Vitamin D in summer: can you really stock up in the sun?

Sun, outdoor strolls, tanned skin... Summer seems to be the perfect season to naturally replenish vitamin D stores. And yet, the reality is a little more complicated. Spoiler: even under a beautiful summer sun, many of us remain below recommended intake levels.

Here's why and, more importantly, how to better manage your vitamin D status throughout the year, without the fuss.

Vitamin D: a quick reminder

Vitamin D is often called the "sunshine vitamin" because our skin can synthesize it under the effect of ultraviolet (UVB) rays. But it is also a fat-soluble vitamin that plays a recognized role in several essential bodily functions.

The EFSA (European Food Safety Authority) officially recognizes that vitamin D:

• contributes to the maintenance of normal bones;
• contributes to the maintenance of normal teeth;
• contributes to the normal absorption of calcium and phosphorus;
• contributes to the normal function of the immune system;
• contributes to the normal function of muscles.

It exists in two main forms: vitamin D2 (ergocalciferol, of plant origin) and vitamin D3 (cholecalciferol), synthesized by the body thanks to UVB and present in certain foods of animal or plant origin, such as reindeer lichen. D3 is the form best assimilated by the body.

🔬 According to the Esteban 2014-2016 study, 6.5% of the general adult population in France has a vitamin D deficiency and 28% have a moderate deficiency. These figures already give food for thought... Even before addressing the issue of summer.

Can we really replenish our stores with the sun in summer?

Theoretically yes... but in practice, it is often insufficient. Here's why:

Sun protection

It is essential to protect the skin. But it also blocks up to 90% of vitamin D synthesis. A very real summer paradox: the more we protect ourselves (and we are right to do so!), the less vitamin D we synthesize.

🔬 A study published in the British Journal of Dermatology confirmed that the regular use of sunscreen significantly reduced cutaneous vitamin D synthesis, even in very sunny regions.

Skin type

Darker skin types naturally contain more melanin, a built-in sunscreen, so to speak. Result: for the same duration of exposure, vitamin D synthesis is significantly slower. 🔬 This is confirmed by several dermatological studies, including a systematic review published in Acta Dermato-Venereologica.

Latitude and time

In mainland France, UVB rays are only intense enough to trigger cutaneous synthesis for a few hours a day in summer, between approximately 10 am and 4 pm. Outside this window, sun exposure does not produce vitamin D, even in bright sunshine.

Lifestyle habits

Working indoors, wearing covering clothes, spending one's days in the shade... These are all perfectly normal habits, but they reduce actual exposure to UVB. The exposed skin surface also counts: face and hands only, or bare arms and legs, the result is not the same.

What the data says

🔬 A study (Holick, 2007, New England Journal of Medicine) recalls that vitamin D deficiency is a global phenomenon, including in sunny countries, and that summer synthesis is not enough to maintain an optimal level throughout the year. In France, Esteban data shows that the prevalence of deficiency remains high regardless of the population profile.

💡 Tips: when the sun appears, maximize the exposed skin surface (bare arms, legs) for 15 to 30 minutes, between 10 am and 4 pm, without sun protection on these areas. The larger the surface, the more vitamin D your body can synthesize naturally. But be careful: no need to exceed 30 minutes, synthesis plateaus, and sunburn, on the other hand, continues.

Dietary sources of vitamin D

Diet can cover some needs, but rarely all of them. The best sources are:

Animal sources (most concentrated)

Food	Indicative content (per 100 g)
Cod liver oil	~250 µg
Wild salmon	~16 µg
Mackerel	~14 µg
Sardines	~11 µg
Whole egg	~2 µg
Enriched dairy products	~1-2 µg

Plant sources

Plants are not highly concentrated in vitamin D. Only certain UV-exposed mushrooms (shiitake, maitake) provide modest amounts. For people following a vegetarian or vegan diet, enriched foods and supplementation remain the most effective options.

💡 Tips: remember to combine your vitamin D with magnesium. 🔬 A study published in the Journal of the American Osteopathic Association shows that magnesium is essential for the activation of vitamin D in the body — without it, even a good level of D3 can remain partly inactive.

How much vitamin D per day?

The guidelines recommended by EFSA for adults are as follows:

Nutrient	General adult population
Vitamin D	15 µg/day (600 IU)

This recommendation is based on the assumption of minimal sun exposure — which means it is designed to cover needs regardless of geographical and seasonal conditions.

A blood test (25-OH-vitamin D) can accurately assess one's status and adapt intake. In case of confirmed deficiency, your doctor can prescribe appropriate supplementation — therapeutic doses are then different from maintenance doses.

⚠️ Important: vitamin D is fat-soluble and is stored in the body. Overdosing is possible in case of excessive and prolonged supplementation. Do not combine several sources without medical advice.

Should you supplement in summer?

Good news: the answer is nuanced. Not everyone necessarily needs it, but certain profiles clearly benefit from supplementation even in summer:

• People with darker skin;
• People who work mainly indoors;
• People following a vegan diet (little to no fish, no dairy products);
• Elderly people (cutaneous synthesis decreases with age);
• People with a confirmed deficiency in blood tests.

The best way to know your status? A blood test, to be requested from your doctor. Depending on your results, they can guide you towards the most suitable supplementation for your profile.

If you are considering supplementation, prioritize vitamin D3, the form best assimilated by the body. It can be of animal origin (lanolin) or plant origin (reindeer lichen), which allows for covering needs even for people following a vegan diet.

Frequently Asked Questions

Can you lack vitamin D even in summer?
Yes. Sun protection, skin type, lifestyle habits, and latitude often limit cutaneous synthesis, even during the summer months. Available data in France show that deficiencies remain frequent regardless of the season.

What is the difference between vitamin D2 and D3?
D3 (cholecalciferol) is the form synthesized by the skin under the action of UVB and the best assimilated by the body. 🔬 Comparative studies show that D3 is more effective in maintaining a stable serum level over time than D2.

Can you stock up on vitamin D in a single day at the beach?
No. Cutaneous synthesis has a natural ceiling: beyond a certain exposure threshold, the skin does not produce more, and sunburn even degrades part of the D3 already formed. Moderate and regular exposures are much more effective than a single prolonged day.

Is vitamin D stored from one summer to the next?
Partially. As vitamin D is fat-soluble, it is stored in adipose tissues. But this stock generally depletes over the winter, and is not enough to maintain optimal serum levels all year round without continuous intake.

How long does sun exposure need to be?
15 to 30 minutes a day, with bare arms and legs, between 10 am and 4 pm, without sun protection on exposed areas — and without getting sunburned. This duration varies depending on skin type, latitude, and time of day.

This article is for informational purposes only and does not constitute medical advice. Consult your doctor for personalized health advice, especially before starting supplementation.