The scientists found that people with the highest blood levels of an omega-3 fatty acid called docosahexaenoic acid, or DHA, were about half as likely to develop dementia as those with lower levels.
The substance is one of several omega-3 polyunsaturated fatty acids found in fatty fish and, in small amounts, in some meats. It is also sold in fish oil or DHA supplements. The researchers looked for a reduced risk associated with seven other omega-3 fatty acids, but only DHA had any effect.
The study, in the November issue of The Archives of Neurology, used data from the Framingham Heart Study to follow 899 initially healthy participants, with a median age of 76, for an average of more than nine years.
The scientists assessed DHA and fish intake using a questionnaire and obtained complete dietary data on more than half the subjects. They took blood samples from all the participants to determine serum levels of fatty acids.
Ninety-nine people developed dementia over the course of the study, including 71 cases of Alzheimerâ€™s disease. The average level of DHA among all the participants was 3.6 percent of all fatty acids, and the top 25 percent of the population had values above 4.2 percent. People in this top one-quarter in DHA levels had a 47 percent reduced risk of developing dementia, even after controlling for body mass index, diabetes, hypertension, smoking status and other known or suspected risks. Risk reduction was apparent only at that top level of DHA â€” those in the bottom three-quarters in DHA level showed no detectable difference in risk.
People who ate two or more servings of fish a week reduced their risk for dementia by 39 percent, but there was no effect on the risk for dementia among those who ate less than that.
The finding that DHA alone reduces risk, the authors write, is consistent with earlier data showing high levels of DHA in healthy brain tissue and low levels in the brains of people with Alzheimerâ€™s disease.
Dr. Ernst J. Schaefer, the lead author of the study, was cautious in interpreting the results.
This study doesn’t prove that eating fish oil prevents dementia, he said. â€œItâs an observational study that presents an identified risk factor, and the next step is a randomized placebo-controlled study in people who do not yet have dementia.â€ Dr. Schaefer is chief of the Lipid Metabolism Laboratory at Tufts University.
The study was financed in part by Martek, a concern that manufactures DHA, and one author received a grant from Pfizer, France.
Eating fish is not a guarantee of having high levels of DHA. In fact, fish intake accounted for less than half of the variability in DHA levels. Other dietary intake and genetic propensities probably account for the rest. Dr. Schaefer pointed out that the kind of fish consumed is important. Fatty fish, he said, is best, and frying will cause DHA to deteriorate.
Supplements may be an additional source of DHA, but an editorial in the same issue, by Dr. Martha Clare Morris, an associate professor of medicine at Rush University Medical Center in Chicago, points out that there are no published human studies of the effects of omega-3 fatty acid supplementation. The Food and Drug Administration does not endorse DHA or fish oil capsules, but recognizes doses of up to 3 grams a day of fish oil as generally safe. High intakes of fish oil can cause excessive bleeding in some people.
Dr. Morris writes that there are few human studies examining the effect of mercury intake from eating seafood, and it is not known if the risks of eating fish outweigh the benefits.
But, she adds, epidemiological studies consistently show positive health effects from fish consumption on mortality, cardiovascular risk factors and, now, dementia
Source:The New York Times