It's well known that being overweight or obese increases the risk of many health issues, including high blood pressure, high cholesterol, type 2 diabetes, heart disease and stroke. But the impact body weight can have on cognitive function gets less attention.
For years, scientists have been trying to understand how body weight – particularly higher body weight – affects the brain. Many studies have found that obesity at all stages of life impacts brain health, leading to declines in the executive functioning needed to plan and carry out tasks, and a higher risk of dementia.
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Research has linked a higher body mass index in young adults with a poorer ability to remember past events. By middle age, the accumulated damage puts people with obesity at higher risk of dementia than their peers, according to the American Heart Association. Research also has linked a higher BMI to lower blood flow in the five regions of the brain that can be affected by Alzheimer's disease.
The most recent study, published Tuesday, found similar patterns of shrinkage in the brain regions responsible for learning, memory and judgment among middle-aged obese people and Alzheimer's patients. This was the first research to make a direct comparison.
The study, which analyzed brain scans from more than 1,300 people, found that both people with obesity and Alzheimer's patients experienced a thinning of the cerebral cortex, the area of the brain that controls speech, perception, long-term memory and judgment.
The scans can't show that obesity causes the thinning of the cerebral cortex, but the study's findings suggest that controlling body weight might reduce the risks of brain degeneration, Sabrina Diano, director of the Institute of Human Nutrition at the Columbia Irving Medical Center, told NBC News. She was not involved in the study.
"We know that if you take a mouse that has a genetic predisposition to develop Alzheimer's and you put that mouse on a diet rich in carbohydrates and fat – similar to the Western diet – you can induce increases in body weight in the animal and as they gain weight, cognitive impairment and the brain degeneration is accelerated," Diano said.
Many scientists believe that losing weight can reverse some damage done to the brain, but some studies suggest there may be a point where the damage is irreversible and the focus needs to be on preserving the cognitive abilities remaining.
One large study found no difference in cognitive performance between people with obesity who participated in a weight loss program and those who did not. However, the researchers noted that all the participants had type 2 diabetes, which is also known to impact brain function.
It's not exactly clear how obesity can cause the loss of brain cells, but scientists believe chronic inflammation plays a role. Obesity-related inflammation in the body breaches the blood-brain barrier between the periphery and central nervous systems, causing neuroinflammation.
But other conditions associated with obesity, including high blood pressure, high cholesterol and type 2 diabetes, also can damage blood vessels in the brain and lead to cell death. This makes identifying direct causal links more difficult. And some studies suggest that lower cognitive functioning leads to poor eating behaviors – not the other way around.