Genome Wide Association (GWA) Study for Early Onset Extreme Obesity Supports the Role of Fat Mass and Obesity Associated Gene (FTO) Variants. Even if this is true, these correlations between particular alleles and obesity hold for the modern German lifestyle. I guarantee you that population level diversity in weight correcting for height was sharply attenuated when all Germans were basically farmers and laborers. It seems possible to me that in pre-modern times “obesity alleles” might have been selected for something different in an environment where gaining a lot of weight and becoming subject to higher risk of various chronic diseases was not a plausible outcome. With the change in environment the whole phenotypic landscape shifted as the environment in which the genes expressed was radically altered.
Note: I’m skeptical that obesity alleles mark a much more efficient metabolism. After all, if LCT can nearly fix in northern Europe why not genes which allow you to more efficiently convert food into energy would not have been swept to fixation long ago? I suppose this could be balancing selection, but there’s only so much of that a genetic architecture can support.