Why vitamin D is different
Most nutrients are easy to get from a varied diet, but vitamin D is unusual: few foods naturally contain it, and the other main source — sunlight — is inconsistent. That combination is why vitamin D is one of the most commonly recommended supplements [1].
Food sources
The NIH Office of Dietary Supplements notes the best sources are fatty fish (trout, salmon, tuna, mackerel) and fish liver oils, with smaller amounts in egg yolks, beef liver, and cheese, and variable amounts in UV-treated mushrooms [1]. In practice, fortified foods — milk, plant milks, and breakfast cereals — provide most of the vitamin D in American diets [1].
Sunlight: real but unreliable
Skin makes vitamin D when exposed to UVB, but NIH lists many factors that limit it: season, time of day, latitude, cloud cover, skin melanin, and sunscreen [1]. Older adults and people with darker skin make less from the same exposure [1]. Relying on sun alone is unpredictable — and more sun also raises skin-cancer risk, so it's not a clean solution.
How much you need
The adult RDA is 600 IU (15 mcg), rising to 800 IU (20 mcg) after age 70 [1]. Because food sources are limited and sun is variable, many people — especially in winter, at northern latitudes, with darker skin, or who are older or homebound — fall short. See vitamin D and blood levels.
When a supplement makes sense
Vitamin D is a reasonable supplement for people who can't reliably get it from food or sun, which is a large share of the population in many regions. But more isn't better — high-dose vitamin D has real toxicity risk, so stay within sensible amounts unless a clinician advises otherwise based on your blood level.
Practical guidance
- Include fatty fish and fortified foods where you can.
- Don't count on sun as a reliable, year-round source.
- Consider a modest supplement if you're in an at-risk group, and check your level rather than guessing at high doses.