When I look back, I suspect that I embraced the so-called "Thrifty Gene Hypothesis" for the whole of my adult life. How I regret that now!
TGH tells us that we are built to put on extra weight when food is plentiful so that we might survive the periods of famine that were common when Homo sapiens first arose. If true, the so-called "obesity epidemic" is just what we should expect. Today food is plentiful, cheap and easily obtained. The Thrifty Gene drives us to eat, and we put on the fat that in a time of famine would prove so advantageous. But since we today experience no times of famine, we continue to accumulate fat throughout the whole of our lives and thus make ourselves vulnerable to the so-called diseases of civilization.
On TGH, the so-called diseases of civilization are really diseases of abundance; wherever we find long periods of caloric abundance, there too we will find the diseases of civilization.
The TGH, though perhaps plausible at first glance, is certainly without ground. In fact, it is most likely false. (My objections are not the only ones possible.)
1. History presents us with many examples of peoples who lived in a time and place of caloric abundance but were not fat. Let us consider one that we all know - mid 20th century United States. It was a time of caloric abundance. Food was cheap and universally available. Yet we were not nearly so fat as we are today. This is a counter-example to the TGH.
2. Peoples exist today who have more than enough to eat yet never grow fat. The Masai are but one of many examples. (They are a favorite example of mine. They are such a beautiful people - fit, healthy, lean and happy. They are a model of what the human animal is supposed to be.) Please, do research them. They eat well (at least those that still adhere to the traditional ways) but they do no grow fat.
3. The Thrifty Gene (if it exists) makes us sick in times of plenty. Thus it would be selected for only if the benefit it bestows outweighs the harm it does. The benefit (allegedly) is the ability to survive longer in times of famine. Here's the problem: we don't know what our species arose in a time when famine was common. Indeed we likely arose in a time of plenty. We are hunters by nature, and game was plenty.
4. Moreover, TGH seems to imply that we would always be sick. In times of plenty, we would grow fat and become sick. In times of famine, we would be sick. (Famine is mass starvation. Starvation ravages the human body.) But I find it implausible in the extreme that we are sick by design.
5. Finally, my last objection, the one that I think most important. TGH gets the human animal dead wrong. We are made to be lean and fit. This is our natural state, and any deviation from it implies pathology. Let the point sink in. (It took a long with me.) If you're fat (as I was for such a very long time), that's pathology. If you're always tired, that's pathology. If you are chronically hungry or if you chronically overeat, that's pathology. What is the cause of that pathology? From where does it come? For the great majority of us, it comes from malnutrition. We eat foods that, if made staples, are poisons. These are the high-carbohydrate, micronutrient poor foods. These are the bread, rice and potatoes. These are the beers and wines. These are the sugars.
Let me put the point this way. Proper weight is as much a part of homeostasis as is temperature. The body seeks to maintain proper weight, and it will if it's able. When we consume foods high in carbohydrate, we render the body ever less capable to maintain proper weight (and we make it ever more susceptible to the diseases of civilization). Feed the body right, and it will of itself bring the weight down; and it will do this whether caloric intake is more or less than calories used. The body will adjust. If caloric intake is restricted, metabolism will be retarded. If caloric intake is greater than needed, metabolism will be ramped up. The body will keep itself lean and fit if only it's allowed to do so.
And why would it do so? Why would this be favored by evolution? We are hunters. We are made to run, to lift, to twist and to jump. We are made to sweat. The body will do its damnedest to keep us ready to do these things. All we need to do is let it.