A substance found in fish oil may be
associated with a significantly reduced
risk of developing Alzheimer’s and other
dementias, researchers reported.
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 Alzheimer’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 — 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 Alzheimer’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.
“It’s 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.”
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.
