Obesity decreases IQ.
The effects on obesity on physical health is widely known; however, we're starting to learn how it affects the human brain, too.
psychcentral.com
It's less healthy on the body, including the brain. Typically the cause of obesity is unhealthy habits, such as poor sleep, processed foods, insufficient exercise, lack of vitamin D and sunlight (sunlight is an appetite suppressant along with other things that decrease obesity), etc. These also damage the function of the brain.
Worse function of the brain = Worse decisions = Worse society. You're contributions to society or lack there of effect every one.
Society is not a zero sum game. The contributions of everyone can benefit every one. With all the people who could have been pushing science and technology forward much faster, who were instead obese, we could have had robots doing all the work by now, a cure for aging, robot wives that cook and clean. We could be living in a relative paradise.
Low IQ also makes you susceptible to scams and being mislead by companies. This financially incentivises evil corporations to do evil. This means that corporations are more likely to be evil and exploit human beings, because that's what's being incentivised.
So sure, it's not a direct cause but the hell hole dystopian we live in was largely caused by obese people.