Sources of Vitamin D
The primary source of vitamin D for humans is sunlight exposure to the skin, with dietary sources playing a secondary but important role, particularly when sun exposure is inadequate. 1
Non-Dietary Sources
Sunlight (Primary Source)
- Endogenous synthesis through UVB radiation (290-315 nm) is the major source of vitamin D for most children and adults 1
- Vitamin D3 (cholecalciferol) is synthesized in the skin when 7-dehydrocholesterol is exposed to UVB rays
- Effectiveness depends on multiple factors:
- Latitude: Above and below approximately 33° latitude, vitamin D3 synthesis is very low or absent during most winter months 1
- Season and time of day: Zenith angle of the sun dramatically influences production 1
- Skin pigmentation: Dark-skinned individuals require 3-5 times longer sun exposure to produce the same amount as light-skinned individuals 1, 2
- Age: Older individuals require more sun exposure than younger people 2
- Sunscreen use: SPF 30 reduces vitamin D synthesis by more than 95% 1
- Clothing: Has a large influence on vitamin D production 3
Dietary Sources
Natural Food Sources (Limited)
Very few foods naturally contain significant vitamin D 1, 2, making dietary intake challenging:
Rich Sources (>4 μg/100g):
- Fatty fish (5-25 μg/100g): salmon, mackerel, herring 1, 4
- Fish liver oils (250 μg/100g), particularly cod liver oil 4
- Mushrooms (21.1-58.7 μg/100g), especially sun-exposed varieties 1, 4
- Reindeer lichen (87 μg/100g) 4
Moderate Sources (1-4 μg/100g):
- Eggs (1.3-2.9 μg/100g) 4
- Cheese (1.3-2.9 μg/100g) 4
- Beef liver (1.3-2.9 μg/100g) 4
- Dark chocolate (4 μg/100g) 4
Fortified Foods
In the United States and Canada, milk is fortified with vitamin D, along with some bread products, orange juices, cereals, yogurts, and cheeses 1. In Europe, most countries do not fortify milk 1.
Common fortified products include:
- Milk and dairy products
- Plant-based beverages
- Breakfast cereals
- Fat spreads/margarines
- Orange juice 4
Vitamin D Forms in Food
- Vitamin D3 (cholecalciferol): Found in animal-derived products, including fish, eggs, and fortified foods 1, 2
- Vitamin D2 (ergocalciferol): Obtained from UV irradiation of yeast sterol ergosterol; found naturally in sun-exposed mushrooms and almost absent in typical diet 1, 2
- 25-hydroxyvitamin D: Also contributes significantly to dietary vitamin D intake from many animal-derived products 4
Supplementation
Since adequate vitamin D intake (15 μg/day) is hard to achieve through diet alone, dietary supplements are usually recommended 4. Both vitamin D2 and D3 are used in supplements, though availability differs between countries 2.
Key Clinical Considerations:
- Commercially available vitamin D3 is synthesized from 7-dehydrocholesterol naturally present in skin or obtained from lanolin 1
- Oral vitamin D is absorbed through the intestinal tract via chylomicrons, passing into lymphatic circulation before reaching central venous circulation 5
- Approximately 35% of ingested vitamin D is carried in lipoproteins rather than vitamin D-binding protein 5
Important Caveats:
- Fat malabsorption syndromes and bariatric surgery patients are often unable to absorb fat-soluble vitamin D 1
- Patients on anticonvulsants and AIDS/HIV medications are at risk due to enhanced catabolism of vitamin D metabolites 1
- Obesity (BMI >30 kg/m²) is associated with vitamin D deficiency due to inverse association with serum 25(OH)D 1
- High dietary calcium intake has a vitamin D-sparing effect by increasing the half-life of 25(OH)D 3