A recent study of 210 elderly men, aged 70 to 89 years, and living in Zutphen, Netherlands found that consumption of the omega-3 fatty acids, EPA and DHA from fish, offered a significant protective effect on cognitive (brain) function over a 5 year follow-up period. This benefit was found to be largely dose dependent. Although all men studied suffered some degree of cognitive decline, those who consumed little or no fish suffered four times greater cognitive loss when compared to those who ate fish regularly. The researchers determined that the group consuming the highest amount of fish consumed at least 400 mg. of total EPA/DHA per day. This is equivalent to roughly six servings of lean fish per week, or a single 140 gram serving of fatty fish such as mackerel or herring.
Van Gleder, B.M.; Tijhuis, M; et al. "Fish Consumption, n-3 fatty acids, and subsequent 5-y cognitive decline in elderly men: the Zutphen Elderly Study." Am.J.Clin.Nutr., 2007;85:1142-7.