Let's consider, though, why humans can become fat. The body stores energy in the form of fat for later conversion to sugars in the event of insufficient sugars to keep the body functioning. Sugar is only part of what the body needs to keep functioning (we also need amino acids we can't synthesize except by deconstructing muscle, trace amounts of vitamins and minerals, and water), but it seems to be the most critical since our bodies are very good at storing it. To me this suggests that outright starvation is, evolutionarily speaking, a far greater threat than lack of access to particular essential comestibles. So our bodies are doing the right thing by making us fat because it protects us against the very real and serious threat of starvation.

That is the standard argument. Maybe it is true. But has it actually been tested? Is it observed that when famine strikes, the fat survive best, or that people with a tendency to fat in times of plenty survive better than the naturally thin in conditions of chronic scarcity? A lot of things with obvious explanations turn out not to be true, and there's surely enough famine in the world to study.

After all, if the standard explanation is true, then we do know how people can make themselves thinner: eat as if food were in chronically short supply. This does not tell us how to make people thinner, short of concentration camp conditions, which certainly did work --- none of the liberated prisoners were fat. But outside of such extremes, making people do anything is difficult.

See also Intermittent Fasting (eat only every other day), reputed to also extend life.