Want to stay sharp well into old age? Keep an eye on your waistline, suggests a new study from the University of Arizona. Having a higher body mass index (BMI) can negatively impact brain functioning in older adults, researchers say, and there’s evidence that inflammation is to blame.

Maintaining a healthy weight can protect against a variety of health issues; it can reduce the risk of cancer, heart disease, and diabetes, to name a few. Previous studies have also linked weight to brain health, but there’s been little research into exactly how one affects the other.

Figuring out that “how” could potentially help scientists develop interventions to better prevent cognitive decline, says Kyle Bourassa, a psychology doctoral student and co-author of the new study.

Bourassa and his co-author suspected that systemic inflammation—a chronic overreaction of the body’s immune system—might be to blame, since previous research has shown that inflammation in the brain can negatively impact cognitive functioning. It’s also well established that being overweight contributes to inflammation throughout the body. "The higher your BMI, the more your inflammation goes up," he says.

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To further explore these connections, Bourassa and his co-author analyzed data from more than 21,000 British people, ages 50 and older, who had their BMI, inflammation levels, and cognition scores tested several times over the course of six years.

BMI, a measure of weight in relation to height, is often used to determine whether a person is normal, underweight, or overweight. For individuals, BMI is not always an accurate measure of healthiness—but for large populations like this, it’s a good way to estimate averages. In general, a BMI of 18 to 25 is considered normal weight, and any number over 25 is considered overweight.

For the study, inflammation was measured by the presence of C-reactive protein (CRP)—a marker of systemic inflammation throughout the body—in the participants’ blood. Cognitive function, meanwhile, was measured with word recall and verbal fluency tests.

The researchers found a clear link between the three factors. “The higher participants’ body mass at the first time point in the study, the greater the change in their CRP levels over the next four years,” Bourassa said in a press release. That change in CRP then predicted a decline in brain functioning—including executive functioning and memory—two years later.

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In other words, the findings suggest that “the body mass of these people predicted their cognitive decline through their levels of systemic inflammation,” said Bourassa.

Co-author David Sbarra, PhD, professor of psychology and director of UA’s Laboratory for Social Connectedness and Health, cautions that the study was unable to prove a cause-and-effect relationship, since it simply monitored people over time. To establish causation, researchers would need to find a way to reduce participants’ body mass under tightly controlled conditions, and examine the subsequent effects on inflammation and cognition.

But the researchers say that their findings may provide valuable insights for further studies and possible interventions. "If you have high inflammation, in the future we may suggest using anti-inflammatories—not just to bring down your inflammation but to hopefully also help with your cognition," Bourassa said.

For now, it provides yet another reason to keep excess weight off. "Having a lower body mass is just good for you, period,” Bourassa said. “It's good for your health and good for your brain.”