Importance of vitamin D via sun exposure

Vitamin D is among the more unusual entries in the vitamin list, and one of the most consequential. It is not a single compound but a group of fat-soluble secosteroids: principally D3 (cholecalciferol), made in skin and found in animal foods, and D2 (ergocalciferol), from plant and fungal sources.

There are three routes to it: synthesis in skin, diet, and supplements.

Strictly, calling it a vitamin is a historical accident. A vitamin is by definition something the body cannot make and must obtain from food. Skin makes vitamin D given ultraviolet light, which makes it far closer to a hormone, and it behaves like one, acting on receptors throughout the body.

How skin makes vitamin D in sunlight

The starting material is 7-dehydrocholesterol, a cholesterol precursor sitting in the plasma membranes of keratinocytes in the epidermis. UV-B photons in the 290–315 nm range break one of its rings, producing previtamin D3, which then isomerises thermally to vitamin D3. UV-A does not do this; only UV-B has the energy for it, which is why glass, which transmits UV-A but blocks UV-B, produces no vitamin D at all.

What comes out of the skin is inactive and takes two hydroxylation steps to become useful. In the liver, vitamin D3 gains a hydroxyl group at carbon 25 to become 25-hydroxyvitamin D: the circulating form, and the one measured in a blood test, because it reflects total status from all three sources. In the kidney a second hydroxylation at carbon 1 yields 1,25-dihydroxyvitamin D, or calcitriol, the active hormone. Kidney conversion is tightly regulated by parathyroid hormone according to the body's calcium needs. Surplus vitamin D is stored in adipose tissue.

Why is sunlight best for vitamin D?

Dietary sources are sparse. Oily fish, egg yolks and liver carry meaningful D3; UV-exposed mushrooms supply D2; and in many countries milk, cereals and spreads are fortified. Outside those, ordinary food contributes little, which makes diet alone a difficult route, though vegan D3 derived from lichen has closed the gap for people avoiding animal products.

Supplements are a reliable alternative and worth stating plainly as such: they are the standard recommendation for winter at high latitudes, for older and housebound people, during pregnancy where advised, and for anyone with darker skin living far from the equator. They carry none of the UV risk, and they raise serum 25(OH)D dependably.

Sunlight remains the most efficient single route, which is why vitamin D is nicknamed the sunshine vitamin, but deficiency is common all the same. One frequently cited US analysis, using a 20 ng/mL threshold, put deficiency at around 42% of adults, with higher rates at northern latitudes and among people with darker skin. Most adults can meet the recommended 600–800 IU daily with roughly 10 to 30 minutes of sun on forearms and face several times a week, longer for darker skin. Importantly, the reaction self-limits: once skin approaches reddening, previtamin D3 photodegrades and no further vitamin D is made. Burning has no benefit.

Benefits of vitamin D from sunlight

The well-established roles are in mineral handling and the musculoskeletal system. Vitamin D regulates intestinal absorption of calcium and phosphate, the building blocks of bone, and severe deficiency causes rickets in children and osteomalacia in adults. It also matters to muscle function, and to nerve and immune signalling. The vitamin D receptor is expressed widely, and there is reasonable evidence for a modest benefit in respiratory infection risk among the deficient.

Beyond that, honesty is more useful than enthusiasm. Vitamin D has been linked observationally to lower rates of cancer, cardiovascular disease, diabetes, autoimmune conditions and depression. The large randomised trials designed to test those links (VITAL and D-Health among them) did not find that supplementation reduces cancer or cardiovascular incidence in generally healthy adults. The most likely reading is that low vitamin D is partly a marker of poor health and limited outdoor activity rather than a cause of disease. Correcting a genuine deficiency is clearly worthwhile; treating vitamin D as a general preventive is not supported by the trial evidence.

For the other, non-vitamin-D effects of sunlight, see stress relief via sunbathing.