The volunteers had to limit their calorie intake
Reducing what you eat by nearly a third may improve memory, according to German researchers.
They introduced the diet to 50 elderly volunteers, then gave them a memory test three months later.
The study, reported in the Proceedings of the National Academy of Sciences journal, found significant improvements.
However, a dietician said the reduction could harm health unless care was taken.
There is growing interest in the potential benefits of calorie restricted diets, after research in animals suggested they might be able to improve lifespan and delay the onset of age-related disease.
However, it is still not certain whether this would be the case in humans - and the the levels of "caloric restriction" involved are severe.
The precise mechanism which may deliver these benefits is still being investigated, with theories ranging from a reduction in the production of "free radical" chemicals which can cause damage, to a fall in inflammation which can have the same result.
The researchers from the University of Munster carried out the human study after results in rats suggested that memory could be boosted by a diet containing 30% fewer calories than normal.
The study volunteers, who had an average age of 60, were split into three groups - the first had a balanced diet containing the normal number of calories, the second had a similar diet but with a higher proportion of unsaturated fatty acids, such as those found in olive oil and fish.
The final group were given the calorie restricted diet.
After three months, there was no difference in memory scores in the first two groups, but the 50 in the third group performed better.
They also showed other signs of physical improvement, with decreased levels of insulin and fewer signs of inflammation.
The researchers said that these changes could explain the better memory scores, by keeping brain cells in better health.
They wrote: "To our knowledge, the current results provide first experimental evidence in humans that caloric restriction improves memory in the elderly.
"The present findings may help to develop new prevention and treatment strategies for maintaining cognitive health into old age."
However, care was taken to make sure that the volunteers, despite eating a restricted diet in terms of calories, carried on eating the right amount of vitamins and other nutrients.
Dr Leigh Gibson, from Roehampton University, said that the drop in insulin levels were one plausible reason why mental performance might improve.
The hormone was known to act on parts of the brain related to memory, he said, and the higher levels found in people with poorly controlled type II diabetes had been directly linked to worse memory and cognitive function.
A spokesman for the British Dietetic Association said that people, particularly those already at normal or low weight, should be "extremely careful" about attempting such a diet.
She said: "There is other evidence that, far from enhancing memory, dieting or removing meals can interfere with memory and brain function.
"A drop of 30% in calories is a significant one for someone who is not overweight, and should not be undertaken lightly.
"It could even be dangerous if the person is already underweight."