Vitamin D (or ‘calciferol’) is a fat-soluble vitamin essential for musculoskeletal health.1,2
In the liver, vitamin D is metabolised to 25-hydroxyvitamin D (25[OH]D), the major circulating form of vitamin D measured in most assays.3
In the kidneys, further hydroxylation takes place to form 1,25-dihydroxyvitamin D, the active form of vitamin D, which promotes the absorption of calcium and phosphate from the gut and facilitates bone mineralisation.3,4
How much vitamin D is needed?
Assuming no or minimal sunlight exposure, the recommended daily adequate intake (AI) of vitamin D in Australia is 5 micrograms (200 IU) for children, adolescents and adults aged 19–50 years, 10 micrograms (400 IU) for adults aged 51–70 years, and 15 micrograms (600 IU) for adults over 70 years of age.5
Vitamin D is found in small amounts in animal-sourced foods, such as oily fish, eggs and meat. Vitamin D is also added to fortified foods such as fortified milk and margarine.5,6
The Food and Nutrient Database for Dietary Studies (FNDDS) Nutrient Values Database (accessible via FoodData Central) can be used as a guide to see how much vitamin D is found in different food sources.
However, for most Australians, vitamin D intake through dietary sources alone is insufficient.5,6
The main source of vitamin D in humans is vitamin D3 (cholecalciferol), which is formed through the action of ultraviolet B (UVB) radiation on 7-dehydrocholestrol in the skin.5,6
How much sun is needed?
The amount of sun needed to produce enough vitamin D depends on the time of day, a person’s skin type and the amount of skin exposed.3
Considerable variation in vitamin D levels can also result from geographical differences and seasonal changes in sun exposure, making it difficult to recommend an amount of sun applicable to the population as a whole.3
The Cancer Council website has more information on vitamin D production and sun exposure.