Vitamin D and the Seasons: Why the Time of Year Matters More Than You Think
Vitamin D synthesis in the skin depends on the intensity of UVB radiation. Radiation intensity depends on the sun's angle in the sky — which in turn depends on latitude and season. Estonia sits at 57–59° north latitude. This means that from October through March, UVB radiation is too weak to synthesise vitamin D, regardless of how much time you spend outdoors.
The Estonian Climate and the Winter Vitamin D Problem
A study among the Estonian population found that more than 60% of adults have vitamin D levels below optimal (< 75 nmol/L) during the winter months (Pilz et al., 2019). The reasons are logical:
- November–February: UVB radiation is practically zero
- March–April: radiation begins increasing but is not yet sufficient
- May–September: active sunlight enables synthesis (but only with exposed skin)
- October: radiation drops quickly below effective levels
Practical Seasonal Dosing Recommendations
Winter (October–March)
Recommended supplementation for all Estonian residents:
- General adult: 2000–4000 IU per day
- Over 60 years: 3000–5000 IU (immune and muscle function decline faster with D deficiency)
- Darker skin tones: 3000–5000 IU (melanin suppresses synthesis)
- Overweight individuals: higher doses needed (vitamin D is sequestered in adipose tissue)
Summer (May–September)
If spending sufficient time outdoors:
- Active outdoor lifestyle: reduce to 1000–2000 IU or take a break
- Primarily indoors: 2000 IU maintenance dose remains sensible
- Sunscreen SPF 30+: blocks ~95% of UVB — supplementation still recommended
Spring and Autumn (April, October)
Transition periods — 2000 IU per day is a universally safe maintenance dose.
How Much Vitamin D Is "Enough"?
Serum 25(OH)D reference values:
| Level (nmol/L) | Interpretation |
|---|---|
| < 30 | Severe deficiency |
| 30–50 | Deficiency |
| 50–75 | Insufficient |
| 75–125 | Optimal |
| 125–200 | Acceptable |
| > 250 | Potential toxicity |
Most experts target 80–120 nmol/L, which is approximately 32–48 ng/mL (Holick, 2011).
Best Supplements for the Estonian Winter
At maxfit.ee you'll find a strong range for seasonal needs. OstroVit Vitamin D3 4000 IU 120 Caps is ideal for a winter maintenance dose — 120 capsules last an entire winter season. For those preferring precision dosing, ICONFIT Softgel D3-Vitamin 4000 IU N90 offers a soft and fast-absorbing capsule form. For those needing a higher dose, MST D3 5000 IU 120 Softgels is an excellent option. Among combination products, OstroVit Vitamin D3 + K2 90 tabs is particularly popular, covering both D and K2 needs in one supplement.
When Should You Test?
Recommended timing for blood 25(OH)D checks:
- In September (before starting winter supplementation): shows how well summer restored your reserves
- In March (at the end of the winter period): shows whether the dose was adequate
If tests show levels below 75 nmol/L despite 4000 IU daily, consider increasing the dose (with medical advice) or adding K2 and magnesium to improve absorption.
FAQ
Does going to the sauna in summer replace sun exposure?
No. The sauna heats the body but there is no UVB radiation indoors. Vitamin D synthesis specifically requires the UVB range (290–315 nm), which penetrates outdoor air but not window glass or sauna walls.
Does using a solarium in winter help?
Some tanning beds emit UVB radiation, which theoretically allows vitamin D synthesis, but most devices primarily use UVA, not UVB. In addition, solarium use increases skin cancer risk. A supplement is a safer and more effective alternative.
Is vitamin D excess dangerous?
Yes, but only at very high doses. Toxicity (hypercalcaemia) typically only occurs with sustained intake above 10,000 IU per day. EFSA's safe upper limit for adults is 4000 IU per day. Risks are minimal within this range.
References
- Pilz, S., März, W., Cashman, K. D., et al. (2019). Rationale and plan for vitamin D food fortification. Frontiers in Endocrinology, 10, 373.
- Holick, M. F. (2011). Vitamin D: a D-Lightful solution for health. Journal of Investigative Medicine, 59(6), 872–880.
- Webb, A. R., Kift, R., Durkin, M. T., et al. (2010). The role of sunlight exposure in determining the vitamin D status of the U.K. white adult population. British Journal of Dermatology, 163(5), 1050–1055.
- Cashman, K. D., Dowling, K. G., Skrabáková, Z., et al. (2016). Vitamin D deficiency in Europe: pandemic? American Journal of Clinical Nutrition, 103(4), 1033–1044.
- Holick, M. F., Binkley, N. C., Bischoff-Ferrari, H. A., et al. (2011). Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism, 96(7), 1911–1930.
