To add further fuel to the debate over obesity, researchers in France are saying that being overweight in middle age may indicate a higher risk of dementia later in life.
In a study involving 2,223 healthy French adults who were between the ages of 32 and 62 in 1996, researchers at the Toulouse University Hospital and the National Institute of Health and Medical Research, have discovered that overweight middle-aged adults had poorer scores in tests of memory, attention and learning ability than their thinner peers.
They say their findings strongly suggest a greater risk of dementia in overweight persons at middle-age.
The researchers believe that higher rates of cardiovascular disease or diabetes might explain the link, but say it is also possible that substances produced by fat cells, such as the hormone leptin, have direct effects on the brain.
The study's lead author Dr. Maxime Cournot, says both obesity and dementia, including Alzheimer's disease, are becoming increasingly common, and even after factors such as age, education and general health were accounted for it did not explain the link.
At the start of the study the participants were given a battery of standard cognitive tests, assessing abilities such as memory, attention and speed of learning, then five years later, they took the tests again.
The researchers found in general that those with a high body mass index (BMI) achieved lower test scores than those with a lower BMI, and they also tended to show greater cognitive decline between the two test periods.
In a test involving word memory recall people with a BMI of 20 remembered an average of 9 out of 16 words, while people with a BMI of 30 remembered an average of 7 out of 16 words.
Cournot says the tests used in the study were sensitive enough to detect "small variations" in cognition, and the weight-related differences seen among these healthy middle-aged adults would probably not be obvious in daily life, whereas over time, there could be more apparent effects on the rate of age-related mental decline.
According to Cournot's team, it is possible that excess fat cells have some direct effect on brain function and other work has also suggested that the "hunger" hormone leptin, which is produced by fat cells, plays a role in learning and memory.
Also although the study participants were in generally good health, disorders such as elevated blood pressure and diabetes could act as a bridge between high BMI and poorer cognitive function, as the thickening and hardening of the blood vessels supplying the brain can contribute to dementia.
Diabetes may also harm cognition by either leading to artery disease or via direct effects of the hormone insulin on brain cells.
Cournot says however regardless of the impact of weight on dementia risk there are already enough reasons to maintain a healthy weight.
Cournot suggests that as both dementia and obesity are increasing in epidemic proportions, the implied effects on mental function may motivate people to change their lifestyle habits.
The study was supported by grants from the National Center for Scientific Research, Institute of Aging, French Heart Foundation, Midu-Pyrénées Region, Ministry of Further Education and Research, Regional Social Security Services, and Ministry of Employment.
The study is published in the current issue of Neurology, the scientific journal of the American Academy of Neurology.