Healthy vitamin D levels can be easily obtained through exposure to UVB radiation in the form of sunlight. However, most of us spend a lot of time indoors, we use sunscreen (for good reasons), wear clothes (also for good reasons), or we simply live in places not so near the equator (places where casual exposure to sunlight will not result in any appreciable vitamin D synthesis during the winter months). All of this can result in low vitamin D levels, leaving us to feel tired and prone for infectious diseases.
Luckily, low vitamin D levels from a lack of sun exposure can be easily corrected, but generally not by our diet alone. The general recommendation to obtain adequate vitamin D levels is vitamin D-supplementation, especially in non-summer months.
Although careful exposure to sunlight is also recommended, specific recommendation for healthy sun exposure cannot be made, whereas it is highly variable on the basis of skin color, the season, the time of day, where you live, cloud cover, and even air pollution. Besides, sun exposure is the most important risk factor for the development of skin cancer and skin aging.
Dietary Vitamin D
Vitamin D is the generic term for both vitamin D2 (ergocalciferol) and D3 (cholecalciferol). Although they differ in structure, it is not clear whether they also have a different function in the body. Currently, it appears that at nutritional doses vitamin D2 and D3 are similar, but at high doses (for instance to correct a deficiency) vitamin D2 is less potent.
Vitamin D3 specifically is the type produced in the skin following sun exposure, and both vitamin D3 and vitamin D2 can be obtained from a varied diet – although few foods naturally contain considerable amounts of vitamin D.
Vitamin D3 is found in some animal‐source foods and few plants, as well as fortified foods and many supplements. Vitamin D3 is also found algae and reindeer lichen – a symbiotic organism consisting of a fungus and an alga – which form the first link in the food chains for many animals.
Vitamin D2 occurs in some plants and fungi, and is less commonly used in supplements or as a fortificant.
How Much Vitamin D Do We Need?
The recommended daily vitamin D intake is set at 15 microgram per day (or 600IU), with an upper limit of 4000 IU/day. Because vitamin D is a fat-soluble vitamin, the absorption of vitamin D is more efficient when dietary fat is present. A supplement is therefore best taken before dinner / a big meal.
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