Mentioning vitamin D is a great way to start an argument, especially what is an optimum level for humans as measured in a blood sample. The controversy certain

Mentioning vitamin D is a great way to start an argument, especially what is an optimum level for humans as measured in a blood sample. The controversy certainly comes up frequently at the medical conferences I attend. The "evidence base" remains unsettled, and there's a wide range of interpretations of the ideal range for vitamin D levels or intake. The conservative recommended level is 20 ng/ml, but there is substantial opinion that the minimal level of 20 ng/ml is way too low, and that 40 to 60 ng/ml is consistent with historic human environmental conditions. Here's my take on the subject. Depending on where you live, it may be important or not. I agree with the comments here: the further north you reside, the greater the risk of inadequate body level of vitamin D and attendant health consequences. At my location just north of the 45th parallel, I order D levels frequently and about half of patients show D level <30. (Reference range: 30-100 ng/ml.) Low levels have been associated with many conditions, but I'd point out cognitive dysfunction and depressed mood as less widely known effects. Vitamin D is an enzyme co-factor at rate-limiting steps in pathways of CNS neurotransmitters synthesis. Negative effects on brain functioning are a logical consequence of D deficiency. Human populations indigenous to arctic regions have only a few months a year of enough UVB exposure for synthesis of D in the skin. Yet they have survived at far north locations for a millennium. How is that possible? Consider their diet, very high in fish and seafood, e.g., 100g of salmon contains around 350 IU of D, so easily intake >=2000 IU/day. We recommend D supplementation of 2000 IU/day and calcium 1g/day for most people who live north of 40th parallel, especially people living in rainy coastal climates. Vitamin D toxicity can develop with super-high doses of D. I've seen it a few times, but only with very high amounts (>10kIU/day) over months of excessive intake. At any reasonable amount, say 5kIU/day, toxicity is very unlikely. In any case it's easy to check blood level if a reason to suspect it and advise accordingly.