I saw this new study which hypothesizes that Vitamin D deficiency is the cause of common obesity. The idea is that a lack of sunlight (and therefore a lack of Vitamin D) triggers a "winter metabolism" in humans. This leads to increased fat accumulation until the return of sunlight in the spring.
I read through the paper, and it's pretty far out there. There is a lot of explanation for how this mechanism works, but not much in the way of data. It does mention how there is often an inverse correlation between Vitamin D status and obesity, which previously researchers thought was the obesity leading to the lowered Vitamin D status. Now it's hypothesized that it's the other way around.
At the very least, it does suggest that Vitamin D deficiency may play a role in weight gain. It's just another good reason to make sure children and adults get adequate sunlight or take Vitamin D supplements.
