Vitamin D
Today
30-day average. Months to years of reserve, so only a sustained trend is meaningful.
What it is
Vitamin D controls how much calcium you absorb from food, which makes it essential for bones. Your skin makes it from sunlight, so how much you need from food depends on where you live, your skin tone, and how much time you spend outside. Very few foods contain much of it — oily fish and fortified milk are the main ones — so a lot of people are low in winter. It's stored in body fat for months, which is why a single low day means nothing.
Reference intake
Your daily target, personalised to your age and sex.
- RDA
- 15 mcgmale_19_30
- Upper limit
- 100 mcgapplies to total
100 mcg equals 4,000 IU. Sunlight cannot push intake past this — excess previtamin D3 is photodegraded in the skin — so the UL is effectively a limit on supplements and fortified foods.
What the evidence says
Wording reflects how strong the evidence actually is.
Prevents rickets in children and osteomalacia in adults
May reduces fracture risk when combined with calcium in older adults
May reduces the risk of acute respiratory infection, though the evidence is thin
Has been associated with: prevents cancer or cardiovascular disease
Deficiency
vitamin D deficiency (rickets in children, osteomalacia in adults)
Roughly 18% of the US population has serum 25(OH)D below 40 nmol/L (at risk of inadequacy), with markedly higher rates in people with darker skin, at higher latitudes, and in winter.
Early signs
- fatigue
- bone aching, especially in the lower back and hips
- muscle weakness, particularly proximal
- low mood
Higher risk
- people with darker skin (melanin reduces cutaneous synthesis)
- residents above roughly 37° latitude in winter
- people with limited sun exposure — housebound, veiled, night-shift workers
- older adults (skin synthesis declines markedly with age)
- obesity (sequestration in adipose tissue lowers circulating levels)
- fat malabsorption: celiac, Crohn's, cystic fibrosis, post-bariatric surgery
- exclusively breastfed infants without supplementation
Excess
vitamin D toxicity (hypervitaminosis D)
Not achievable from unfortified food. Reported toxicity is essentially always from supplements or from manufacturing errors in fortified products.
From supplements
- hypercalcemia
- nausea and vomiting
- excessive thirst and urination
- confusion
- kidney stones
At extreme doses
Sustained hypercalcemia causes vascular and soft-tissue calcification and renal failure. Typically requires prolonged intake well above 10,000 IU/day; sunlight alone cannot cause it, because excess previtamin D is photodegraded in the skin.
Absorption
~50% of dietary intake; cutaneous synthesis is the dominant source for most people
Helps absorption
- consumption with a fat-containing meal
- adequate magnesium status (required for the activating hydroxylases)
- UVB exposure to skin — the main source, not food
Hinders absorption
- fat malabsorption of any cause
- obesity (sequestration in adipose tissue)
- sunscreen, clothing, glass, and darker skin pigmentation (all reduce synthesis)
- corticosteroids and some anticonvulsants (accelerate catabolism)
Top sources
Per 100 g, in mcg. FoodData Central IDs are resolved by the importer, not written by hand.
- 250 mcg
Cod liver oil
5 mL (1 tsp)
- 16.2 mcg
Rainbow trout, farmed, cooked
85 g fillet
- 14.2 mcg
Salmon, sockeye, cooked
85 g fillet
- 28.1 mcg
Mushrooms, UV-exposed
70 g (1 cup)
- 5.4 mcg
Egg yolk
17 g (1 yolk)
- 1.3 mcg
Milk, fortified
240 mL (1 cup)
Ranking these by what you already eat needs the real log — see docs/STATUS.md.
Sources
Last reviewed 2026-08-11