French researchers from Toulouse University Hospital conducted a five-year study of more than 2,200 adult men and women who were examined using four mental ability tests administered at the beginning and the end of the study.
When the trial began, people with a Body Mass Index (BMI) of 20 or less -- within the "healthy" range of the BMI scale -- could recall 56 percent of the words in a vocabulary test, while those classified as obese, with a BMI of 30 or higher, only remembered 44 percent of vocabulary words.
When the subjects were re-tested five years later, the participants with a healthy BMI had the same level of vocabulary recall, while obese participants' recall abilities fell to 37.5 percent.
Lead researcher Dr. Maxime Cournot, an assistant professor of clinical epidemiology at Toulouse University Hospital, said hormones secreted by excess fat could possibly damage brain function.
"Another explanation could be that since obesity is a widely known cardiovascular risk factor, due to the thickening and hardening of the blood vessels, that the same happens with the arteries in the brain," she said.
Dr. David Haslam, clinical director of the UK's National Obesity Forum, called the research "alarming," and said, "It goes to show obesity affects every single organ in the human body."
Nutritionist Mike Adams, author of "Natural Appetite Suppressants for Safe, Effective Weight Loss," said the study's findings are not surprising, since many of the foods that cause obesity can also be damaging to the nervous system.
"The brain is a physical organ, and like the rest of the body, it also needs supportive nutrients and protection from dietary and environmental chemicals," Adams said. "When it doesn't get the nutrients it needs, such as when a person avoids natural foods and consumes mostly processed junk foods, the brain begins to suffer a drop in performance."