With advanced age, a brain's prowess can wane -- in some cases to the point of dementia. However, a new study suggests that eating fish may keep mental faculties keen well into our twilight years. By contrast, diets high in corn oil and certain related polyunsaturated fats appear to let cognition cloud.
These findings, the latest from a heavily studied population of very elderly Dutch men, suggest that food may offer one means of modifying the extent to which an individual's intellectual capacity declines with time.
Amongst the very elderly, thoughts can appear muddled, memory may come and go, and reasoning can languish. Many scientists have attributed such symptoms to stroke, to microscopic tissue-killing clots in the brain, and to a nutrient starvation of brain cells. The latter can be caused by a reduced blood flow through arteries and vessels that are becoming clogged with atherosclerotic plaque.
Since 1960, Dutch scientists have been studying the development of chronic disease among men in Zutphen, a town in the Netherlands. Born between 1900 and 1920, many of the study's original participants are pushing 100 years old.
In the most recent study, Sandra Kalmijn, a physician and epidemiologist at Erasmus University Medical School, investigated the extent to which certain foods that have been associated with stroke and atherosclerosis might affect cognition. Her research team focused on three dietary components.
Linoleic acid is a polyunsaturated fat that predominates in corn oil and several other vegetable oils. In most persons, it comprises 90 percent of the polyunsaturates in low-density-lipoprotein (LDL) cholesterol, the so-called bad cholesterol.
Being highly susceptible to oxidation -- the chemical reaction responsible for making fats rancid -- linoleic acid may render any LDLs that shuttle it around in the blood more vulnerable to transformation into foam cells. Foam cells serve as the base for atherosclerotic deposits that develop inside arteries and blood vessels.
Dietary antioxidants are compounds such as vitamins E and C, beta carotene, and flavonoids. By fighting oxidation, these compounds can spare LDLs from that transformation into foam cells.
Long-chain omega-3 fatty acids (such as EPA and DHA), which are found in fish, compete with linoleic acid. Omega-3 fats reduce the production of chemicals (such as thromboxane A2) that make blood cells develop into sticky clumps.
Kalmijn's team assessed linoleic's contribution from data on its presence in foods. "The most important food groups that predicted absolute intake of linoleic acid in this population were margarines, butter, baking fats, sauces, and cheeses," she says. The source of this linoleic acid? "In Holland, we use mostly sunflower oil," Kalmijn told Science News Online.
Kalmijn and her colleagues found that men whose diets contained a lot of fats rich in linoleic acid were more likely to exhibit cognitive impairments on a standard psychological test than were the men who ate the smallest amounts of that component. The test assessed each man's performance on activities that ranged from copying complex geometric shapes and recalling lists of words to answering questions that depended on his awareness of time and location.
In the January American Journal of Clinical Nutrition, her team concludes that these "findings are consistent with the hypothesis that linoleic acid is atherogenic."
Because oxidative reactions can play an important role in atherosclerosis, Kalmijn suspected that diets rich in antioxidants might temper any adverse brain-functioning problems associated with consuming linoleic. "The surprise," she says, "was finding that antioxidants were not related to cognitive impairment" in the 940 elderly men that her group studied.
In contrast, she notes, last year Lenore J. Launer, a colleague (and coauthor in this Zutphen study), was part of a team that indicated that antioxidants offer some protection against age-related declines in mental skills among a similarly large group of aging Rotterdam men.
The difference in their findings, Kalmijn says, may reflect different antioxidant consumption patterns among the two groups. The Zutphen men eat diets low in these compounds and seldom take supplements of antioxidant vitamins.
The most rewarding finding, says Kalmijn, was the positive effect of fish in the diet. Men who, on average, ate more than 20 grams of fish daily faced only about 60 percent the risk of cognitive impairment compared to those who ate none.
"Because this is one of the first studies on this subject, caution is called for in the interpretation of our findings," Kalmijn's team says.
The findings extend those of a study last year by these researchers. It found that the incidence of cognitive impairments was higher in the elderly Zutphen men who smoked cigarettes or drank low to moderate amounts of alcohol -- but only in subjects with cardiovascular disease or diabetes.
In a separate study, Kalmijn showed that Zutphen men with diabetes or preclinical stages of this disease (such as glucose intolerance or high blood levels of insulin) were more likely to suffer cognitive impairments than men without those conditions. Again, this all ties in, she notes, because diabetes is a risk factor for heart disease and a condition that appears to foster oxidative damage.
Kalmijn's study is not the first to show a possible neurological benefit to eating fish. Two years ago, scientists with the National Institute of Health found evidence that the long-chained omega-3 fats in fish also may play a role in warding off depression.
So, what's your pleasure? Will it be tuna salad casserole, a salmon steak, or stew made from sea lamprey?
Jama, W.J., Launer, L.J., et al. 1996. Dietary antioxidants and cognitive function in a population-based sample of older persons: The Rotterdam Study. American Journal of Epidemiology 144:275.
Kalmijn., S., et al. 1997. Polyunsaturated fatty acids, antioxidants, and cognitive function in very old men. American Journal of Epidemiology 145(January):33.
Kalmijn, S., et al. 1995. Glucose intolerance, hyperinsulinaemia and cognitive function in a general population of elderly men. Diabetologia 38:1096.
Launer, L.J., Feskens, E.J.M., Kalmijn, S., et al. 1996. Smoking, drinking, and thinking: The Zutphen Elderly Study. American Journal of Epidemiology 143:219.
Fackelmann, K.A. 1993. Lipoprotein link to heart disease revealed. Science News 143(June 12):375.
Hertog, M.G., et al. 1995. Flavonoid intake and long-term risk of coronary heart disease and cancer in the seven countries study. Archives of Internal Medicine 155:381.
Raloff, J. 1996. Lamprey: A taste treat from prehistory. Science News Online (Aug. 10).
_____. 1996. Antioxidants: Confirming a heart-y role. Science News 150(July 6):6.
_____. 1996. Vitamin E slows artery 'aging.' Science News 149(May 4):287.
_____. 1995. New support for tea's heart-y benefits. Science News 148(Dec. 9):399.
_____. 1993. Fat may spur spread of prostate cancer. Science News 144(Oct. 9):228.
Rimm, E.B., et al. 1996. Relation between intake of flavonoids and risk for coronary heart disease in male health professionals. Annals of Internal Medicine 125(Sept. 1):384.
_____. 1996. More on those "really bad" LDLs.... Science News Online (Sept. 21, 1996).
Wu, C. 1995. Got them low-fat, polyunsaturated blues. Science News 148(Sept. 2):153.
Department of Epidemiology and Biostatistics
Erasmus University Medical School
Rotterdam, the Netherlands
This week's Food for Thought is prepared by Janet Raloff, senior editor of Science News.
Illustration: Wendy Temple.