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

I think the best way to understand why is to start with the fact that storing excess glucose as fat isn’t an actual decision in the first place. It’s simply the result of chemical reactions. Many innate behavior patterns are pretty much just the animal taking advantage of what chemical reactions their body is capable of performing.

So to answer your question, fat will continue to accumulate because there is no mechanism to stop it if there continues to be excess carbohydrate consumption. If blood glucose is above a certain level in the vessels that pass by pancreatic beta cells, insulin is released. Insulin in the blood above a certain level blocks glucagon release. If there is no insulin, then glucagon is constitutively released. Insulin triggers a series of reactions that lead to eventual production of fat. Glucagon leads to breakdown of fat into more glucose. Notice how this system doesn’t leave any room for simple excretion of the excess.

To add on to this, normal kidney function at normal blood glucose leads to complete retention of all glucose (it gets filtered out of the blood into the nephron and then reabsorbed down the line). You do end up with (Edit: ~~urine in the blood~~ oops) sugar in the urine if you have diabetes as the reuptake transporters get completely saturated. But this only happens because the normal balance between insulin and glucagon has been disrupted. We would also rather have normal insulin function than this because excess glucose can be very damaging, including things like kidney disease, eye disease, nerve disease, etc. High blood sugar will directly kill you so blocking the effects of insulin for the sake of stopping fat production is not an option.