Vitamin D is the one vitamin the body can make itself: in sunlight, skin turns a form of cholesterol into its precursor. In summer that’s usually enough. But in Ukraine — and anywhere at similar latitudes — there are months when even a clear midday sky doesn’t give skin what it needs.
It’s not the sun, it’s the ozone
Vitamin D synthesis is triggered only by ultraviolet B at 290–315 nm, most effectively 295–300 nm (MacLaughlin et al., 1982). The sun emits it all year, but on the way down the rays pass through the atmosphere, and ozone absorbs exactly this part of the spectrum. The lower the sun, the longer the path through the air — and the less UVB reaches the ground. Clouds, haze and thicker ozone cut it further (Engelsen et al., 2005).
Webb and colleagues (1988) tested this directly, exposing skin samples and a precursor solution to sunlight on clear days. In Boston (42° N) nothing formed from November through February, and in Edmonton (52° N) the “empty” period ran from October through March. Ukraine lies right between them, at 44–52° N — the same band as much of Central Europe, southern Canada and the northern US — so a “vitamin D winter” there lasts roughly four to six months. Modelling for the whole of Europe shows the same pattern: from none at 35° N to eight months at 69° N (O’Neill et al., 2016).
From November to February, a walk on a sunny day won’t add vitamin D, however long it is. At these latitudes the sun is simply too low for that.
What studies in Ukraine show
In Transcarpathia, vitamin D levels were measured in 1,823 people in 2019. In winter, 52% of adults were below 20 ng/mL — the cut-off most guidelines treat as deficiency — and 13% were below 12 ng/mL. Women had lower levels than men on average (Shchubelka, 2020). Among pregnant women in the Rivne and Khmelnytskyi regions, half were deficient (Carlson et al., 2017).
Why bones need it
Without vitamin D the gut absorbs calcium poorly. In a study by Heaney and colleagues (2003), calcium absorption was 65% higher in people with vitamin D around 86 nmol/L than in those around 50 nmol/L — even though both values are formally within the normal range. So calcium from cheese or milk works less well than it could without enough vitamin D.
But more isn’t better. In the large VITAL trial, nearly 26,000 people aged 50 and over took 2,000 IU of vitamin D a day or a placebo for more than five years — and fractures were equally common in both groups (LeBoff et al., 2022). Participants weren’t selected for deficiency: if you already have enough vitamin D, extra doses don’t strengthen bones.
Can you get it from food?
The daily target for adults is 15 µg (600 IU), or 20 µg (800 IU) after 70. Few foods are truly rich in vitamin D, and nearly all of them are oily fish.
| Food | Vitamin D per 100 g |
|---|---|
| Carp | 24.7 mcg |
| Mackerel | 16.1 mcg |
| Salmon | 11 mcg |
| Pink salmon | 10.9 mcg |
| Egg yolk | 5.4 mcg |
| Sardines in oil | 4.8 mcg |
| Herring | 4.2 mcg |
| Egg | 2 mcg |
100 g of mackerel or carp covers a day’s target, while you’d need about fifteen eggs a day to do the same. Unless oily fish is on your table several times a week, reaching the target from food in winter is close to impossible.
So do you need supplements?
Here guidelines differ, and that’s worth knowing. Polish doctors — neighbours with the same latitudes and the same sun — recommend preventive vitamin D3 for the whole population (Płudowski et al., 2023). A review of international guidelines, with Ukrainian researchers among its authors, lists usual preventive doses of 400–2,000 IU a day and notes that sun and food usually can’t keep levels in the desired range all year (Płudowski et al., 2018).
The US Endocrine Society is more cautious: it suggests supplements for children, pregnant people and adults over 75, and advises healthy adults under 75 not to exceed the usual daily target or test “just in case” (Demay et al., 2024). Still, it too acknowledges that natural sources of vitamin D are scarce, so the target often has to be met with supplements or fortified foods.
The takeaway: in the cold months skin makes almost no vitamin D, and food provides little. A preventive dose within the daily target — 15–20 µg (600–800 IU) a day — is a simple way to get it. Megadoses without a blood test aren’t needed, and if you’re pregnant or have osteoporosis or kidney disease, discuss the dose with a doctor.
To see how much vitamin D you get from food, log your meals in the diary: the calculator counts it along with other vitamins, and the weekly history shows if you’re consistently short.