Quite some time ago, I mentioned the fact that one of the reasons humans need about 2000 calories a day to maintain their bodies is because that is what evolution resulted in. Neanderthals, on the other hand, required 3000 calories per day and one of the reasons for that branch of humanity's perishing was the difficulty in acquiring so much energy on a regular basis. It was a lot harder to eat 3000 calories per day than 2000 when man was struggling to survive.
Lately, I've been reading blogs and comments by people who mention that even on relatively low-calorie diets that they lose little weight. A lot of people who are of average weight or who are losing weight cast a doubtful eye in the direction of such folks. They believe that there is something such people are doing "wrong". They aren't eating the correct foods. They aren't counting calories properly. They don't exercise enough. They have an undiagnosed metabolic issue like a thyroid condition, insulin resistance, etc. Doubters can't accept that it is possible for a person to eat fewer than 2000 calories and not lose weight.
A big part of the reason for this doubt is that people have a vested interest in believing in "personal responsibility." They want to believe in a "just world" where correct actions are rewarded. If you work hard to lose weight, justice would dictate that you will lose weight. To not embrace the notion of a just world would cause people to feel anxious about the consequences of their choices and actions. If they have to embrace the idea that people may work hard and do their best to lose weight yet still remain fat, they fear that they may some day put in the effort and similarly fail. The ultimate outcome of "just world" thinking is that victims are blamed so that those who sit in judgment of them can feel better about the way things work in the world.
Evolution would seem to suggest that it is unlikely that, when it comes to weight, that the world operates in a just manner. We can assume that we are all products of various mutations of humanity. Some of us have genes that may fall within the range of "normal". That means that we need about 2000 calories per day to maintain a relatively healthy weight. People who eat a lot and whatever they want may have "won" the genetic lottery from a modern viewpoint and can maintain weight on far larger calorie allotments.
Considering that we all have different genes and are descended from different lines with sometimes relatively little variation in genetic mixes, it is certainly possible that some people have "lost" the genetic lottery from a modern lifestyle viewpoint. These people could have had ancestors who could survive on substantially fewer calories than the average human. That is, they might be capable of sustaining their weight with as little as 1500 calories (or even less). The ancestors of such people would have been fortunate creatures indeed, and have had an even better chance for survival and passing on their genetic material than those with higher calorie needs.
This theory should not be so far-fetched. After all, it is a fact that larger, heavier human sub-species required more calories for their survival. There must be broad variations in basic calorie requirements based on genetic history that could operate relatively independent of bone structure and height. People who are vastly atypical in terms of calorie needs likely fall outside of the mean on the bell curve. They may be one or even two standard deviations from what is statistically "normal", but they surely do exist, and their claims about their struggles should not be dismissed by those who need to believe in a just world for their own selfish reasons.