And what if the sun isn’t enough to fill up on vitamin D? The question may seem absurd in the heat of summer, when terraces are crowded, sunscreen spills from beach bags, and endless sunny days set the pace for holidays. And yet, one in two French people has a vitamin D deficit, even after weeks of exposure. How can we explain such a paradox? In reality, it may not be a paradox at all, given how the mechanisms at play are more complex than they appear. Between lifestyle habits, diet, and individual peculiarities, several factors combine to slow the production of this vitamin that is essential to our body.
The sun isn’t enough: why our UVB rays aren’t doing their job
Contrary to common belief, spending time outdoors does not guarantee a good synthesis of vitamin D. For the skin to manufacture it, it must be exposed directly to UVB rays, without any protective screen. Yet in summer, sun-protection reflexes naturally take precedence: sunscreen applied from the first rays, light but covering clothing, seeking shade during the hottest hours. These habits, essential to protect against skin cancers, mechanically reduce the amount of UVB that actually reaches the epidermis. To this is added an increasingly urban and sedentary lifestyle: air‑conditioned offices, car or public transport commutes, days spent indoors. As a result, even in the midst of a heatwave, the useful exposure time to the skin often remains very limited, far from the common notion of a summer inevitably generous in vitamin D.
The plate, an insufficient source faced with real needs
If the sun does not do everything, could diet take over? Vitamin D does exist in certain foods: fatty fish such as salmon, mackerel or sardines, egg yolks, or enriched dairy products and margarines. But these sources remain largely insufficient to cover the daily needs of the body. One would have to consume substantial and regular amounts to hope to close the gap, which does not align with the eating habits of the majority of French people. Vitamin D is indeed one of the rare vitamins whose dietary intake remains secondary to cutaneous synthesis. In other words, even a balanced and varied plate usually does not suffice, on its own, to guarantee an optimal level. This observation partly explains why deficiency affects a sizeable portion of the population, regardless of season.
Age, skin, weight: profiles more exposed to deficiency
Beyond the lack of sun exposure and the limits of diet, some profiles accumulate risk factors that further complicate the synthesis and storage of vitamin D. This is notably the case for people with tan or dark skin: melanin, which naturally protects against UV rays, also acts as a filter that slows vitamin D production. With equal exposure, these skin tones require more time in the sun to synthesize the same amount of vitamin D as lighter skin. Age also plays a non-negligible role: with age, the skin loses part of its ability to transform UVB rays into active vitamin D, making older individuals particularly vulnerable to deficiencies. Finally, overweight is another factor to consider. Vitamin D being fat-soluble, it tends to store in adipose tissue, reducing its availability in the blood and complicating its use by the body. These various mechanisms explain why vitamin D deficiency is ultimately frequent mainly due to lack of UVB exposure, a diet that provides little of it, and individual factors such as age, skin pigmentation, or overweight. A combination of causes that makes this deficit far more widespread than one might imagine, even in the height of the summer season.
Facing this reality, it’s impossible to rely solely on sunny days to boost vitamin D. Between adapting sun exposure, targeted dietary choices, and, if needed, supplementation supervised by a health professional, several levers exist to correct this too-common deficit. A blood test remains the best way to know exactly where you stand, rather than trusting appearances of a sunny summer.
So, the next time you enjoy a beautiful summer day, perhaps it’s worth asking whether your body is truly getting all the benefits you expect. A simple blood test could well shed light on this paradox that quietly concerns one in two French people.
