side, the Tolerable Upper Intake Level for ages 9+ is 100 mcg (4,000 IU) per day. [1] Overt toxicity signs are unlikely below 250 mcg (10,000 IU)/day, but authorities note that intakes even below the upper limit could carry risks over time, so more is not automatically better. [1] Importantly, toxicity comes from high-dose supplements, not from sun or food — excess sun does not cause vitamin D toxicity. [1] The main danger of overdose is hypercalcemia (too much calcium in the blood). Never take high-dose vitamin D long-term without medical supervision.
What vitamin D does not reliably do
Vitamin D has been studied for cancer, heart disease, depression, immunity, and more. While low blood levels are often associated with poorer health in observational studies, large randomized trials have generally not shown that supplements prevent these conditions in people who are not deficient. Expert reviews conclude the strong, consistent benefit is for bone health, while other areas remain uncertain. [1] In short: correct a genuine deficiency, but don’t expect vitamin D to be a cure-all.
Key takeaways
- Vitamin D is activated in two steps (liver → kidney) and mainly supports bone and calcium health.
- Status is measured by blood 25(OH)D; < 30 nmol/L (< 12 ng/mL) signals deficiency, ≥ 50 (≥ 20) is sufficient for most.
- Routine testing isn’t advised for healthy people — assays vary and evidence for population screening is insufficient.
- Highest-risk groups: infants, older adults, low sun exposure, darker skin, fat-malabsorption, obesity.
- Sources: sunlight (variable, not risk-free), fatty fish, UV-mushrooms, and fortified foods.
- D3 raises blood levels more than D2 and is a sensible default; both work.
- More isn’t better — upper limit 4,000 IU/day (ages 9+); high-dose only under medical advice; toxicity comes from supplements, not sun.
Frequently Asked Questions
Should I get my vitamin D tested?
Not routinely if you’re healthy — major bodies don’t recommend population screening, partly because lab results vary. Test if you have risk factors or symptoms, guided by a clinician.
Is D3 better than D2?
For raising and holding blood levels, most evidence favours D3, though both are effective.
Can I get enough vitamin D from the sun?
Sometimes, but it’s highly variable (season, skin tone, latitude, sunscreen) and UV is a skin-cancer risk, so it isn’t a reliable or risk-free source.
Can you take too much?
Yes — from supplements, not sun. The upper limit is 4,000 IU/day for ages 9+; high doses need medical supervision.
Which foods have the most vitamin D?
Fatty fish and cod liver oil are highest; UV-treated mushrooms, egg yolks, and fortified milk/cereals also contribute.
References
- NIH Office of Dietary Supplements — Vitamin D Fact Sheet for Health Professionals. ods.od.nih.gov (updated 2024)
- MedlinePlus (NLM) — Vitamin D. medlineplus.gov
- Tripkovic L, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D: a systematic review and meta-analysis. Am J Clin Nutr 2012. PubMed
- Endocrine Society — Vitamin D for the Prevention of Disease: Clinical Practice Guideline. J Clin Endocrinol Metab 2024. PubMed
Disclaimer
Medical disclaimer: This article is for general information only and is not medical advice. Do not start, stop, or change any supplement or medicine based on it. Consult your doctor or pharmacist about your vitamin D needs, testing, and dosing — especially during pregnancy, in infancy, or with kidney disease or other conditions.
Emergency: In an emergency, contact your local emergency services immediately.
Last reviewed: 31 August 2026