Being overweight is certainly risky for your physical health, but new evidence suggests that it may carry an added mental-health burden as well.
Studies have linked overweight to a host of health problems, including heart disease, diabetes, hypertension and stroke, but research led by Dr. Diana Kerwin at Northwestern University now shows that extra weight may also contribute to lower cognitive performance. Culling data from the Women's Health Initiative, a long-term, multicenter study of postmenopausal women between the ages of 65 and 79, Kerwin's team found that for every one-point increase in body mass index (a ratio of height and weight used to measure overweight and obesity), study volunteers experienced an accompanying one-point drop in cognitive performance scores.
Although the women's memory scores were within the normal range, heavier women still did worse than their thinner peers.
Kerwin says she was surprised to discover that the association persisted, even after controlling for factors such as hypertension, stroke and diabetes. Initially, she had expected to find that obesity's effect on cognition was mediated by heart and circulatory pathways such as blood flow and blood pressure. "Our results tell women who are obese that they shouldn't feel falsely confident that it's okay if you are obese even if you don't have high blood pressure or diabetes," says Kerwin, who published her results in the Journal of the American Geriatric Society. "Just being obese could still affect your cognition as you get older."
The authors speculate that obesity itself may affect the brain, and that the mechanisms may include genetic factors that predispose people to obesity, or hormones such as cytokines and estrogen that are released by fat tissue.
In fact, Kerwin's team is already seeing some hints that estrogen may play a role in cognition in obese women. In a closer analysis of the study's 8,745 volunteers, researchers found that women who carried more weight in their abdomen (the "apple" shape) experienced slightly less cognitive decline than those who were heavier in their lower body (the "pear" shape). In postmenopausal women, says Kerwin, abdominal fat still produces small amounts of estrogen, which may have a protective effect on the brain, preserving cognitive function.
Still, she says that theory needs to be tested with additional research. Overall, the study supports the idea that overweight tends to reduce people's cognitive reserve, thus leading to poorer intellectual performance. While the new findings do not link this cognitive decline to long-term consequences, such as a higher risk of dementia, previous studies have shown that diminished cognitive reserve can indeed predict future dementia. "This study opens up new ways of thinking about how obesity can affect cognition," says Kerwin.