I wonder if they controlled this for activity levels? The higher the BMI, generally, the more sedentary the individual. I know I went outside more when I was thinner and more active.
Either way, fascinating research on our ability to convert and metabolize the vitamin breaking down with obesity. I wonder if chemical or hormonal interference is the culprit?
Living so far north and being a house mouse I supplement myself liberally with vitamin D, as well as working at getting lots of dietary sources of it. I supplement my kids for the same reason. It's really necessary when there is only sunlight a few hours a day, and it never gets decently above the horizon