People with higher levels of the omega-3 fatty acids found in fish oil may have larger brain volumes in old age equivalent to preserving one to two years of brain health, according to the study.
Shrinking brain volume is a sign of Alzheimer's disease as well as normal ageing, researchers said.
For the study, the levels of omega-3 fatty acids in red blood cells were tested in 1,111 women who were part of the Women's Health Initiative Memory Study.
Those with higher levels of omega-3s had larger total brain volumes eight years later. Those with twice as high levels of fatty acids (7.5 vs 3.4 per cent) had a 0.7 per cent larger brain volume.
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"These higher levels of fatty acids can be achieved through diet and the use of supplements, and the results suggest that the effect on brain volume is the equivalent of delaying the normal loss of brain cells that comes with ageing by one to two years," said study author James V Pottala, from the University of South Dakota in Sioux Falls.
In Alzheimer's disease, the hippocampus begins to atrophy even before symptoms appear.
The study was published in Neurology, the medical journal of the American Academy of Neurology.