Older women with higher levels of omega-3 fatty acids in their blood had slightly less brain shrinkage than women with low fatty acid levels in a new study.
The results may suggest that omega-3s protect the brain from the loss of volume that happens with normal aging and is seen more severely in people with dementia, the researchers say.
"The brain gets smaller during the normal aging process - about 0.5 percent per year after age 70, but dementia is associated with an accelerated and localized process of brain shrinkage," said James Pottala, who led the study.
Pottala is an assistant professor at the University of South Dakota Sanford School of Medicine in Sioux Falls and chief statistician for the Health Diagnostic Laboratory in Richmond, Virginia.
He and his colleagues analyzed data from the Women's Health Initiative Memory Study to see whether omega-3s were associated with brain shrinkage in general, and in specific brain regions involved in memory and other cognitive processes.
The data covered 1,111 women who were, on average, 70 years old and had no signs of dementia at the beginning of the study. At that time, the amounts of the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in their red blood cells were measured.
DHA accounts for 30 percent to 40 percent of the fatty acids found in brain cell membranes, and it's especially concentrated near the synapses where the cells communicate with one another, Pottala and his colleagues note in their report, published in the journal Neurology.
Red blood cell levels of the omega-3s are good indicators of how much a person has consumed, the researchers add.
The researchers used an omega-3 index to describe the fatty acid levels seen among women in the study and to divide them into four groups: women with the highest levels had an average index reading of around 7.5 percent, while women with the lowest levels had an average of 3.4 percent.
Eight years after the women's blood was tested, they underwent MRIs to measure the volume of gray matter and white matter in their brains.
The researchers found that women with the highest EPA and DHA blood levels at the study's outset had brains that were about two cubic centimeters larger overall than women with the lowest levels.
In addition, the hippocampus, a brain region critical to forming and storing memories, was 2.7 percent larger in women who had fatty acid levels twice as high as the average.
Of 13 specific brain regions the researchers looked at, the hippocampus was the only one where they saw a significant difference.
The analysis adjusted for other factors that could influence the women's brain size, including education, age, other health conditions, smoking and exercise.
The researchers didn't measure cognitive function, only brain volume, so they cannot say whether the size differences they saw had any link with differences in memory or dementia risk.
The authors acknowledged other limitations in their report, including that they did not look at whether the women's omega-3 consumption had changed over time.
It's possible that some of the participants had changed their diets or started taking fish oil or other forms of omega-3 fatty acids, Pottala told Reuters Health in an email.
But in previous study, he and his colleagues showed red blood cell EPA and DHA levels and peoples' dietary fish intakes generally don't change over time.
"If some subjects in our MRI study began taking fish oil supplements, then the reported benefits would be underestimated," Pottala said.
Pottala says higher blood levels of omega-3 fatty acids can be achieved by dietary changes, such as eating oily fish twice a week or taking fish oil supplements.
Since the study does not prove that blood levels of omega-3s are the cause of the brain-size differences observed, or that those differences have any effect on cognitive function, the researchers caution that more research is needed to know whether raising omega-3 levels would make any difference to brain health.