Does Fish Oil Prevent Alzheimer's Disease?

When I was young I was told that eating fish made you clever, in the same way that eating carrots made you see in the dark. Luckily I always liked fish, though whether it had the desired effect upon my intellect it is not for me to say.

At any rate, the connection between eating fish and brain function has long been part of common folklore, and it is probably for this reason that large numbers of people approaching old age take fish oil capsules in the hope of warding off Alzheimer’s disease. This, no doubt, is an example of the belief or superstition that, if only we got our diet right, we should never fall ill. Immortality is but a diet away.

A meta-analysis of trials of fish oil capsules or margarine in the prevention of cognitive decline has just been published in, or on, the Cochrane Library, a website devoted to examining the evidence for (or of course against) the use of drugs and medical procedures in the prevention and treatment of illnesses. The quality of the analyses published in, or on, the Cochrane Library is generally accepted as the best possible.

The authors aggregated the results of three trials that met their methodological criteria. The trials had to be double-blind and placebo-controlled, and involved 3,536 participants who were cognitively unimpaired and over the age of 60. Subjects took fish oil capsules or margarine (or placebo) for 6, 20 or 40 months.

There was no evidence that fish oils prevented cognitive decline as measured by the Mini-Mental State Examination, the measuring instrument usually used in such trials, and other simple tests. The only significant side-effect of taking fish oil was mild gastrointestinal disturbance; overall levels of side-effects were as great among those taking placebo as those taking fish-oil.

There are severe limitations in what can be concluded for the meta-analysis, however. Because the cognitive decline in both treatment and control groups was so small, the trials did not have sufficient power to detect any possible benefit, much less to draw definite conclusions about the ability of fish oils to prevent Alzheimer’s disease. Moreover the fish oil was taken by subjects for at most 40 months; perhaps if it were taken for longer, and the follow-up were also for longer, a difference would manifest itself. Since the trials excluded people with dementia that was already manifest, it was impossible to conclude from the results whether or not fish oil is of benefit in established dementia.

The authors did not conclude that people should not take fish oil supplements, because they said that such fish oils might have benefits other than in the prevention of cognitive decline. And since longer trials might be necessary to establish definitively the uselessness (or otherwise) of fish oils in the prevention of cognitive decline, those who put their faith in them are not yet forced by the evidence to abandon it or risk joining the ranks of the irrational.

For the moment, nutritionists recommend the consumption of fish twice a week, including of oily fish — salmon, herrings, mackerel or sardines — at least once a week. I confess I find such recommendations suspect: how do the nutritionists know that three times a week would not be better, or once a week as good as twice?

The decision as to what to eat cannot be taken on the basis of placebo-controlled double blind trials, first because such trials usually establish very little (and that little is often contradicted by subsequent trials), and second because there are purposes to eating other than the preservation or improvement of health. Meals are not medical procedures, and the dinner table is not, or not yet, an operating table.