Blanca Martín, nutritionist: “If you have a vitamin D deficiency, it may be affecting your immune system and you don’t even know it”

Vitamin D stands out from most nutrients because our bodies don’t rely primarily on food to get it. While it can be found in certain foods — like cod liver oil, eel, herring, smoked salmon, and egg yolks — the body mainly produces vitamin D through sunlight exposure.

That’s why getting a bit of sun each day matters. About 15 minutes of sunlight is often enough for the body to start making vitamin D. When ultraviolet rays hit the skin, they interact with a compound called 7‑dehydrocholesterol, breaking it down and converting it into vitamin D3, also known as cholecalciferol.

Of course, not all sun exposure produces the same amount of vitamin D. Sunscreen can significantly reduce vitamin D production, though it also protects the skin from UV damage. Skin tone also plays a role: people with more melanin generally need more time in the sun than those with lighter skin to produce the same amount of vitamin D.

Why vitamin D matters

Vitamin D plays a key role in supporting the immune system. Nutrition expert Blanca Martín highlighted this in a recent TikTok video, noting that a deficiency may affect the body’s defenses without obvious symptoms. Vitamin D also helps the body absorb and use calcium and phosphorus, and it contributes to maintaining healthy bones, muscles, and teeth.

When supplementation may be recommended

Some groups may have a harder time producing vitamin D naturally due to biological or lifestyle factors. In these cases, healthcare professionals sometimes recommend supervised supplementation. This can be especially relevant for:

However, taking too much vitamin D can lead to serious problems, including nausea, muscle weakness, dehydration, and long‑term damage to the kidneys or soft tissues. Because of that, supplements should always be taken under medical guidance.

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