If you're 70 or older and you eat more than about 2200 calories (9210kj) per day - slightly more than a man needs to maintain his weight - you increase your risk of developing mild cognitive impairment (MCI), which often precedes Alzheimer's disease, according to a study at the Mayo Clinic.
The more calories you consume, the higher your risk.
"We observed a dose-response pattern, which simply means that the higher the amount of calories consumed each day, the higher the risk of MCI," said Dr Yonas E. Geda, lead author of the study.
Dr Geda studied 1233 people between the ages of 70 and 89 who were free of overt dementia.
After testing their memory, language and sense of direction, he and his colleagues concluded that 163 participants already had MCI.
The study participants fell into three groups of roughly equal size based on how much they ate.
Members of the first group consumed a less-than-average 600 to 1526 calories per day. The second group consumed a typical diet containing 1526 to 2143 calories. The third group consumed 2143 to 6000 calories a day.
Dr Geda and his colleagues found that the 163 people with MCI had a higher caloric intake than those who tested normal, and the higher the caloric intake, the higher the chances of having MCI.
Those in the third group faced double the risk of MCI as those in the first group.
How does a higher caloric intake contribute to MCI?
Dr Geda and his colleagues don't know exactly, but a growing body of research suggests that a calorie-rich diet may be hard on the brain; for example, the Personality and Total Health Through Life Project, conducted by researchers at Australian National University, found that excessive caloric intake and high intake of monounsaturated fats (found in meat, whole dairy products, nuts and avocados) predicted MCI.
Also, animal studies have consistently shown that decreasing caloric intake by about 30 per cent seems to protect the brain.
In fact, it was the research on rhesus monkeys at the Wisconsin National Primate Research Center in Madison that got Dr Geda interested in conducting his study.
The monkeys at the centre that have been limited to about 70 per cent of the number of calories they would normally consume remain strikingly younger and more vigorous than their peers.
Last year, Italian researchers at the Catholic University of Sacred Heart in Rome found that production of a molecule known as CREB1, which regulates memory, learning and anxiety control and normally declines with age, is increased by calorie restriction, and activates several genes linked to longevity and to the proper functioning of the brain.
Researchers at the National Institute on Aging, in Baltimore, have found that fasting for one or two days a week seems to protect the brain against Alzheimer's, Parkinson's and other brain disorders. The intermittent fasting, which involves limiting food intake to about 500 calories on fasting days (about five medium apples), conferred benefits without causing the intense food cravings typical of traditional diets, in which food consumption is restricted every day.
"Cutting calories and eating foods that make up a healthy diet may be a simple way to prevent memory loss as we age," Dr Geda said.