Mayo Clinic researchers found that those aged 70 plus who consume a lot of protein and fat relative to carbohydrates are less likely to become cognitively impaired.
"We think it's important that you eat a healthy balance of protein, carbohydrates and fat, because each of these nutrients has an important role in the body," lead author Rosebud Roberts said.
Researchers tracked 1,230 people ages 70 to 89 who provided information on what they ate during the previous year.
At that time, their cognitive function was evaluated by an expert panel of physicians, nurses and neuropsychologists. Of those participants, only the roughly 940 who showed no signs of cognitive impairment were asked to return for follow-up evaluations of their cognitive function.
About four years into the study, 200 of those 940 were beginning to show mild cognitive impairment, problems with memory, language, thinking and judgment that are greater than normal age-related changes.
Those who reported the highest carbohydrate intake at the beginning of the study were 1.9 times likelier to develop mild cognitive impairment than those with the lowest intake of carbohydrates.
Participants with the highest sugar intake were 1.5 times likelier to experience mild cognitive impairment than those with the lowest levels.
However, those whose diets were highest in fat - compared to the lowest - were 42 per cent less likely to face cognitive impairment, and those who had the highest intake of protein had a reduced risk of 21 per cent.
When total fat and protein intake were taken into account, people with the highest carbohydrate intake were 3.6 times likelier to develop mild cognitive impairment.
"A high carbohydrate intake could be bad for you because carbohydrates impact your glucose and insulin metabolism," Roberts said in a statement.
"Sugar fuels the brain - so moderate intake is good. However, high levels of sugar may actually prevent the brain from using the sugar - similar to what we see with type 2 diabetes," Roberts said.
The study was published in the Journal of Alzheimer's Disease.
