Why you need 15 minutes of sun

Vitamin D is, strictly speaking, a hormone that the body synthesises through the skin when it is exposed to ultraviolet B radiation from the sun. Unlike other vitamins, diet provides relatively small amounts, making sun exposure the primary source for most people. The problem is that modern living conditions, indoor work, systematic use of sunscreen and life at high latitudes, dramatically reduce that exposure.

Estimates of vitamin D deficiency prevalence in the global population are around 40%, with peaks in regions with prolonged winters and among people with darker skin living at high latitudes, since melanin reduces skin synthesis. Symptoms of severe deficiency include muscle weakness, bone pain and increased susceptibility to infections. Moderate deficiency is more silent but also more common, and is associated with higher risk of autoimmune and cardiovascular disease in long-term studies.

Fifteen minutes of direct sun exposure on face and arms, without sunscreen, during peak radiation hours, is enough for most fair-skinned people to synthesise the vitamin D needed for the day. That threshold varies depending on skin type, latitude and season. Oral supplementation is the alternative for those living with insufficient sunlight, and blood tests make it easy to verify levels.