When measuring blood levels of vitamin D (as 25-hydroxy vitamin D), what constitutes a desirable level?
There's no study that directly examines this question, no study that enrolled thousands of people and assigned a placebo group and groups receiving escalating doses of vitamin D and/or achieved higher levels of vitamin D, then observed for development of cancer, diabetes, depression, heart disease, multiple sclerosis, osteoporosis, osteoarthritis, etc. Such a study would requires many thousands of participants (particularly to observe cancer and multiple sclerosis incidence), many years of observation, and many tens of millions of dollars. Nope, only a drug company could afford such costs.
So we have to piece together various observations and extrapolate what we believe to be the ideal level of vitamin D. Epidemiologic observations in several cancers (breast, colon, prostate, and bladder) suggest that a 25-hydroxy vitamin D level of 30 ng/ml or higher is desirable (with less cancer incidence above this level). Other data suggest a level of 52 ng/ml or greater is desirable. Unfortunately, much cancer research looked at intake of vitamin D from food and supplement sources, rather than actual blood levels. We also have to factor in the great individual variation in vitamin D metabolism, with a single dose yielding variable blood levels (as much as a 10-fold difference). There's also the variation introduced by vitamin D-receptor variation (genetic polymorphisms).
A new study using vitamin D administration helps chart the desirable levels of vitamin D.
In this New Zealand study, 42 women (23 to 68 years old) were given 4000 units vitamin D, 39 women given placebo. Median 25-hydroxy vitamin D levels increased from 21 nmol/L (8.4 ng/ml) to 75 nmol/L (30 ng/ml). Both HOMA (a measure of insulin sensitivity) and fasting insulin levels improved, with greatest improvement seen at 25-hydroxy vitamin D levels of 80-119 nmol/L (32-47.6 ng/ml) or greater.
We also know that a vacation on a Caribbean beach in a bathing suit will increase vitamin D blood levels to the 80-110 ng/ml range without ill-effect (at least in young people who maintain the capacity to activate vitamin D in the skin, a phenomenon that declines as we age).
So do we really know the truly ideal level of vitamin D to achieve? I believe that, given the above observations, it is reasonable to extrapolate that the ideal vitamin D blood level likely lies somewhere above 50 ng/ml. We also know that vitamin D toxicity (i.e., hypercalcemia) is virtually unheard of until vitamin D blood levels approach 150 ng/ml, and even then is inconsistent. The health benefits of vitamin D supplementation are so tremendous, that I am not willing to wait for the prospective data to explore this question fully. For now, I aim for a blood level of vitamin D of 60-70 ng/ml (150-175 nmol/L).