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Published On March 8, 2018


How Poor Diet Shapes the Brain

A high-fat, high-sugar diet can cause harm to the hippocampus—and that may lead, perversely, to even worse impulses around unhealthy food.

Henry Molaison, famous in the annals of psychology, underwent surgery in 1953 that removed a large part of his hippocampus, which curtailed his ability to form new memories. An unusual appetite was one of his many notable characteristics. Presented with a plate of food, Molaison would tuck in even if he had finished a full meal minutes before. He just couldn’t remember having eaten.

Damage to the hippocampus doesn’t need to be as severe as Molaison’s to affect how people respond to food. New research suggests that changes to this area of the brain may affect decisions around what and how much to eat. Making matters worse, those threats to the hippocampus may come from poor diet—creating a vicious cycle that can intensify over a lifetime.

Dozens of animal and human studies have strongly suggested that a diet high in saturated fatty acids and simple carbohydrates—what researchers call a Western-style diet—can alter the function of the hippocampus, which is important for memory, learning and decision-making. Obesity also has an established link to changes in this brain region.

Terry Davidson, director of the Center for Behavioral Neuroscience at American University in Washington D.C., is looking into how those alterations occur. His published studies over the past decade suggest that a fatty, high-sugar diet leads to a more permeable blood-brain barrier (BBB). That leaky BBB is associated with both inflammation of brain cells and a shortage in the hippocampus of brain-derived neurotrophic factor, a protein important to neuron development and plasticity. A more permeable BBB also makes it easier for environmental toxins and heavy metals to damage the hippocampus.

One striking implication of this impairment, Davidson notes, is that it can lead to deficits in memory, learning and attention—and those, in turn, can play a major role in the way people eat. “The Western diet can cause changes to the brain that disrupt regulatory control. Among other behavioral changes, this may make it more difficult for people to stop eating reflexively,” he says. “This can contribute to even more intake of the Western diet.”

Suzanne Higgs, professor of psychology at the University of Birmingham in the United Kingdom, is also concerned about the role that an impaired hippocampus plays in responding to food cues. “A healthy hippocampus is important for inhibitory learning, which is what keeps us from always responding to the sight of food with hunger,” she says. “When you alter the hippocampus, it affects those immediate responses.”

Higgs says that this can manifest as a more habitual response to food, rather than one that involves a more conscious decision-making process that takes into account whether the person has eaten recently, and whether eating at that moment is consistent with their long-term health goals.

Other research shows that animals and humans who are fed a Western diet respond more readily to external cues to eat—seeing food—and less to signals that come from within their own bodies, such as satiety. In a study published in February 2017, researchers in Australia found that people who ate a sugary breakfast every day were less sensitive to hunger and fullness cues. They also performed worse in hippocampal-dependent learning and memory tasks, effects that kicked in after only four days.

Developing brains may be particularly vulnerable. A diet high in saturated fats has been associated with poorer performance in hippocampal-related learning and memory tasks in children under the age of 10. Further, the effects of these cognitive changes caused by a leaky BBB can snowball over a lifetime and may even lead to an excess of beta amyloid proteins circulating in the brain. Those proteins are a primary component of the amyloid plaque implicated in Alzheimer’s disease.

Indeed, Davidson says, new data shows a link between the breakdown of the BBB and the beginning of cognitive deficits and Alzheimer’s pathophysiology. “This part of our research seems to tie together two of the most pernicious problems that twenty-first-century humans face: obesity and cognitive dementia,” he says.

While it’s not clear whether an ineffective BBB can be repaired, animal studies suggest that exercise may reduce memory deficits and stabilize levels of brain-derived neurotrophic factor, Davidson notes. Higgs and other psychologists are also studying the effectiveness of cognitive training strategies that could help compensate for physical damage by making the act of eating more of a conscious process.

It may also be possible to train people to resist the mental and physiological urge to eat high-fat, high-sugar foods. Still, it appears that the surest way to stop the vicious cycle is certainly the hardest: Get people to stop eating junk food in the first place.