"Subjects with baseline plasma PC DHA levels in the upper quartile experienced a significant 47 percent lower risk of dementia compared with participants with levels in the lower three quartiles," wrote lead author Ernst Schaefer.
The study, published in the Archives of Neurology, comes only a month after scientists from Karolinska University Hospital Huddinge in Sweden reported -- in the same medical journal -- that omega-3 acids might slow the mental decline of people with mild Alzheimer's, although no effect was observed in people with advanced cases.
The Tufts study analyzed the blood of 899 men and women with an average age of 76 -- all participating in the population-based Framingham Heart Study -- and looked at the association between dementia and DHA levels. All participants were free of dementia at the beginning of the study, and underwent neuropsychological testing after giving their initial blood sample. A subgroup of 488 participants also filled out a semi-quantitative 126-item food frequency questionnaire (FFQ) that assessed their diet, especially their level of fish consumption.
Roughly nine years of follow up studies later -- and after taking other risk factors such as age and homycysteine levels -- the participants were split into quartiles based on their levels of DHA in the blood. The quartile with the highest DHA levels had a 47 percent lower risk of developing dementia and 39 percent lower risk of developing Alzheimer's disease than the remaining three quartiles.
The participants in the top quartile reported on their FFQ that they ate an average of 0.18 grams of DHA daily, and an average of 2.9 servings of fish a week. The other three quartiles reported substancially less fish in their diets, eating between 1.3 and 2.3 servings a week.
"In our study, the correlation between (blood) DHA content and fish intake was significant, indicating that fish intake is an important source of dietary DHA," wrote the authors.
While the Tufts study did not use mechanistic processes to elucidate the benefits of DHA, previous studies have shown that DHA is involved in the membrane of ion channels in the brain, which allows them to change shape easier and transmit electrical signals. Other limitations of the study include only taking DHA measurements once and only having dietary data on a subset of the participants.
"In the future, it will also be important to determine whether combined dietary supplementation with DHA can decrease further mental deterioration in patients with established dementia," concluded the researchers.
Rush University Medical Center, Chicago's Martha Clare Morris noted in an accompanying editorial that, while fish consumption was linked with DHA levels, there was no significant correlation between fish consumption and Alzheimer's risk, possibly because the study was not capable of observing such an effect if it existed.
Morris also noted that oily fish often contains high levels of pollutants such as methyl mercury, dioxins, and polychlorinated biphenols, which have caused some experts to recommend a reduced intake of fish, especially for pregnant women.
"The only way to resolve the risk-benefit question is to examine, directly in humans, mercury intake from fish and its effect on various health outcomes relative to the beneficial effects of the omega-3 fatty acids consumed," said Morris.
"Omega-3 fatty acids are emerging as nothing short of miraculous in terms of their protective health effects," said Mike Adams, a holistic nutritionist and author of "The Seven Laws of Nutrition." "They prevent heart attacks, depression, diabetes and obesity, as well as supporting the nervous system, cognitive function and longevity factors. We should all strive to increase our consumption of omega-3s."
While a proponent of consuming oily fish for its omega-3 content, Adams added that consumers should look to chia seeds as a rich source of DHA without the fishy taste or the risk of heavy metals. Those who prefer fish can follow their meal with chelating formulas such as Baseline Nutritionals Metal Magic, he said.