For most of human history, the "natural" level of vitamin D in the body was likely 50 ng/ml or higher. Nowadays, many people have but a fraction of that, 10 or 15 ng/ml.
We don't get as much sun exposure as we used to, and when we do, we often slather on sunscreen, which, if used as advised, reduces vitamin D synthesis in skin by 99 percent.
The currently recommended amount of vitamin D for adults up to age 50 is 5 micrograms, or 200 IU daily. For those 51 to 70, it is 10 micrograms, or 400 IU. But those amounts were set before scientists understood the multiple biological roles of D and the enormous variation in how much we make, which depends on where we live, the season, time of day, how much of our body is exposed, and our skin pigmentation.
Those living at higher latitudes make much less of the vitamin. Fair-skinned individuals generate abundant vitamin D with short periods of exposure; people with darker skin can require six to 10 times as much time in the sun to make the same amount. This is because melanin effectively absorbs UVB, the very rays that trigger vitamin D production. Sunscreen has the same effect.
Harvard's Edward Giovannucci sees a "high probability of benefit" and "low likelihood of harm" from vitamin D supplements but points out that slugging 800 IU a day would still not raise blood levels to 30 ng/ml. Cholecalciferol (vitamin D3) is more effective than ergocalciferol (D2), he says.