The concept of vitamin D as a nutrient arose during the early part of this century, when small quantities were detected in butter, at a time when many of the micronutrients were being discovered (1). However, it soon became clear that in nature cholecalciferol is supplied principally through the action of ultraviolet light on skin (source of about 90% of the body's vitamin D) (2).
- Skin synthesis:
- approximately 90% of the body's vitamin D comes from the action of UVB (290-320nm) sunlight on 7-dehydrocholesterol in the skin, which results in the formation of vitamin D3 (cholecalciferol)
- a reduction in synthesis is caused by a lack of exposure to sunlight, due to extensive covering with clothing, failure to venture outdoors, or the use of anti-UVB sunscreens
- in the winter, very little UVB radiation reaches the Earth's surface
- this results in, little or no vitamin D being synthesised in the skin of people living at latitudes above or below about 35º (such as in the UK)
- in the UK, in those with fair skin, exposure (for about 15 minutes) of the hands, arms, face, or back to suberythemal doses of sunlight from April to September, two or three times a week, results in sufficient skin synthesis of vitamin D (1)
- people with darker skin may need longer exposure than those with white skin to synthesise the same amount of vitamin D in their skin
- vitamin D can be replenished by sun exposure without burning and no sunscreen use (2)
- another review has noted that 5-15 minutes daily sun exposure between 10am and 3pm from April to October would be sufficient to improve vitamin D status (2)
- in a fair skinned person, 20-30 minutes of sunlight exposure to the face and forearms at midday generates about 2000 units of vitamin D (2)
- UK guidance on skin cancer prevention emphasises the importance of appropriate sun protection, including use of sunscreen and spending time in the shade between 11am and 3pm
- two or 3 sunlight exposures per week are sufficient to achieve vitamin D levels in the summer if individuals have adequate levels to begin with (2)
- however, those with a deficiency, pigmented skin and the elderly need increased exposure time or frequency to get the same level of vitamin D synthesis
- sun exposure should be avoided if someone has a history of skin cancer, or conditions such as xeroderma pigmentosum or actinic keratosis
- the National Diet and Nutrition Survey results from Years 1 to 4 (combined) of the rolling programme for 2008 and 2009 to 2011 and 2012 (Public Health England and Food Standards Agency) shows that vitamin D status is highest among all age groups in the summer months and lowest in the winter
- for example, only 8% of adults aged 19-64 had a low vitamin D status in July to September, compared with 39% in January to March. Similarly, around 2% of children aged 4 to 10 years had a low vitamin D status in July to September,compared to 32% in January to March (2)
- from mid-October to the beginning of April in the UK there is no ambient ultraviolet sunlight of the appropriate wavelength for skin synthesis of vitamin D. During this time, the population relies on both body stores from sun exposure in the summer and dietary sources to maintain vitamin D status (2)
- Diet:
- there are in fact very few natural foods which contain substantial amounts of vitamin D in either the D2 or D3 form. They include:
- fish-liver oils
- egg yolk
- fortified milk
- oily fish such as salmon, sardines, herring, mackerel, trout, fresh tuna
- ergosterol within plant matter can be converted to vitamin D
- average adult daily diet in the UK provides only 3.7mcg of vitamin D for men and 2.8mcg for women (10mcg = 400 units)
- excellent food sources (greater than 5mcg per portion) of vitamin D include 2 teaspoons cod liver oil, 70g sardines, 100g tinned salmon, pilchards or tuna, 110g of cooked mackerel or herring and 130g cooked kipper (2)
- these intakes alone, in the absence of skin synthesis, will not provide optimal vitamin D status
- in most developed countries, including the UK, some foods such as infant formulas, margarine and breakfast cereals are fortified with vitamin D
- liver and liver products are good sources of vitamin D, but are rich in vitamin A, so should not be eaten more than once a week, and should be avoided by pregnant women, due to the risk of vitamin A toxicity (1)
Notes:
- children from age 1 year and adults need 10 µg of vitamin D a day; this includes pregnant and breastfeeding women, and people at risk of vitamin D deficiency
- babies up to age of 1 year need 8.5 to 10 µg of vitamin D a day (3)
- to convert doses in international units to micrograms, divide by 40.
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