David, I think you are right in a sense. Serum levels DO seem to be associated with the risk of several disease, including cardiovascular ones, colorectal and breast cancer, rheumatoid arthritis, multiple sclerosis, poor bone health, macular degeneration, depression, and more (even as multivariable analyses attempt to control for co-covariables such as smoking and obesity). What remains unclear is whether supplementation helps. I think one challenge is that these diseases may take a decade or more to develop, and so clinical trials are challenging. A February week in Hawaii for me buys me 4 months of normal vit D levels (and then the Seattle sun emerges)! While I am not obese, I am older (and with darker skin and work inside). Have we gone too far in avoiding sun completely (for skin CA risk)? Another challenge is looking at the genetics of vitamin D creation (try getting a drug company to pay for that study). Thank you for your thoughts. I do not disagree and have written about latitude and health historically.