Researchers at Utah State University in the US found that over-65s on a diet full of green leafy vegetables, oily fish and the odd glass of red wine scored higher in mental tests.
A separate study at the University of California found that moderately physically active older adults might experience slower rates of mental decline.
In the first study, Heidi Wengreen, an assistant professor of nutrition at Utah State University, asked 3,831 adults, aged 65 and older, to complete a food survey. They then tested their cognitive skills over an 11-year period, beginning in 1995.
The researchers looked to see how well the participants followed the DASH diet, an eating regimen that protects against hypertension and heart trouble.
Those who followed the DASH diet more closely had higher scores on the cognitive tests at the start of the study and over time, Wengreen found.
In the second study, Deborah E. Barnes, of the University of California, San Francisco, followed more than 3,000 adults aged 70 to 79.
Those who were sedentary had the lowest level of cognitive function at the start and higher rates of decline over the course of the seven-year study.
The two studies were reported at the Alzheimer's Association 2009 International Conference on Alzheimer's Disease in Vienna.