Vitamin D in Winter: What Actually Works
TL;DR: the 30-second version
If your shadow is longer than you are tall right now, the sun is not feeding you vitamin D and will not for months. Food helps, but it rarely carries the whole load. Supplements do the job quietly: 1,000 to 2,000 IU of vitamin D3 with a meal is the dose most of us default to through the dark months. And skip the bright lamps — they make mood light, not vitamin D.
The short answer
The short answer: winter at northern latitudes pulls the plug on skin-made vitamin D, food alone will not reliably replace it, and a modest D3 supplement is the workaround almost everyone ends up on. The one number worth chasing is a 25(OH)D blood test — everything else is a guess in a winter coat.
The shadow rule tells you everything
Sun-safety guidance has a two-second diagnostic: if your shadow is longer than you are, the sun is too low to make vitamin D. Stand outside at noon in New York or London in January and you lose by a mile. Above roughly 35° north, the UVB your skin needs simply does not arrive between November and February. It has nothing to do with how cold it is — a bright day on a snowy slope at altitude can still do the job, while a grey day at latitude 50 cannot.
What the supermarket can actually do
Food is real, but do the math before you expect it to save you. Farmed salmon: roughly 250 IU per 100 g. One egg yolk: about 40 IU. Fortified milk in the US and Canada: about 100 IU per cup. Canned sardines and UV-exposed mushrooms are the other usual suspects. Against a typical winter target of 600 IU a day, that means planning your whole plate around it twice a week, every week, until March. Most of us do not hold that pattern for three months straight — which is why the jars aisle exists.
The winter sun grind, for the south-leaning
Not everyone lives above the line. Around Madrid, Los Angeles or below, midday winter sun can still work. Two rules of thumb: UV index of 3 or higher, and the shadow rule flipped (shadow shorter than you). Ten to twenty minutes with skin out, done daily, beats one heroic hour a month. Two fine points: sunscreen with SPF blocks the UVB you are after, so if you are deliberately sun-hunting, leave the legs bare for the short window and cover up after. And windows are not a workaround — standard glass stops UVB, which is also why your car windshield keeps you warm and D-less.
Lamps, beds, bottles: the fast version
The cheerful light boxes sold for winter mood are not helping your vitamin D — they emit no UVB, and the label usually says so if you read it. Tanning beds do emit UVB, and we are not going to recommend a skin-cancer trade for a vitamin. So: the bottle. Vitamin D3 (cholecalciferol) is what most people buy, it costs a few dollars a month, and taking it with a meal that has some fat in it helps absorption. Common winter dosing is 1,000 to 2,000 IU a day; the tolerable upper intake for healthy adults sits around 4,000 IU. If you are thinking about more than that, or you have kidney or bone conditions, that is a clinician conversation — an article is not your pharmacist.
The only test that ends the guessing
Ask your doctor for a 25-hydroxyvitamin D test — the 25(OH)D one. It is a standard blood panel that tells you what shadow-gazing cannot: your actual level. Your lab’s reference range decides what “low” means where you live, so do not cross-check against a number some blog threw out. One pattern we like: test once in early winter, once in early spring. We are usually surprised the spring number is the lower one.
Evidence table
What we say, how confident we are, and where it comes from:
| Claim | Tier | Source type |
|---|---|---|
| Sun below the shadow-rule angle makes little to no vitamin D | Tier B | Consensus physiology + national sun-safety guidance |
| Vitamin D content of common foods in the ranges above | Tier A | Government food-composition databases (USDA, EFSA) |
| 1,000–2,000 IU/day covers most winter needs; adult upper intake 4,000 IU/day | Tier B | National dietary-reference guidance |
| Garden-variety light-therapy lamps emit no UVB | Tier A | Device specifications, engineering fact |
| 25(OH)D blood test reflects true status | Tier B | Clinical practice guidance |
We are writers, not your doctor, and this is general information rather than medical advice. If you are pregnant, on medication, or managing a condition, run your plan past a clinician. Full terms: Health Disclaimer.
Quick answers
Q: Can I get vitamin D through a window?
A: No. Standard glass blocks UVB, and in winter the sun is usually too low anyway.
Q: Will my winter mood lamp raise vitamin D?
A: No — it gives off no UVB. It can help your mood; it cannot make vitamin D.
Q: What dose is the usual starting point?
A: 1,000–2,000 IU of vitamin D3 daily with a meal, with 4,000 IU as the adult upper intake. Higher doses are a clinician’s call.
Editorial review: EasternReset Team · Evidence-graded per our Editorial Standards