Older people (greater than age 50) have a higher risk of developing vitamin D deficiency. The ability of skin to convert 7-dehydrocholesterol to pre-vitamin D3 is decreased in older people. The kidneys, which help convert calcidiol to its active form, sometimes do not work as well when people age. Therefore, many older people may need vitamin D supplementation.
Newborn infants who are exclusively breastfed may require vitamin D supplements. Breast milk does not contain significant levels of the vitamin, and although infants could receive this vitamin from sunlight, it is usually not recommended that small infants be exposed to sunlight in the levels required to produce a sufficient amount of vitamin D. Infant formula is generally fortified with vitamin D, so this requirement only applies to breastfed infants.
Those who avoid or are not exposed to the midday sunshine may also require Vitamin D supplements. In particular, recent studies have shown Australians and New Zealanders are Vitamin D deficient [1], particularly after the successful "Slip-Slop-Slap" health campaign encouraging Australians to cover up when exposed to sunlight to prevent skin cancer. Ironically, a vitamin D deficiency too may lead to skin cancer.
Recent research suggests that adult human beings can handle up to 5,000 international units (IU) per day long-term, although the metabolic pathways have not yet been identified. Given this, and given that humans on vitamin pills of 200 IU per day become deficient if not exposed to sunlight there is reasonably convincing evidence that the Recommended Dietary Allowance (RDA) of 200 IU (5 micrograms) may be rather too small. The fact that human skin produces such large quantities as 20,000 IU without any apparent toxicity also gives strong support for this position.
In addition, merely being exposed to sunlight does not guarantee that vitamin D is produced, only the UVB in sunlight triggers vitamin D3 production; UVB only reaches ground level when the sun is very high in the sky. At many lattitudes this only occurs in the summertime close to solar midday (1 pm summertime). In the US, those living above a line from San Francisco to Atlanta will not be able to produce it at all for 3 to 6 months a year. Still, only a few minutes of exposure is all that is required; the production is very rapid.
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