Two people take the same 1,000 IU supplement daily and get wildly different blood results. The variable is not the dose. It is the latitude, the month, and how much skin was exposed to a sun that was actually high enough in the sky to produce vitamin D. Season is not a detail in vitamin D math; for most of the world it is the dominant term.
Vitamin D is the one nutrient where the body manufactures its own supply, and the factory shuts down seasonally. Understanding that shutdown window is the practical core of any supplement decision.
The Physics That Drives the Seasonality
UVB radiation converts 7-dehydrocholesterol in the skin into previtamin D3, which then isomerizes to vitamin D3. That reaction only proceeds when the sun is above a certain elevation. The threshold is a solar zenith angle of about 45 degrees, or equivalently a UV Index around 3.
Below that angle, the atmosphere filters out nearly all UVB, and no meaningful vitamin D is produced no matter how long you stay outside. You can get a sunburn from UVA in winter and still make zero vitamin D, because the wavelengths responsible for each are different.
| Latitude | Months with little or no cutaneous synthesis | Practical window |
|---|---|---|
| 0–20° (Singapore, Nairobi) | None | Year-round synthesis possible |
| 20–35° (Los Angeles, Cairo, Shanghai) | 1–2 months | Brief winter dip in December–January |
| 35–45° (New York, Rome, Beijing) | 3–4 months | November through February |
| 45–55° (London, Seattle, Berlin, Toronto) | 5–6 months | October through March |
| 55°+ (Oslo, Moscow, Edinburgh) | 6–8 months | September/October through April |
At 52° north, which covers London, Berlin, and most of Canada’s population, cutaneous vitamin D production effectively stops from roughly October to March. That is five to six months of the year during which diet and supplements are the only inputs.
What the Blood Numbers Show
Population studies track a consistent seasonal curve. Serum 25-hydroxyvitamin D peaks in late summer, typically August to September in the northern hemisphere, reflecting a lag of several weeks behind peak sun. It bottoms out in February to March, also lagged, which means the lowest levels arrive well after the darkest day.
A typical swing for a mid-latitude adult without supplementation is on the order of 15 to 25 nmol/L between the August peak and the February trough. For someone whose summer level sits at 75 nmol/L, that is comfortable. For someone whose summer level is 50 nmol/L, the winter trough can land near 30, which is the deficiency boundary used by many clinical guidelines.
Thresholds vary by body, but the commonly cited bands are: under 30 nmol/L as deficient, 30 to 50 as insufficient, 50 to 125 as sufficient for most healthy adults, and above 250 as potentially harmful.
Why Summer Does Not Always Bank Enough
Several factors reduce how much a summer produces, and they compound:
- Skin pigmentation. Higher melanin content requires substantially longer exposure to synthesize the same amount. A person with deeply pigmented skin at 50° north may not produce adequate vitamin D even in summer with typical clothing and indoor work.
- Age. Skin concentrations of 7-dehydrocholesterol decline with age, so an 70-year-old produces a fraction of what a 20-year-old does from identical exposure.
- Sunscreen and clothing. Correctly applied SPF 30 reduces cutaneous synthesis by more than 95 percent. This is not an argument against sunscreen, only a note that it moves vitamin D responsibility to diet and supplements.
- Indoor work. A midday lunch break of 15 minutes is often the entire sun exposure an office worker gets in a week.
- Body fat. Vitamin D is fat-soluble and sequestered in adipose tissue, so higher body fat is associated with lower circulating levels for the same intake.
How to Estimate Your Own Winter Risk
The practical calculation is a two-point one: know your late-summer level and know how many months your latitude spends below the synthesis threshold. If your August level is 80 nmol/L and you face five dark months, an unsupplemented February trough is likely to land somewhere near 45 to 55. If your August level is 55, the same five months will likely put you under 30.
If you have never had a test, the latitude table above is a reasonable proxy. Above 45 degrees latitude, routine winter supplementation is the mainstream clinical recommendation for most adults, typically 400 to 1,000 IU daily, with higher doses for older adults and those with darker skin.
One useful companion number is body composition, because vitamin D status correlates with it and because the same seasonal logic applies to weight and body fat measurements taken at different times of year. The body fat calculator using the Navy method gives a quick estimate that makes the sequestration effect concrete rather than abstract, and pairing it with a BMI reading shows how body composition and vitamin D status tend to move together.
What to Do With This
Four concrete steps, in order of value:
Test once, in late winter. A February or March measurement captures the annual low point, which is the number that actually matters for deficiency risk. Testing in August tells you almost nothing about your winter.
Match dose to latitude, not to a label. Someone in Singapore and someone in Oslo do not need the same plan. Above 50 degrees latitude, year-round supplementation is the reasonable default.
Take it with fat. Vitamin D3 absorption improves substantially when taken with a meal containing fat. A supplement taken on an empty stomach may deliver far less than the label states.
Do not megadose to compensate. Daily doses above 4,000 IU without monitoring risk hypercalcemia. The correction of a documented deficiency is a clinical decision with a defined protocol, not a self-directed escalation.
Frequently Asked Questions
Can I make vitamin D through a window? Standard glass blocks UVB almost entirely. Sitting by a sunny window produces no vitamin D.
How long in the sun is enough in summer? For light-skinned adults at midday in summer, 10 to 20 minutes on arms and face is typically sufficient. Longer adds little; the synthesis self-regulates.
Does a sunbed help? Not recommended. The UV exposure carries skin cancer risk that outweighs any vitamin D benefit.
Is vitamin D2 as good as D3? D3 raises and maintains serum levels more effectively at equivalent doses. Most guidelines favor D3.
The seasonal curve is one of the most predictable patterns in nutrition. If you live above 45 degrees latitude, treat October through March as a period when your body is not making vitamin D at all, and plan accordingly.
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