UK researchers recently proposed 10 to 15 minutes of unprotected exposure to the midday sun as a good source of the vitamin. In the US, where over 1.5 million people are diagnosed with skin cancer every year, experts are pushing supplements, claiming recommendations for sun exposure are "highly irresponsible".
Some studies have shown that sunshine levels in some northern countries are so weak during the winter months that the body makes no vitamin D at all, leading some to estimate that over half of the population in such countries have insufficient or deficient levels of the vitamin, prompting some people to recommend tanning beds.
In the new review published in the Journal of the American Academy of Dermatologists (on-line, doi:10.1016/j.jaad.2005.11.1057), Deon Wolpowitz and Barbard Gilchrest from the Boston University School of Medicine wrote: "At noon in June in Boston, a fair-skinned individual will maximize his or her vitamin D photosynthesis in well less than five minutes, and additional sun exposure will produce only photageing."
From an evolutionary point-of-view, the trade-off between obtaining vitamin D from sunlight exposure and the effects of photoageing and skin cancer was sensible since lifespans were not very long. It does not make sense, the scientists argue, in a society where life expectancy has doubled and thirty per cent of Caucasians will develop skin cancer.
"Fortunately, there is a noncarcinogenic alternative - intestinal absorption of vitamin D fortified foods and/or dietary supplements," said Wolpowitz and Gilchrest.
The authors also discussed calls to increase the RDA for the vitamin. A study from the University of California Moores Cancer Research Center said the RDA should be increased from the current 400 IU per day for adults to 1000 IU per day. Other investigators have called for a RDA of 2000 IU per day.
The study is yet more music to the ears of supplement makers, coming quickly after reports linked vitamin D to lower risks of certain cancers and osteoporosis. Vitamin D sales have rocketed in the UK with some retailers reporting 400 per cent increases in supplement sales.
However, British charity Cancer Research UK played down the use of supplements. Sara Hiom, head of health information, told NutraIngredients.com: "In general, most people should be able to achieve a healthy balance that enables them to get enough UV radiation to make adequate Vitamin D but not enough to increase their skin cancer risk."
Hiom suggests that supplements may only be necessary for people who don't get adequate sun exposure, such as elderly people who are house bound or women who cover most of their skin for cultural reasons.
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former is produced in the skin on exposure to UVB radiation (290 to 320 nm) and can also be consumed from oily fish. The latter is derived from plants and only enters the body via the diet.
Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D, the non-active 'storage' form, and 1,25-dihydroxyvitamin D, the biologically active form that is tightly controlled by the body.