Davidson realized these rats didn't know they were full. He says something similar may happen in human brains when people eat a diet high in fat and sugar. Davidson says there's a vicious cycle of bad diets and brain changes. He points to a 2015 study in the Journal of Pediatrics that found obese children performed more poorly on memory tasks that test the hippocampus compared with kids who weren't overweight.
He says if our brain system is impaired by that kind of diet, "that makes it more difficult for us to stop eating that diet. ... I think the evidence is fairly substantial that you have an effect of these diets and obesity on brain function and cognitive function."
The evidence is growing. Research from the Cambridge Centre for Ageing and Neuroscience published in July found that obese people have less white matter in their brains than their lean peers — as if their brains were 10 years older. A more recent study from researchers at the University of Arizona supports one of the leading theories, that high body mass is linked to inflammation, which affects the brain.
But if we understand how obesity affects the brain and memory, then maybe we could use that relationship to prevent people from becoming obese in the first place.
Lucy Cheke, a psychologist at the University of Cambridge, says her study in The Quarterly Journal of Experimental Psychology this November gives her some idea of how to do that.
Her researchers asked obese and lean people to do a memory task that's a virtual treasure hunt. The subjects had to hide something in a scene across various computer sessions, then they were asked what they hid, where they hid it and in which session.
The obese people were 15 to 20 percent worse than lean ones in all aspects of the experiment. The finding confirmed what other researchers had already seen in rodents. "This really picks apart spatial, item and temporal memory, as well as, crucially, the ability to integrate them," which Cheke says is "one of the most fundamental aspects of memory."
If you're obese, she says, you might just be "10 to 15 to 20 percent more likely to not quite remember where you put your keys."
Diet isn't necessarily destiny. People can compensate. As American University's Davidson puts it, "Let's say I had a kid and I gave him a high-fat diet and he showed hippocampal dysfunction. That kid may not do worse in school."
But, Davidson adds, the processes that help the kid do well in school may be impaired. When that happens, the kid would have to work harder and be more motivated and would "have a tougher go of it."
Cheke says with the link between obesity and the brain growing as a field of research, we could see more ways of targeting obesity.
For example, if the issue is that the diet of obese people degrades their memory and makes them more likely to overeat, then maybe making their meals more memorable would help them eat less of the bad stuff. Cheke says there's already some research showing that if you watch TV while you eat lunch, you'll eat more and also be more likely to get hungry in the afternoon and later to eat more at dinner.
She says not watching TV while you eat is one of the "small easy changes that people can make that don't involve a lot of self-control and that don't involve a lot of sacrifices, but that can still make a significant difference into how much you're eating."
However, even though we are beginning to understand that obesity affects the brain, we don't exactly know how, says John Gunstad, professor and director of the Applied Psychology Center at Kent State University in Ohio.
He points out that obesity changes a lot about the body: blood sugar levels, the cardiovascular system, inflammation levels throughout the body. Any one of those things could affect the brain.
"Most likely, the effect of obesity on the brain is related to not just one cause but a combination of causes," Gunstad says.
Davidson is also moving forward by studying how to break the vicious cycle of a Western diet, obesity and brain changes. But he says the underlying idea that obesity affects the brain is clear.
"It's surprising to me that people would question that obesity would have a negative effect on the brain, because it has a negative effect on so many other bodily systems," he says, adding, why would "the brain would be spared?"
Alan Yu is a freelance reporter who also contributes to the South China Morning Post in Hong Kong. You can follow him on Twitter: @Alan_Yu039.
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