Vitamin D: dosage, the right form and interactions

In Estonia’s latitude, vitamin D is more of a basic necessity than a trendy supplement. Our country lies at 59° north — on the same line as Alaska and southern Greenland — where for half a year the radiation needed for vitamin D synthesis doesn’t reach the surface of the skin. In this guide we look at what the right dose is, which form to choose and which supplements vitamin D works best with.

Why is vitamin D so important in Estonia?

From October to March, the angle of the sun’s rays in Estonia is so low that type B ultraviolet radiation (UVB) practically doesn’t reach the ground. It is UVB that triggers vitamin D synthesis in the skin, so for half the year your skin physically can’t produce it — no matter how much time you spend outdoors. Even in summer, production is limited: office work, clothing and SPF 30 sunscreen (which blocks up to 97% of UVB) noticeably reduce synthesis.

It’s hard to get the necessary amount from food. Fatty fish is the best source, but even 200 g of salmon provides only about 600–1000 IU — to reach 4000 IU you would have to eat nearly 800 g of salmon a day. That’s why deficiency is more the norm at our latitude. You’ll find a more detailed overview in our article vitamin D, and the topic is explored further in vitamin D. EFSA officially recognizes the claims that vitamin D contributes to the normal function of the immune system and to the maintenance of healthy bones and muscles.

What is the right vitamin D dose?

An adult’s preventive dose depends on the season. In summer, when you’re regularly out in the sun, 1000–2000 IU per day is enough (or you can take a break). From October to March, 2000–4000 IU per day is justified for Estonian residents. For confirmed deficiency, 4000 IU per day is used, but preferably under medical supervision and with a repeat test after 2–3 months. The EFSA safe upper limit is 4000 IU per day — don’t exceed it without a doctor’s prescription, because for most vitamins the principle “more isn’t better” applies.

The most accurate feedback is a blood test. The target value for 25(OH)D in the blood is 40–60 ng/ml (100–150 nmol/l); a level below 20 ng/ml means deficiency and 20–30 ng/ml is insufficient. It makes sense to check your level roughly every half year.

Vitamin D is especially important for some groups: people with dark skin (whose skin produces the vitamin more slowly), office workers who spend the day indoors, and people over 50, whose skin’s synthesis capacity declines. These are the people who should monitor their dose and test results particularly closely, and, if a deficiency is confirmed, repeat the test in a couple of months.

D3 vs D2 — which form should you choose?

Vitamin D is sold in two forms. D3 (cholecalciferol) is of animal origin (lanolin, fish oil) and raises blood 25(OH)D levels more effectively and more durably — according to Tripkovic et al. (2012), nearly 87% more effectively than D2. D2 (ergocalciferol) is plant-based (mushrooms, yeasts), but less effective and less stable in storage. The practical conclusion: prefer D3, leaving D2 only for cases where strict veganism rules out the animal-derived form.

Taking it is simple, but it matters: vitamin D is fat-soluble, so take it with fatty food (avocado, nuts, olive oil, fish) — this increases absorption by 50% or more. Prefer the morning or daytime, because there is evidence that taking it in the evening may affect sleep.

Which supplements does vitamin D work well with?

Vitamin D doesn’t work alone. Magnesium takes part in converting it into its active form — without adequate magnesium levels, vitamin D remains largely inactive. Vitamin K2, in turn, directs calcium into the bones rather than the blood vessels, and dietary fats improve absorption. Omega-3 also gives good synergy, as its fats support the uptake of fat-soluble vitamins. If you want to get your whole vitamin picture in order, also take a look at our B vitamin overview, because B6 in turn supports the transport of magnesium into cells.

Reliable further reading is offered by the NIH Office of Dietary Supplements vitamin D fact sheet. Do keep in mind, though, that this article is informational — if you take medication, are pregnant or are breastfeeding, consult a doctor before you start.

References

• Health Board. Vitamin D recommendations for Estonian residents.
• EFSA (2012). Scientific Opinion on the Tolerable Upper Intake Level of Vitamin D.
• Tripkovic et al. (2012). D2 vs D3 and the rise in 25(OH)D levels.
• BMJ meta-analysis (2017). Vitamin D and the risk of acute respiratory infections.
• NIH Office of Dietary Supplements. Vitamin D Fact Sheet.
• Source: e-book “Vitamins and supplements: what actually works” (Fitness.ee, 2026).

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