Vitamin D in autumn: when should you start supplementing?

The end of summer is when we start thinking about vitamin D again. The days get shorter, we spend more time indoors, and the skin's ability to make it gradually falls away. Which raises the question: should you start supplementing in September, wait until October, or simply take it all year round?

In Poland the answer depends on age, body weight, diet, sun exposure and any conditions that raise the risk of deficiency. Current Polish recommendations do not come down to a single dose that suits everyone.

Why does autumn bring vitamin D up?

Vitamin D comes from food and supplements, but the body also makes it in the skin under UVB radiation. In spring and summer that skin synthesis can account for a large share of what is available to you.

EFSA sets adequate intake at roughly 15 µg a day, or 600 IU, for everyone from the age of one, assuming minimal skin synthesis. Dietary needs can be lower when the body is producing vitamin D in the skin effectively.

Polish recommendations assume that from May to the end of September, adequate sun exposure can be enough for some healthy people to maintain a proper vitamin D supply. Those conditions are not always met. Time spent outdoors, how much skin is uncovered, time of day, age and individual characteristics all matter.

None of this means you should deliberately tan or skip sun protection to raise your vitamin D level. I wrote about using the sun sensibly in the piece on whether sunlight is healthy.

When should you start supplementing?

For most people living in Poland, early autumn is a good moment to check whether vitamin D is being supplemented regularly.

Polish guidelines say that healthy adults aged 19 to 65 whose skin synthesis is insufficient can take cholecalciferol at 1000–2000 IU a day. Supplementation can run all year, with the dose adjusted to body weight and dietary vitamin D intake, among other things.

So there is no single date, no 1 October, on which everybody should start. If you spend most of the day indoors, get little sun on your skin, or already supplement year-round, there is no reason to wait for late autumn.

How much vitamin D?

The preventive dose depends mainly on age and health status. Current Polish recommendations give the following ranges:

These are doses for preventing deficiency, not for treating a diagnosed one. Where a deficiency has been confirmed, the approach can look different and should be tailored individually.

Do people with overweight and obesity need more?

Body weight matters when setting supplementation. Current Polish recommendations pay particular attention to people with obesity, whose cholecalciferol requirement can be higher than that of people of similar age with a normal body weight.

The guidelines indicate that obesity may call for roughly twice the dose normally recommended for a given age group. In that situation the specific dose is worth setting individually, especially when larger amounts are taken over a long period.

What about older adults?

The skin's ability to produce vitamin D declines with age. Polish recommendations therefore advise year-round supplementation from the age of 65.

For people aged 65 to 75 the usual recommendation is 1000–2000 IU a day, and after 75 it is 2000–4000 IU a day.

Vitamin D in pregnancy and breastfeeding

Vitamin D status is worth attention as early as the stage of planning a pregnancy.

Polish recommendations indicate that during pregnancy and breastfeeding the dose can be set on the basis of 25(OH)D concentration. Where that measurement is not available, the recommendation is cholecalciferol at 2000 IU a day through pregnancy and lactation.

Do you need a test before you start?

Not every healthy person needs to open the autumn with a 25(OH)D measurement.

Both the Polish recommendations and the Endocrine Society guideline published in 2024 indicate that routine 25(OH)D testing in all healthy people is not necessary. The test matters more when risk factors for deficiency are present or there are clinical indications.

Groups that deserve closer attention include people with malabsorption, after bariatric surgery, with liver or kidney disease, with certain metabolic conditions, taking particular medicines, older adults, and people with very little exposure to sunlight.

In those situations a 25(OH)D measurement can help guide what comes next. On how often to repeat routine tests at all, I wrote in the piece on how often to repeat lab tests.

Can diet alone cover it?

Vitamin D occurs naturally in relatively few foods. The best sources are oily sea fish. Smaller amounts are found in egg yolks, some dairy products and fortified foods.

Diet alone does not always cover the requirement, particularly when skin synthesis is limited. That is one reason vitamin D supplementation appears so often in recommendations for European populations. EFSA set adequate intake for everyone from the age of one at 15 µg, or 600 IU a day, assuming minimal skin synthesis.

Vitamin D is the exception here rather than the rule: for most nutrients the plate remains the starting point. I covered that at greater length in the piece on whether supplements can replace a diet.

Does vitamin D protect against autumn infections?

Vitamin D takes part in the normal functioning of the immune system, but that does not automatically mean supplementing it shields you from colds or other respiratory infections.

A meta-analysis published in 2021 found a small reduction in the risk of acute respiratory infections among people supplementing vitamin D.

A newer meta-analysis published in 2025, covering 40 trials and more than 61,000 participants, no longer showed a statistically significant reduction in overall respiratory infection risk.

So vitamin D is not worth treating as an "immunity" product. Supplementing it is above all about ensuring an adequate supply and preventing deficiency. Most household remedies for the infection season come out much the same way, as I found with ginger for immunity.

More is not always better

Vitamin D is fat-soluble, so taking very large doses over a long period can lead to excessive accumulation and adverse effects.

Polish recommendations treat 4000 IU a day as the tolerable upper level of daily preventive intake for healthy adults with a normal body weight. That does not make it the dose recommended to everyone. Standard prevention in most healthy adults involves lower amounts.

The high doses used to treat a deficiency are a different matter from daily preventive supplementation. If you are choosing a product for the autumn, it also helps to know how to assess supplement quality.

In short

Autumn is a good moment to think about vitamin D, but supplementation should not rest on "from October we all take 2000 IU".

For most healthy adults in Poland who do not get enough sun on their skin, current Polish recommendations point to 1000–2000 IU of vitamin D a day. Children, older adults, pregnant women, people with obesity and patients with conditions that raise the risk of deficiency may need a different approach.

Vitamin D matters for the body to work properly, but supplementation is above all there to make up for insufficient intake and synthesis. It does not replace a balanced diet, sleep, physical activity or any other part of preventive care.

Frequently asked questions

Which month is best to start vitamin D?

There is no single date that suits everyone. In Poland, supplementation deserves particular attention in autumn and winter, when skin synthesis is limited. If you spend most of the year indoors or get little sun exposure, year-round supplementation can be justified too.

Do you have to start in September?

You can. September is a transitional period, and actual skin synthesis depends on sun exposure and many individual factors. There is no need to wait for October or November purely because of the calendar.

Does vitamin D need to be taken all year?

For some people, yes. That applies especially to older adults, people with little sun on their skin, and selected groups at raised risk of deficiency. For adults aged 19 to 65, Polish recommendations also allow year-round supplementation where the conditions for adequate skin synthesis are not met.

Is 2000 IU of vitamin D a day a lot?

For a healthy adult it is the upper value of the standard preventive range currently recommended in Poland where skin synthesis is insufficient. That does not make 2000 IU the right dose for everyone.

Do you need a 25(OH)D test before starting?

For a healthy person with no additional risk factors, routine testing is not required. The measurement is more useful in groups at raised risk of deficiency, or where there are medical indications.

Does vitamin D need to be combined with vitamin K2?

There is no standard recommendation that every healthy person supplementing vitamin D must also take vitamin K2. Current guidelines on preventing vitamin D deficiency do not impose it.

Is vitamin D better taken in the morning or the evening?

Current recommendations do not point to one obligatory time of day. In practice, taking the right dose consistently matters more than whether you take it in the morning or the evening.

Will supplementing vitamin D protect me from a cold?

That cannot be guaranteed. The most recent meta-analysis of randomised trials did not show a statistically significant reduction in overall risk of acute respiratory infections from vitamin D supplementation. Supplementing it is above all about preventing deficiency, not about warding off an infection.

References

1. EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) (2016). Dietary reference values for vitamin D. EFSA Journal, 14(10), 4547. doi:10.2903/j.efsa.2016.4547
2. Jolliffe, D.A., Camargo, C.A. Jr, Sluyter, J.D. et al. (2021). Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of aggregate data from randomised controlled trials. The Lancet Diabetes & Endocrinology, 9(5), 276–292. doi:10.1016/S2213-8587(21)00051-6
3. Płudowski, P., Kos-Kudła, B., Walczak, M. et al. (2023). Guidelines for Preventing and Treating Vitamin D Deficiency: A 2023 Update in Poland. Nutrients, 15(3), 695. doi:10.3390/nu15030695
4. Demay, M.B., Pittas, A.G., Bikle, D.D. et al. (2024). Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism, 109(8), 1907–1947. doi:10.1210/clinem/dgae290
5. Jolliffe, D.A., Camargo, C.A. Jr, Sluyter, J.D. et al. (2025). Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of stratified aggregate data. The Lancet Diabetes & Endocrinology, 13(4), 307–320. doi:10.1016/S2213-8587(24)00348-6