A commenter noted how this theory might be similar to those of Gary Taubes. I agree, and as I thought about this theory, I thought about how this theory is Taubes-like.
Taubes wrote that weight gain was a function of hormones. In his view, carbohydrate intake changed the hormonal balance which led to weight gain. Overall, I think his theory is flawed, but the same type of principle applies here: weight gain is hormonally controlled, except that I'm saying it has to due with the body reacting to seasonal factors.
Let's compare the two theories. Taubes paints carbohydrates as the villian, even though there are cultures who eat high-carb and who are not overweight. Even in the U.S., if you go back to the year 1909, Americans were eating 500 grams of carbohydrate per day. If you look back at old photos from that period, I don't see too many overweight people.
With the winter metabolism, you possibly have a mechanism that has been around for millions of years. With a normal food supply and sunlight, the mechanism worked as intended. With a variety of factors in the modern diet (which I will further detail) and a continual lack of sunlight, the mechanism may be running virtually all year round.
