Where does vitamin D actually come from — and why is food barely on the list?
Vitamin D's natural-sources chapter is unique in this library: the primary natural source is not food — it is the sun. Ten to thirty minutes of midday summer sun on arms and legs can synthesize 10,000–20,000 IU; no food on earth delivers that in a serving. The food list is short because evolution never needed a dietary route — the sun did the work. Modern indoor life broke the arrangement, and the supplement became the replacement for a source that was never on the plate.
UVB photons (290–315 nm) strike 7-dehydrocholesterol in skin and convert it to previtamin D3, which thermal-isomerizes to cholecalciferol over hours. The system is self-regulating — extended exposure doesn't overdose, it photodegrades the excess. The engineering worked perfectly for forty thousand generations of outdoor humans.
The variables that govern the arithmetic: latitude (above ~37°, winter sun is too weak for months), time of day (midday UVB is many times morning/evening), skin tone (melanin slows synthesis several-fold — a built-in SPF), age (older skin makes ~75% less), glass and sunscreen (both block UVB almost entirely). A light-skinned person at midlatitude needs minutes of summer midday exposure; a darker-skinned person at high latitude in winter needs hours the sun doesn't have. This arithmetic — not dietary choice — is why a billion people run low.
Evolution built the sun route so reliably that almost no food carries meaningful vitamin D — and what does, reflects either the marine food chain's concentration (fatty fish) or human engineering (fortification). Wild salmon leads (~600–1,000 IU per serving) — the fish's own diet of vitamin-D-rich plankton concentrating up the food web. Farmed salmon carries roughly a quarter of the wild amount (feed-dependent). Cod liver oil — the historical rickets cure, still one of the densest sources — delivers ~1,000+ IU per tablespoon alongside its omega-3s.
Everything else on the list is fortification or trace: milk (100 IU per cup, added by law in fortified countries since the 1930s), orange juice and plant milks (fortified to match), egg yolks (~40 IU — the hen's sun exposure in embryo form), and UV-exposed mushrooms (the plant exception — they make D2, the lesser form, when deliberately irradiated). The honest summary: a perfect vitamin-D diet might deliver 600–1,000 IU — roughly the RDA floor, and only with deliberate effort. The blood level most clinicians target (30+ ng/mL) is hard to sustain on food alone at modern indoor sun exposure. This is why the supplement exists as the standard, not the exception.
600–1,000 IU per 3.5 oz
The densest common food source — the marine food chain's plankton concentration. Farmed salmon carries roughly a quarter of the wild amount.
~1,100 IU per tbsp
The historical rickets cure — still one of the densest sources, with EPA/DHA omega-3s alongside. The reason 'liver and cod liver oil' predate the vitamin's discovery.
~150–250 IU per serving
The pantry route — meaningful D alongside the omega-3s and calcium (sardines with bones).
100 IU per cup
The 1930s public-health engineering that ended the Western rickets epidemic — vitamin D added by law to the food children already drank.
100 IU per cup (varies)
The modern fortification expansion — matching milk's number for the lactose-free and plant-based. Read labels: fortification varies by brand.
~40 IU per egg
A modest dose — hens fed vitamin D pass some to the yolk. Pasture-raised hens with sun exposure lay slightly D-richer eggs.
Variable (D2 form)
The plant exception — deliberately UV-irradiated mushrooms make D2. Check labels: the exposure, and the dose, varies enormously.
~50 IU per 3.5 oz
A trace source — the reason the historical rickets diet included liver, and why cod liver oil out-delivered it.
Variable — read labels
Dairy fortification beyond milk — inconsistent across products; the label is the source.
~300–600 IU per serving
The Northern-European fatty fish — the traditional sunless-winter D sources of the Nordic diet.
40–100 IU per serving
The breakfast fortification route — modest, consistent, and counting toward the daily total.
10,000–20,000 IU per session
The original source, listed here for honesty: minutes of midday summer sun out-delivers the entire food list. The source modern life scheduled away.
Vitamin D's discovery story runs backwards from most nutrients: the cure was found before the vitamin. Cod liver oil was a folk remedy for 'the English disease' — rickets, the bent-bone deformity of industrial-revolution children in sunless cities — for a century before biochemistry caught up. In 1919–1922, researchers demonstrated that either cod liver oil OR ultraviolet light cured rickets, and Elmer McCollum's team isolated the antirachitic factor: the fourth vitamin discovered, named D in the alphabetical sequence of the era.
The public-health sequel became one of nutrition's greatest wins: milk fortification, beginning in the 1930s, ended the rickets epidemic in fortified countries within a generation — a disease that had deformed urban childhood for centuries, eliminated by adding a sunlight-derived vitamin to a breakfast staple. The modern epilogue is quieter: rickets returned at the margins (dark-skinned infants at high latitudes, exclusively breastfed babies), and the adult version — osteomalacia, the soft-bone disease — persists where deficiency does. The deficiency spectrum didn't end; it moved from bent bones in children to falls, fractures, and diffuse symptoms in the indoor-living elderly.
The logic seemed airtight: if daily vitamin D helps, an annual megadose should deliver the year's benefit in one clinic visit — perfect for patients who forget pills. Australian researchers (Sanders et al., JAMA 2010) tested it: 500,000 IU once yearly versus placebo, in women aged 70+ — the exact population the regimen was designed to protect.
The result inverted the premise: the megadose group fell MORE and fractured MORE — roughly 15% higher fall and fracture rates than placebo. The hypothesis for the mechanism: the supraphysiologic spike dysregulates the receptor system, paradoxically impairing the muscle function and balance the vitamin supports — the body's tightly-regulated hormone machinery responding to a flood the way it never evolved to handle.
The trial changed practice worldwide: annual megadose protocols were abandoned, and the field's dosing rule crystallized — daily, moderate, sustained: the rhythm the skin's own system always used.
Your skin can make 10,000–20,000 IU of vitamin D in a single sunlit session — roughly the content of twenty supplement bottles — in a self-regulating reaction that can never overdose, because extended sunlight photodegrades the excess. The engineering worked perfectly. The indoor calendar broke it.