To be fair, conducting a randomized clinical trial of vitamin D supplementation may be difficult. This is because most of us get most of our vitamin D from sunlight. Our skin converts UVB rays into a form of the vitamin that our body can use. We also get it from our diet, but not in large quantities. (Main sources include fatty fish, egg yolks, mushrooms, and some fortified cereals and milk substitutes.)
The standard way to measure a person's vitamin D levels is to look at blood levels of 25-hydroxycholecalciferol (25(OH)D), which is formed when the liver metabolizes vitamin D. But not everyone can agree on what the “ideal” level is.
Even if everyone agreed on the figure, it is not obvious how much vitamin D a person would need to consume to reach this goal, or how much exposure to sunlight it would require. One complicating factor is that people react differently to UV rays—this can largely depend on the amount of melanin in your skin. Likewise, if you sit down to fatty fish and mushrooms and wash them down with a glass of fortified milk, it's hard to know how much more you might need.
There is more consensus regarding the definition of vitamin D. flawAlthough. (In case you're wondering, that's a blood level below 30 nanomoles per liter.) And until we learn more about what vitamin D does in our bodies, we should focus on avoiding it.
For me this means taking supplements. UK government advises All residents of the country should take a vitamin D supplement of 10 mcg in the fall and winter. This advice does not take into account my age, my blood levels, or the amount of melanin in my skin. But that's all I have at the moment.
