Vitamin D From Sun vs Food: What Differs, What Doesn't

The two routes converge faster than most people expect. Skin makes vitamin D3; food delivers D3 or D2; and neither is active until the liver and then the kidney have each added a hydroxyl group. What genuinely separates them is not the molecule — it is how reliably each route supplies you, and what each one costs.

The chemistry, in two paragraphs

Ultraviolet B striking the skin converts 7-dehydrocholesterol, a cholesterol precursor sitting in the epidermis, into previtamin D3, which becomes vitamin D3 — cholecalciferol. That is the entire contribution of sunlight: it makes one molecule, and it makes it in one place.

Dietary vitamin D, being fat-soluble, is absorbed alongside other dietary fats in the small intestine. From there the two sources are indistinguishable. The Institute of Medicine describes vitamin D in either form as biologically inactive until two enzymatic hydroxylations occur, the first in the liver, producing 25-hydroxyvitamin D — the compound your blood test measures, whether it originated on a beach or in a capsule.

D2 and D3 are not a sun-versus-food split

A common misreading treats D2 as the “plant” version and D3 as the “sun” version. The real division is different. D3 is what skin synthesizes and what animal foods contain: fatty fish, fish liver oil, egg yolk. D2, ergocalciferol, is largely human-made and added to foods, and occurs naturally in mushrooms exposed to ultraviolet light.

Both are commercially synthesized and both appear in supplements and fortified foods. Product labels surveyed by the Institute of Medicine mostly showed D3 as the added form, though some milk substitutes are fortified with D2. On the measure that matters clinically — potency for curing vitamin D-deficiency rickets — the IOM reports the two as the same.

Why unfortified food was never going to be enough

Here is the asymmetry that decides the practical question. Naturally occurring vitamin D exists in a very short list of foods, and not in large amounts outside fatty fish. Salmon, the richest common source, carries above 13 micrograms per 100 grams — around 520 IU in a 3.5-ounce portion, so a single serving nearly covers a day’s 600 IU. Beef liver, cheese and egg yolks contribute small amounts, and everything else contributes essentially none.

Fortification is what closes the gap. Most US fluid milk carries about 400 IU per quart, and cereals, juices, spreads and cheeses are fortified in the 40 to 100 IU per serving range. That engineering, not the natural food supply, is why “get it from your diet” is realistic advice in the United States.

Salmon happens to be the overlap point of two nutrient stories — it is also the leading source of EPA and DHA among common foods, which is a reasonable argument for putting it on the menu twice a week regardless of which nutrient you came for.

Which route the guidance actually recommends

Given equivalent chemistry, the choice comes down to side effects, and that is where the two routes stop being equivalent. The American Academy of Dermatology recommends healthy adults obtain adequate vitamin D from a diet including naturally rich and fortified foods, describing that as the way to get the vitamin without increasing skin cancer risk. MedlinePlus adds that exposure for more than a few minutes without sunscreen is not recommended.

Sun exposure remains a real physiological source, and for many people a substantial one in summer. It is simply not the one the guidance asks you to engineer deliberately — see how much sun for vitamin D a day for why no minutes figure exists, and does sunscreen block vitamin D for the trade-off people most often get backwards.

This content is informational and does not replace medical or dietetic advice. Supplement dosing should follow a 25-hydroxyvitamin D blood test, not a guess about sun and diet.