The fact that GLP-1 seems to have roles not just in satiety but that agonists seem to reduce other types of impulsiveness (e.g. gambling, shopping) is interesting. That's not something you'd predict as a consequence, and perhaps is downstream of some gut-brain connection.

Of course we already manipulate brain chemistry in other more direct ways with antidepressants so perhaps any unwanted second-order effects could be minor in comparison to the profile of existing antidepressants .

> Typically, if something "works", there often appear to be side effects

Unless it’s literally a deficit. There isn’t a downside to treating vitamin C deficiency with vitamin C.

> But if the changes it causes are universally good, why didn't evolution already make your body work that way?

That’s not how evolution works.

> But if the changes it causes are universally good, why didn't evolution already make your body work that way?

We evolved in an environment where every bit of food took hours of effort and food preservation was impossible, so the only logical thing to do with extra food was feast and store up as much fat as possible for lean times. We're still many generations away from evolving to compensate for the discovery of fire, let alone everything that came after that.

There are people with more or less sensitive GLP-1 receptors today, who have lesser or greater impulsivity. If some event occurs where only those with lower impulsivity survive, then the future population will have more sensitive GLP-1 receptors.

This is the only way that happen - noticeable evolution is always driven by population bottlenecks or strong selective pressure. In the absence of those, mutation just keeps expanding the gene pool so more different candidates are available for the next bottleneck.

In case an event happens where only red-haired people survive, good thing some are available, otherwise there's no reason to think everyone will have red hair in the future.