Low blood levels of omega-3 fatty acids are associated with smaller brain volume and poorer performance on tests of mental acuity, even in people without apparent dementia, according to a new study.

In the analysis, published online Monday in the journal Neurology, scientists examined 1,575 dementia-free men and women whose average age was 67. The researchers analyzed the fatty acids of the subjects’ red blood cells, a more reliable measurement than a plasma blood test or an estimate based on diet. They used an M.R.I. scan to measure brain volume and white matter hyperintensities, a radiological finding indicative of vascular damage.

People in the lowest one-quarter for omega-3 levels had significantly lower total cerebral brain volume than those in the highest one-quarter, even after adjusting for age, body mass index, smoking and other factors. They also performed significantly worse on tests of visual memory, executive function and abstract memory than those in the highest one-quarter. There was no significant association with white matter hyperintensity volume.

“We feel that omega-3’s reduce vascular pathology and thus reduce the rate of brain aging,” said Dr. Zaldy S. Tan, the lead author and associate professor of medicine at the University of California, Los Angeles.