Why should you keep your vitamin D level around 50 ng/ml? Four different sources, using four different rationales, and four different lines of reasoning, all lead to the same conclusion.
First, what is the vitamin D level of our closest simian relatives, such as chimpanzees living wild in Africa? Professor Reinhold Vieth reports the answer is between 40 and 60 ng/ml. This, by itself, does not prove we need such levels, but it certainly raises that question.
Second, what is the vitamin D level of humans who work in the sun without clothes, such as lifeguards, and without supplementing? We lived in the sun for 2 million years, so certainly lifeguards have more natural vitamin D levels than do people who work indoors. Again, the answer is between 40-60 ng/ml. Here, we have stronger naturalistic evidence unless one assumes the vitamin D levels of indoor workers are natural.
Third, what vitamin D levels do women have to achieve to convert from having little to having lots of vitamin D in their breast milk? Professors Bruce Hollis and Carole Wagner recently answered that question, again 40-60 ng/ml, enough to sustain the infant’s vitamin D levels. One could claim breast milk is not supposed to have vitamin D in it, and that primitive man was supposed to expose newborns to sunlight. But then you would be arguing that primitive man was supposed to expose their infants to predators, which I find unlikely. Besides, we know from the second reason that any woman receiving consistent full body sun exposure would have vitamin D in her breast milk.
Finally, what is the vitamin D level of people who show no evidence of substrate starvation? That is, at what level do people begin to store the parent compound (cholecalciferol) in their fat and muscles? Professor Robert Heaney answered that question: around 40 ng/ml. I remember seeing several patients in the hospital who had vitamin D levels of 40-50 ng/ml in February. Both had worked as roofers the summer before and both had worked with their shirts off. The mechanism for humans who migrated away from the equator must have been the same, to store the parent compound in muscle and fat during the summer for use in the winter. The body stores it well before it turns on the enzymatic machinery to get rid of excess vitamin D.
So we have the above four questions, questions from four very different sources. Chimps, outdoor workers, lactating women, and clinical subjects all lead to the same answer: 40 ng/ml is the lower limit of a natural level. Taking into account errors in laboratory testing and variations in human techniques, we must accept what the Endocrine Society recently recommended, that healthy vitamin D levels are somewhere around 50 ng/ml, levels the Vitamin D Council has advocated for the last 8 years.