Vitamin D3 in summer – do you need to take supplements during the sunniest months?

Summer, sunshine and short sleeves – does the body produce enough vitamin D3 on its own in these conditions? Find out what actually influences skin synthesis, why the Polish summer does not always guarantee optimal vitamin D levels, and when it is worth continuing supplementation even during the summer months.

Vitamin D3 is one of those nutrients around which the most oversimplifications have arisen. The common belief goes something like this: “in summer we have sunshine, so supplementation is unnecessary, whilst in autumn and winter we need to go back to taking capsules”. In reality, the matter is clearly more complex. The European Food Safety Authority has approved several specific health claims for vitamin D: contributes to the normal absorption and utilisation of calcium and phosphorus, to the maintenance of normal bones and teeth, to the normal functioning of muscles, and to the normal functioning of the immune system. It performs these functions regardless of the season – and natural synthesis in the skin is subject to a number of limitations that are worth being aware of before deciding to stop taking the supplement in summer.

How vitamin D3 is produced in the skin

The mechanism of vitamin D synthesis in the skin is well described biochemically. Under the influence of ultraviolet radiation of a specific wavelength (UVB, 290–315 nm), the 7-dehydrocholesterol present in the skin is converted into previtamin D3, which then, through a thermal reaction, is converted into cholecalciferol – that is, vitamin D3. This form is transported to the liver, where it is hydroxylated to 25-hydroxyvitamin D (25(OH)D), and subsequently converted in the kidneys to the active form, 1,25(OH)₂D.

The key term in this description is UVB. Not all sunlight that reaches the skin enables the synthesis of vitamin D. For this to occur, radiation of a specific wavelength is required, which in Poland reaches the Earth’s surface within a limited time window – both on an annual and daily basis.

The Polish summer and the angle of incidence of the sun’s rays – why latitude matters

Poland lies at a latitude of 49–54 degrees – which is relatively high on the map of Europe. The angle at which the sun’s rays reach the Earth’s surface is crucial to the amount of UVB that actually reaches the skin. When the sun is low on the horizon, the rays travel a longer distance through the atmosphere, and a significant proportion of UVB radiation is absorbed by the ozone layers and air particles.

In practice, this means that vitamin D synthesis in Polish skin is only realistically possible between May and September and only around midday (usually between 10 am and 3 pm). Outside this window, UVB intensity is too low to stimulate effective skin synthesis. Even in the height of summer, on cloudy days or during the morning and evening hours, the skin may not receive a sufficient dose of UVB to synthesise physiologically significant amounts of vitamin D3.

Factors that significantly limit cutaneous synthesis in summer

The mere fact that it is summer and the sun is shining does not automatically mean that the body produces adequate amounts of vitamin D. The effectiveness of cutaneous synthesis is influenced by a whole range of factors – and in everyday life, many of these act simultaneously.

UV-filtering sun creams are one of the most significant limiting factors. Sunscreens with SPF 15 block around 93 per cent of UVB radiation, those with SPF 30 around 97 per cent, and those with SPF 50 over 98 per cent. Protecting the skin from sunburn and long-term photoageing damage is sensible, but it comes at a cost in the form of a significant reduction in vitamin D synthesis.

Covering clothing works in a similar way – skin not exposed to the sun does not synthesise vitamin D. Even on a hot July day, someone wearing long trousers and a long-sleeved shirt has only a small area of skin exposed (mainly the face and hands).

Age has a significant impact on the efficiency of skin synthesis. With age, the amount of 7-dehydrocholesterol in the skin gradually decreases, meaning that a person over the age of 60 synthesises significantly less vitamin D than a 25-year-old when exposed to the same amount of sunlight.

Skin phototype is also a factor. People with darker skin (higher melanin content) need longer exposure to the sun to produce a comparable amount of vitamin D.

Lifestyle – office work, travelling by car (windscreens block UVB rays), spending free time indoors. In modern urban life, even summer exposure to the sun can be significantly limited.

What the Polish data show – vitamin D deficiency even in summer

Population-based studies in Poland have for years shown the prevalence of vitamin D deficiency – and this is not limited to the autumn and winter months. Analyses of serum 25(OH)D levels indicate that a significant proportion of the Polish population has concentrations below the levels considered optimal, even during the summer months. In groups with limited sun exposure (older people, those who work indoors, and those who use sun cream), deficiencies in summer are almost as common as in winter.

Polish expert recommendations on vitamin D supplementation for adults suggest doses of 800–2000 IU per day during the autumn and winter months, and during the summer only if there is insufficient exposure to sunlight. The problem is that, in practice, ‘insufficient exposure’ describes the situation of a significant proportion of the adult population in Poland – for the reasons outlined above.

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Who should consider continuing supplementation during the summer as well

There are groups of people for whom summer vitamin D supplementation is particularly justified. If you fall into one of these categories, stopping your supplement from May to September may be a premature decision.

People over the age of 60 – reduced efficiency of skin synthesis, combined with spending more time indoors, means that many older adults remain at increased risk of deficiency even in summer. Vitamin D helps maintain healthy bones – a function that is particularly important after the age of 60.

People with darker skin tones – higher melanin content increases the time needed to produce adequate amounts of vitamin D. Given the level of sunshine in Poland, this can lead to a persistent deficiency even at the height of summer.

People who regularly use UV filters – sensible protection of the skin against photoageing and sunburn, but at the same time an effective reduction in the skin’s ability to synthesise vitamin D. If you cannot imagine summer without SPF 50, summer supplementation is justified.

People who work indoors – office work, factory production, indoor services. Even at times of day when it is theoretically sunny, actual exposure to UVB can be minimal.

Residents of northern Poland – the higher the latitude, the shorter the window for effective synthesis. In Koszalin or Olsztyn, the ‘sunny’ season, from a vitamin D perspective, is significantly shorter than in Kraków.

You can find out more about the role of this vitamin in calcium metabolism in the article how to use vitamins K2 and D3 and in the guide on calcium supplementation.

Vitamin D3 dosage in summer – how much and in what form

The decision on the summer dose of vitamin D3 depends on the individual’s circumstances. For people with good sun exposure (regular time spent outdoors between 10 am and 3 pm without UV filters, for at least 15–20 minutes on summer days), it may make sense to reduce the dose or switch to a maintenance dose. For people with limited exposure, maintaining a standard dose throughout the year is justified on practical grounds.

Typical supplement doses for adults range from 1,000 to 4,000 IU per day, although it is always advisable to consult a doctor before making a final decision – particularly if you are planning long-term supplementation at higher doses. It is best to have your serum 25(OH)D level tested and adjust your supplementation accordingly.

[tip: Vitamin D3 is fat-soluble – take it with your main meal of the day that contains fats. It is best absorbed alongside oils, avocados, nuts or oily fish. Fasting and low-fat meals can significantly reduce its bioavailability.]

Vitamin D3 with K2 – a duo for calcium metabolism

Many people taking vitamin D3 choose to supplement with vitamin K2 MK-7 at the same time. There is a physiological rationale for this. Vitamin D3 aids the absorption of calcium from the digestive tract, whilst vitamin K2 activates the proteins responsible for its proper utilisation – including its incorporation into bone tissue. Both components work together as part of the body’s wider calcium metabolism. You can find the full range of products containing vitamin D3 in various forms and doses in the Vitamins category.

[warning: Vitamin D3 is fat-soluble and may accumulate in the body with chronic use of very high doses (over 10,000 IU per day without medical supervision). Before starting high doses or long-term supplementation with higher amounts, consult your doctor and consider having your serum 25(OH)D levels tested. People with kidney disease, hyperparathyroidism, sarcoidosis or those taking thiazide diuretics should always consult a doctor before taking supplements.]

FAQ – frequently asked questions about summer vitamin D3 supplementation

How much time do you need to spend in the sun to produce a daily dose of vitamin D

For a young person with fair skin, with their arms and face exposed, around midday on a clear summer’s day – usually 15–20 minutes. For older people with darker skin, in partly cloudy conditions or when using sunscreen – significantly longer or practically impossible.

Is a sunbed sufficient for vitamin D synthesis?

Some sunbed lamps emit UVB radiation, which theoretically enables skin synthesis. However, this is not a recommended method of supplementing vitamin D due to the risks associated with regular use of sunbeds.

Can you check your vitamin D levels in the blood?

Yes – measuring 25(OH)D in blood serum is a standard test available at diagnostic laboratories. It is the most reliable way to assess your current vitamin D levels and adjust your supplement dose. People planning long-term supplementation or higher doses should have this test carried out at least once.

Does food provide enough vitamin D?

Natural sources of vitamin D in the diet include oily sea fish (salmon, mackerel, sardines, herring), fish oil, eggs and certain fortified products. In a typical Polish diet, the intake of vitamin D from food rarely meets the daily requirement – which is why a combination of sun exposure, diet and, where necessary, supplementation is a sensible year-round strategy.

Summary: The popular belief that one can completely stop taking vitamin D3 in summer is a significant oversimplification. In the Polish context – given the latitude, widespread use of UV filters, and a lifestyle that limits actual sun exposure – many adults remain at risk of deficiency even during the summer months. Older people, those with darker skin tones, those who regularly use SPF, those who work indoors or live in the north of the country should particularly consider continuing supplementation during the summer as well. The most reliable basis for a decision is to measure serum 25(OH)D levels and adjust the dose to the individual’s situation – ideally after consulting a doctor.