MINNEAPOLIS (Knight Ridder/Tribune Business News) -- Older people who load their plates with carbohydrates have four times the risk of developing mild cognitive impairment -- often a precursor to Alzheimer's disease -- a new study at Mayo Clinic indicates.

"That was a bit of a surprise," said Dr. Rosebud Roberts, lead epidemiologist on the research team. "I thought the big problem would be eating too little protein."

The study -- part of her work to identify risk factors contributing to pre-dementia -- ultimately may help physicians and dietitians better advise patients about proper diets, she said. It was published recently in the Journal of Alzheimer's Disease.

The carb culprit appears to be the sugar that is created when carbohydrates are digested, said Roberts, a researcher for 20 years at Rochester's Mayo Clinic.

The study is based on four years of data from 1,230 Olmsted County, Minn., residents age 70 to 89 who volunteered in 2004.

While those who get most of their calories from pasta, rice, bread and other carbohydrates were associated with much higher risk, people loading up with protein such as chicken and fish lowered the risk by 21 percent. Those with high use of fats from nuts and oils dropped the risk by 42 percent.

That brain benefit came from all fats, not just unsaturated "good fats" like those from olive oil, nuts and avocados, Roberts said.

"This may be one of the benefits of growing older," she said. "Cholesterol is bad for middle-aged hearts, and what's bad for hearts generally is bad for brains. But with older people, that may not be so true. All fats seem to be good for the brain, and perhaps not as bad for the heart as when they were younger."

Still, the lesson from the study is "you need a balanced diet of protein, fats and carbohydrates," Roberts said, perhaps one like the Mediterranean diet, which relies heavily on poultry, vegetables and healthy fats.

"Sugar fuels your brain, so you need some carbohydrates," she said. "But too much may stop the brain from using sugars effectively."

Sugar also may affect blood vessels in the brain or spur development of amyloid plaques, protein fragments associated with Alzheimer's disease that clog the spaces between brain nerve cells, she said.

Food with fats and protein slow the digestive process of converting carbs to sugars, and so do some complex carbohydrates like whole grains or some vegetables.

"When we get older, sometimes we simplify how they eat -- reach for a piece of fruit, drink more juice, that kind of thing. But those are simple carbohydrates, and they convert to sugars very easily," she warned. "It's better to eat more vegetables and less fruit."

Now Roberts is examining the impact of diabetes -- the body's inability to adequately control blood sugar -- on brain health, using some of the same older residents of Olmsted County.

(c) 2012 Star Tribune (Minneapolis) Distributed by McClatchy-Tribune News Service.