In a large Mayo Clinic study, self-reported diet was found to be significantly associated with the risk of seniors developing mild cognitive impairment or dementia over a four-year period.

The study involved 1,230 older adults (70-89) who completed a 128-item food-frequency questionnaire about their diet during the previous year. Of these, around three-quarters (937) showed no signs of cognitive impairment at the beginning of the study period, and were asked to return for follow-up cognitive assessments. These assessments took place every 15 months. After about four years, 200 (21%) had developed mild cognitive impairment (MCI) or dementia.

The likelihood of cognitive deterioration was significantly affected by the type of diet. Those with the highest carbohydrate intake were nearly twice as likely to develop cognitive impairment compared to those with the lowest carbohydrate consumption, and when total fat and protein intake were taken into account, they were 3.6 times likelier to develop impairment.

Those with the highest sugar intake were 1.5 times more likely to develop cognitive impairment.

But — a finding that will no doubt surprise many — those with the highest fat consumption were 42% less likely to develop cognitive impairment, compared to those with the lowest level of fats.

Less surprisingly, those with highest intake of protein had a reduced risk of 21%.

In other words, the worst diet you can have, if you want to keep your brain healthy, is one that receives most of its calories from carbohydrates and sugar, and relatively little from fats and protein.

The findings about carbs, sugar, and protein are consistent with other research. The finding regarding fats is somewhat more surprising. The inconsistency may lie in the type of fat. Research implicating high-fat diets as a risk factor in Alzheimer’s have used saturated fats. Diets high in olive oil, on the other hand, have been found to be beneficial.

It seems likely that the danger of carbs and too much sugar lies in the effects on glucose and insulin metabolism. Saturated fats also interfere with glucose metabolism. Alzheimer’s has sometimes been called Type 3 diabetes, because of its association with insulin problems.