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Fishy diet could help prevent Alzheimer’s disease

Nebraska Deborah Josefson
 

Researchers from Rush-Presbyterian-St Luke’s Medical Center in Chicago have found that elderly people who ate fish at least once a week had a much lower rate of Alzheimer’s disease than people who ate fish infrequently.

Led by Dr Martha Clare Morris and Denis Evans, of Rush’s department of internal medicine and Alzheimer’s disease centre, the investigators enrolled 815 Chicago residents aged between 65 and 94 years who had no evidence of Alzheimer’s disease at the outset of the study in 1993 (Archives of Neurology 2003;60:940-6). The participants were taken from the Chicago health and ageing project, a larger population based, prospective study designed to find risk factors for Alzheimer’s disease.

The original cohort was 62% black (and 38% white) and 61% female. The mean number of years of education was 11.8. The participants were followed for a mean of 3.9 years, and the study lasted for seven years, to 2000.

A self administered food frequency questionnaire was used to assess diet. On average, the participants were given the questionnaire 1.9 years after the start of the study and 2.3 years before clinical evaluation at follow up. Participants were asked about their consumption of tuna, fish cakes and fingers, fresh fish, shrimps, lobster, and crab. They were also asked about their use of dietary supplements, cereals and margarine, types of cooking oil, milk consumption, and fat preferences.

The researchers estimated nutrient intake by multiplying the nutrient content of individual food items by their frequency of consumption. Less than 1% of the participants were taking omega 3 fatty acid supplements. Of the participants, 39.2% reported eating fresh fish at least once a week, 13.9% ate tuna weekly, and 8.4% ate fish cakes or fingers at least once a week. Only 2.4% ate shellfish weekly.
Over the course of the study 131 participants developed Alzheimer’s disease. Another 11 people developed dementia of other types. After controlling for confounding factors of hypertension, educational level, heart disease and stroke, ethnic group, sex, and apoliprotein E genotype, the researchers found that fish consumption was strongly inversely associated with Alzheimer’s disease.

Participants who ate fish at least once a week had a 60% lower risk of developing Alzheimer’s disease than people who rarely or never ate fish (relative risk 0.4 (95% confidence interval 0.2 to 0.9)). Total intake of omega 3 polyunsaturated fatty acids was also inversely associated with the development of the disease, with people whose consumption was in the top 20% having a risk that was 70% lower than the risk among people in the lowest 20%.

While it is not known exactly why fish is protective, the secret may lie in the high content of omega 3 fatty acids in fish. These fish oils may stabilise phospholipid concentrations in the brain membrane and may influence neurotransmitter function. Several epidemiological and animal studies have supported a protective role for omega 3 polyunsaturated fatty acids and suggested that they have a positive effect on learning and memory.

However, in an accompanying editorial Dr Robert Friedland, of the Department of Neurology at Case Western Reserve University in Cleveland, Ohio, while lauding the study, warns of the risk of mercury and lead poisoning from high fish consumption.