Here's an interesting excerpt from Men's Health article called "How Being Fat Makes You Stupid":
Packing on the pounds isn’t just bad for your body—it’s bad for your brain. According to researchers at Carnegie Mellon University, the brains of obese people work harder than those of normal weight people to achieve the same results.
Researchers quizzed 29 people who were having fMRIs and noticed a trend: The connections between the brain parts responsible for memory and decision-making were hyperactive in overweight people, but functioned normally in average weight people, says study author Timothy Verstynen, Ph.D., a professor at Carnegie Mellon University. That’s because high blood pressure and inflammation—which go hand-in-hand with obesity—irritate your brain’s communication systems, making it harder for messages to come through, says Verstynen. It’s like your brain is trying to make a call when the phone lines are down.
And it’s a vicious cycle: The damage is particularly bad in areas responsible for controlling impulsive behaviors, like skipping dessert, says Verstynen. So your brain is making you fat (giving into the dessert), and then being fat is changing your brain, he adds.
The whole idea of being fat and stupid may or not be possible. Like every scientific journal there's always something erroneous that can get punched in. It's best to keep looking for other scientific journals from the experts. I don't want to conclude anything either but it seems that there's a connection between them. Since the heart can't pump blood properly because it's overworked because the person is fat then wouldn't that also mean the brain doesn't get enough oxygen in the blood to function properly? If the brain lacks oxygen then it can't think properly which may contribute to why some overweight people can't think properly. Take note that it doesn't always guarantee that a fat person is guaranteed to be an idiot. Some of them may just take longer to learn. But the problem might be if the person's already delusional and fat at the same time.