eli5: Why will the body continue to store excess fat to the point where it jeopardizes the person’s health?

Question

I understand food wasn’t always so abundant, and humans were well served to store excess consumed energy as fat for later use. What I don’t understand is why the body keeps storing fat to the point where a person becomes morbidly obese and it puts their entire health at risk. Why isn’t there a point where the body just let’s the extra calories pass through without saving for later?

In: 1194

Answer 1

Just very, *very* basic evolutionary biology here.

Evolution doesn’t choose anything. Natural selection is a dumb phrase.

Instead, it’s better to think of it as “if a mutation causes an individual to be worse at surviving, the mutation won’t carry on to future generations.”

Except that’s not true either. Cardiac disease, dementia, and liver failure tend to be fairly common, so shouldn’t they go away?

Well, no. Because the important bit is “carry on to future generations.”

If your family has Alzheimer’s, it’s likely not going to impact how many kids you have (and thus how common it can be in society) because it doesn’t make it harder to reproduce.

As humans live longer, as medicine solves other problems, cancer increasingly becomes a common cause of death. Cancers we don’t have much experience treating, because they don’t really happen in young and healthy people. And there’s no genetic pressure *not* to get these cancers, because they’re typically not happening early enough to interfere with reproduction.

Any disease that causes reproductive issues (physical ailment, sterility, or obviously death) before an individual can reproduce doesn’t carry forward.

So onto body fat.

Excess body fat has no biological “stopper” since it doesn’t (biologically, ignore social aspects) impact reproduction. It’s *possible* for a person to be extremely obese through childhood which causes reproductive difficulty into adulthood, but the last few decades have been the only time in the development of our species where there was enough surplus food for that even to be possible. The majority of people that get very obese won’t have severe complications until after childbearing age.