Vitamin D: Why It Matters and How to Get Enough
Vitamin D is unusually easy to be low on and unusually easy to fix. The honest version of the story sits between hype and dismissal.
The takeaways
- Roughly 30–40% of adults in temperate latitudes are deficient or insufficient by serum 25(OH)D.
- Risk factors stack: dark skin, indoor work, latitude above ~37°, age, obesity, some medications.
- 1,000–2,000 IU daily is enough to maintain status in most adults; 4,000 IU/day is the tolerable upper level for adults.
- Test, don't guess, if you are unsure — a single 25(OH)D blood test is inexpensive and definitive.
Vitamin D is not a vitamin in the usual sense. It is a steroid hormone precursor synthesised in skin from UVB exposure, then converted in the liver and kidneys into its active form. The body uses it for calcium and phosphate handling — which is why classical deficiency causes rickets and osteomalacia — and for a long list of other regulatory roles that are still being mapped.
The state of evidence sits in an uncomfortable middle. Mainstream guidelines are clear that adequate vitamin D status supports bone health and prevents falls in older adults. The bigger claims — that supplementation prevents respiratory infections, slows cognitive decline, or reduces cancer mortality — have a more mixed and modest evidence base. The right way to think about vitamin D is to make sure you are not deficient, and stop short of treating it as a panacea.
Who is at risk of being low?
- People living above ~37° latitude in winter (most of Europe, the northern US, all of Canada).
- People with darker skin pigmentation, who synthesise less vitamin D per unit of UVB.
- Indoor workers and night-shift workers.
- Older adults — skin synthesis efficiency declines with age.
- People with obesity — vitamin D is fat-soluble and partitions into adipose tissue.
- People on chronic corticosteroids, anticonvulsants, or some antiretrovirals.
- Exclusively breastfed infants without supplementation.
How testing works
Status is measured as serum 25-hydroxyvitamin D, written 25(OH)D. The Endocrine Society defines deficiency as <20 ng/mL (50 nmol/L) and insufficiency as 20–29 ng/mL (50–75 nmol/L), with sufficiency at 30 ng/mL or above. The Institute of Medicine's threshold is more conservative (sufficiency at 20 ng/mL). Either way, a single inexpensive blood test gives you a clear answer; this is not a nutrient where you need to guess from symptoms.
How much should you take?
| Status | Typical dose | Notes |
|---|---|---|
| Deficient (<20 ng/mL) | Often 4,000–5,000 IU/day for 8–12 weeks, then maintenance | Re-test after the loading phase. |
| Insufficient (20–29 ng/mL) | 2,000 IU/day | Often enough to push into the sufficient range. |
| Sufficient maintenance | 1,000–2,000 IU/day | Year-round in temperate climates; lower in summer if outdoors. |
| Tolerable upper level | 4,000 IU/day | Per the Institute of Medicine for healthy adults. |
Vitamin D is fat-soluble and absorbs best with a fat-containing meal. Cholecalciferol (D3) is the form most commonly used and slightly outperforms ergocalciferol (D2) in raising blood levels. Drops, gummies, capsules — the form does not matter much; consistency does.
Sun exposure: the messy half of the answer
Sunlight is a real source. About 10–20 minutes of midday summer sun on arms and legs is enough for many fair-skinned adults to make a useful dose; darker skin takes longer. Above ~37° latitude, the sun is too low between roughly October and March for the skin to synthesise meaningful amounts at all. Sunscreen blocks UVB strongly, although in practice people apply it thinly enough that some synthesis still occurs. The realistic picture for most adults in temperate climates is: sun in summer, supplement in winter, test once a year.
What about food?
Food sources are real but limited. The richest natural sources are fatty fish (salmon, mackerel, sardines) and egg yolks. Many countries fortify milk, plant milks, and breakfast cereals; in places without fortification (most of continental Europe), dietary intake alone rarely covers requirements. Tracking apps that surface vitamin D intake — Cronometer, NutriShot AI, and other micronutrient-aware trackers — make the gap visible quickly.
Frequently asked
Should I take vitamin D every day?
If you live in a temperate climate, work indoors, or have any of the listed risk factors, daily supplementation in the 1,000–2,000 IU range is reasonable for most healthy adults. The most accurate path is to get a 25(OH)D blood test once and adjust from there.
Can I get enough vitamin D from food alone?
In countries with milk and cereal fortification, possibly — but only with a deliberate diet. Without fortification, food alone rarely meets requirements year-round. Fatty fish twice a week plus fortified products gets you closer but not always all the way.
Is vitamin D2 or D3 better?
D3 (cholecalciferol) raises blood 25(OH)D modestly more efficiently than D2 (ergocalciferol). Both work; D3 is the more common form in modern supplements and the simpler default.
References & further reading
- Holick MF (2007). Vitamin D deficiency. NEJM.
- Institute of Medicine (2011). Dietary Reference Intakes for Calcium and Vitamin D.
- Bouillon R. et al. (2022). Skeletal and extraskeletal actions of vitamin D. Endocrine Reviews.
Editorial note. Articles on The Pantry Notes are written for general informational purposes and are not medical advice. See our editorial principles for how we work.
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