Sun
Vitamin D is the only vitamin your body manufactures rather than eats. The process runs on a narrow band of ultraviolet light, and it stops itself before it can hurt you.
Call it a vitamin and you have already misdescribed it. Vitamins are compounds the body cannot synthesise and must obtain from food. Vitamin D fails that test in a way no other member of the list does: given a few minutes of the right kind of sunlight, human skin builds it from scratch. The naming is a historical accident from the 1920s, when researchers curing rickets assumed they were isolating a dietary factor. What they had actually found was a hormone precursor with a photochemical trigger.
Understanding the chain matters, because almost every practical question about sun exposure — how long, what time of day, what season, how much skin — is answered by one of its steps.
In the epidermis, particularly the stratum spinosum and stratum basale layers, sits a cholesterol derivative called 7-dehydrocholesterol. It is present in significant concentration and it is doing nothing in particular until a photon of the right energy hits it.
That energy requirement is the pivot on which everything turns. The bond that breaks in 7-dehydrocholesterol absorbs light between roughly 290 and 315 nanometres — a slice of the ultraviolet B band. Longer wavelengths pass through without effect. UVA, which makes up the overwhelming majority of ultraviolet reaching the ground, does nothing here at all. It tans, it ages skin, it penetrates glass, and it produces no vitamin D whatsoever.
When a UVB photon lands, the B ring of the sterol opens. The product is previtamin D3, an unstable intermediate. Over the following hours, body heat drives a thermal isomerisation that converts previtamin D3 to cholecalciferol — vitamin D3 proper. This step is slow. Peak conversion occurs several hours after the exposure that started it, which is one reason a single measurement of blood vitamin D taken immediately after sun exposure understates what that exposure will eventually produce.
Cholecalciferol is inert. It binds to vitamin D-binding protein and travels to the liver, where the enzyme CYP2R1 attaches a hydroxyl group at carbon 25. The result is 25-hydroxyvitamin D, or calcidiol. This is the compound a blood test measures, because it has a circulating half-life of two to three weeks and therefore reflects your accumulated status rather than yesterday's sunshine.
Calcidiol is still not the active hormone. The kidney — and, as it turns out, many other tissues including immune cells, the prostate, and the colon — performs a second hydroxylation at carbon 1, producing 1,25-dihydroxyvitamin D, or calcitriol. That is the molecule that binds the vitamin D receptor and changes gene transcription in hundreds of target genes.
The distinction is worth holding onto. Renal production of calcitriol is tightly regulated by parathyroid hormone and serves calcium homeostasis. Local production in other tissues appears to depend more directly on how much calcidiol is circulating, which is the mechanistic argument behind interest in vitamin D status beyond bone health.
This is the most useful and least known part of the system. Previtamin D3 is itself photosensitive. Continued UVB exposure converts it into two biologically inert compounds, lumisterol and tachysterol, rather than allowing it to accumulate without limit. Cholecalciferol that has already formed is also degraded by further UV.
The practical consequence is a plateau. Synthesis rises steeply during the first portion of an exposure, flattens, and then goes essentially flat. Published work using controlled whole-body irradiation puts the inflection well below the dose that causes visible redness — commonly cited at around a quarter to a half of the minimal erythemal dose, the amount of UV that produces barely perceptible pinkness twenty-four hours later.
Sit in the sun past that inflection and the arithmetic turns against you. Additional minutes deliver DNA damage, photoaging, and eventually a burn, while adding almost nothing to the vitamin D total. This is why oral vitamin D toxicity is well documented and solar vitamin D toxicity has never been observed. The skin has a built-in stop. Supplement bottles do not.
The single most common error in sun exposure is treating vitamin D synthesis as though it accumulates linearly with time. It does not. It saturates, and everything after saturation is pure cost.
Four variables dominate, and none of them are how long you sat outside.
Solar zenith angle. UVB is scattered and absorbed by atmospheric ozone far more aggressively than UVA. When the sun sits low, sunlight travels a longer slant path through the ozone layer, and the UVB fraction is filtered out almost entirely. This is why the same latitude produces abundant vitamin D in June and none in January, and why 8 a.m. and noon are not interchangeable even on the same day.
Melanin. Melanin is an effective broad-spectrum UV absorber, which is precisely its evolutionary function. It competes with 7-dehydrocholesterol for incoming photons. Deeply pigmented skin therefore requires substantially longer exposure — estimates range from three to ten times — to produce an equivalent amount of vitamin D. It also confers meaningful protection against UV damage. The trade-off is real in both directions.
Exposed surface area. Synthesis is a function of irradiated skin, not of time. Face and hands represent roughly six percent of body surface. Torso, arms and legs together approach sixty. The difference between those two configurations is an order of magnitude in output, and no amount of extra time on the six percent version closes it.
Age. Concentration of 7-dehydrocholesterol in the epidermis declines steadily across the lifespan. A seventy-year-old produces roughly a quarter of what a twenty-year-old produces from an identical exposure. Older adults also tend to spend less time outdoors and to cover more skin when they do, which compounds the effect.
The chain suggests a specific strategy: short, frequent exposures of a large skin area near solar noon during months when the sun climbs high enough. Long exposures do not help. Late-afternoon exposures barely register. Small exposed areas cap the output regardless of duration.
It also explains why supplementation exists as a serious option rather than a marketing product. Above roughly 37 degrees of latitude, the sun does not climb high enough between November and February for meaningful UVB to reach the ground at all. During those months the photochemistry described above simply does not run, and stored calcidiol from summer is what carries you through.
Use the vitamin D calculator to see how the four variables interact for your own skin type, latitude and season — including where your particular exposure crosses from productive into merely damaging.