Vitamin D, Explained: Sunlight, Food, and Supplements
Vitamin D occupies a strange place in nutrition: it's technically a hormone your skin manufactures from sunlight, it's genuinely deficient in large parts of the population, and it's credited with everything from bulletproof immunity to boundless energy. The first two are true. The third is where things get murky — vitamin D has one of the widest gaps between proven benefits and claimed benefits in all of nutrition.
This guide sticks to what's solid: what vitamin D actually does, who's likely to run low and why, the sensible ways to fix it through sun, food, and supplements, and where the evidence genuinely ends. Educational content, not medical advice — dosing decisions, especially higher ones, belong in a conversation with your clinician.
Vitamin D's firmly established job is calcium absorption and bone health — preventing rickets in children and protecting against bone loss and fractures in adults, especially older adults. Deficiency is common in northern latitudes, in winter, and in people with darker skin, limited sun exposure, or older age. Sensible sun exposure without burning, vitamin-D-rich foods (fatty fish, fortified dairy), and modest supplements (the adult RDA is 600–800 IU daily) cover most people. Claims about immunity, mood, and chronic disease are mostly unproven — large trials haven't confirmed them. Don't megadose: the upper limit is 4,000 IU daily, and excess causes real harm.
What vitamin D actually does
Vitamin D's core, non-controversial role is managing calcium: it enables intestinal calcium absorption and maintains the blood calcium and phosphate levels that bones need to mineralize properly. In children, severe deficiency causes rickets — soft, deformed bones. In adults, it causes osteomalacia (soft bones) and contributes to osteoporosis and fracture risk. This is textbook physiology backed by a century of evidence, including the eradication of rickets through fortification. When anyone tells you vitamin D "does nothing," this is the part they're ignoring.
Beyond bone, vitamin D receptors appear in immune cells, muscle, and many other tissues, which launched a thousand hypotheses: that deficiency raises risks of respiratory infections, autoimmune disease, depression, cancer, and cardiovascular disease. Observational studies dutifully found associations with all of these. But association kept collapsing under testing — most notably in VITAL, a large randomized trial of 2,000 IU daily in 25,000+ adults, which found no reduction in cancer, cardiovascular events, or (in most analyses) meaningful infection benefit. Low vitamin D in observational studies looks increasingly like a marker of poor health — less outdoor activity, more adiposity, chronic illness — rather than a cause.
The fair summary: vitamin D is essential for bone, plausibly relevant to muscle function and fall risk in deficient older adults, and unproven for nearly everything else despite enormous research effort. That's not nothing — bone health alone justifies attention — but it should discipline the hype.
Who's likely to run low
Your skin makes vitamin D when UVB photons hit it — which means geography and season dominate. Above roughly 37° latitude (San Francisco, roughly), winter sunlight is too weak for meaningful synthesis; many people in northern regions spend half the year making essentially none. This isn't a lifestyle failure; it's physics. Darker skin, which is superb sun protection, needs substantially more exposure to make the same vitamin D — a genuine tradeoff with real deficiency consequences.
Modern life adds layers: indoor jobs, sunscreen (which works as intended, blocking the UVB that makes vitamin D), covering clothing, and age — older skin synthesizes vitamin D less efficiently. Obesity is associated with lower blood levels, partly because vitamin D gets sequestered in fat tissue. Breastfed infants, people with fat-malabsorption conditions, and those who've had bariatric surgery are additional higher-risk groups clinicians watch.
The result: insufficiency is genuinely common, with estimates varying by definition and population but consistently high in winter at northern latitudes. "Common" doesn't mean "everyone needs pills" — it means it's worth knowing your risk factors and, if concerned, getting the actual blood test (25-hydroxyvitamin D) rather than guessing from symptoms, which are nonspecific to the point of uselessness.
Sun, food, and supplements — in that order
Sensible sun exposure is the ancestral source and still the most potent: brief, regular, non-burning exposure of arms and legs, with the dose depending on skin tone, latitude, and season. Nobody can give you a universal minute count because the variables genuinely differ — a fair-skinned person in Arizona and a dark-skinned person in Seattle live in different UV universes. The principle is "a little, often, never burning," and burning is never worth it: skin cancer risk is the hard tradeoff here.
Food contributes modestly but reliably. Fatty fish — salmon, sardines, mackerel — are the best natural sources; fortified milk, plant milks, and cereals carry most of the dietary load in countries with fortification programs; egg yolks and UV-exposed mushrooms contribute smaller amounts. Diet alone rarely corrects deficiency, but it meaningfully supplements sun and pills.
Supplements fill the gap simply and cheaply. The adult RDA is 600 IU daily up to age 70 and 800 IU after — amounts that maintain bone health in most people. Vitamin D3 (cholecalciferol) raises blood levels slightly more efficiently than D2 (ergocalciferol), so D3 is the usual choice. Take it with a meal containing some fat, since it's fat-soluble. And respect the upper limit of 4,000 IU daily: vitamin D toxicity is real, causing high blood calcium with nausea, confusion, kidney problems, and calcium deposits. The people taking 10,000+ IU daily on internet advice are running an uncontrolled experiment on themselves.
Testing and working with your doctor
The 25-hydroxyvitamin D blood test is the standard measure, and it's worth getting if you have risk factors, bone concerns, or are considering sustained supplementation — it turns guessing into knowing. Definitions vary slightly, but deficiency is generally under 20 ng/mL (50 nmol/L), with 20–30 considered insufficiency by many guidelines. Note the units confusion: ng/mL and nmol/L differ by a factor of 2.5, a perennial source of internet panic over "low" numbers that are actually fine.
If you're deficient, doctors typically prescribe a repletion course — often higher doses for weeks — then a maintenance dose, with retesting. This supervised approach beats self-directed megadosing in both safety and effectiveness. Certain medications (some seizure drugs, glucocorticoids, certain weight-loss drugs) affect vitamin D metabolism, another reason supplementation decisions belong in a clinical conversation.
Special populations deserve tailored advice, not general articles: pregnant people, infants, older adults with osteoporosis, people with kidney or liver disease, and those with granulomatous conditions like sarcoidosis (where vitamin D metabolism goes awry). If you're in one of these groups, consider everything above as background for a doctor's visit, not a plan.
Frequently asked questions
How much sun do I need for vitamin D?
There's no universal number — it depends on skin tone, latitude, season, time of day, and how much skin is exposed. The workable principle is brief, regular, non-burning exposure; many dermatology-informed sources suggest something like 10–30 minutes of midday sun on arms and legs a few times weekly for lighter skin, more for darker skin, less in intense sun. Never burn for vitamin D — the skin cancer tradeoff isn't worth it, and supplements exist precisely for when sun isn't sensible.
Is D3 better than D2?
Slightly. D3 (cholecalciferol, usually from lanolin or lichen) raises and maintains blood levels a bit more effectively than D2 (ergocalciferol, from fungi) in most comparisons. For routine supplementation, D3 is the standard choice; D2 is the common prescription form in some countries and works fine too. Vegans should look for lichen-based D3.
Should I take vitamin D with food?
Yes — it's fat-soluble, so absorption improves with a meal containing some fat. This isn't a huge effect, but it's free. Morning vs. evening doesn't matter much; some people report vivid dreams with nighttime dosing, which is anecdotal but harmless to accommodate by switching to mornings.
Can you get too much vitamin D?
Yes, and it's unpleasant: toxicity causes hypercalcemia — high blood calcium — with nausea, vomiting, confusion, excessive thirst, and kidney damage in severe cases. It essentially never happens from sun (your skin self-regulates) or food; it's a supplement phenomenon, typically from months of very high doses. Stay at or below the 4,000 IU/day upper limit unless a doctor supervises otherwise.
Educational content only — not medical advice. Talk to a qualified healthcare professional about personal health decisions.