It seems like its easy to identify the harms of extremes. Too little of X causes Y and too much of W causes Z. But with so many variables affecting the human body(nutritional, genetic, environmental, microbial, mental, etc) how can we even approach being certain about optimal levels in the middle? I don't see how we could control for enough variables for long enough to know how much vitamin D is best for the body, and exactly how it interacts with other variables in regards to various diseases.