Eating broiled or baked fish high in omega-3 fatty acids at least once a week is linked to a lower risk of memory loss and stroke in older adults.

That bit of health news you may know.

What you may not know is why. What is it about certain types of fish that protects the aging brain?

A new study published this week in Neurology (the medical journal of the St. Paul-headquartered American Academy of Neurology) offers an intriguing answer. It found that the regular consumption of some fish significantly reduces the risk of subclinical brain infarcts, or "silent strokes."

This finding "helps connect the dots," said David Siscovick, M.D., professor of medicine and epidemiology at the University of Washington in Seattle and a co-author of the study.

Silent strokes are known to increase the risk of cognitive impairment (loss of thinking skills), dementia and full-blown strokes. If something as simple as eating fish can help prevent these silent strokes, that would be very good news indeed, particularly since some types of fish are also known to be good for your heart.

An insidious danger
Silent strokes are nasty pieces of work. Most likely triggered by interruptions of blood flow in the brain, they destroy tiny clumps of brain cells — areas too small to cause the classic visible symptoms of stroke, such as slurred speech, blurred vision, and numbness or weakness on one side of the body.

In fact, the brain injury caused by silent strokes can be detected only by a brain scan. Yet silent strokes are far from innocuous. Over time their damage can accumulate and lead to memory lapses and other cognitive problems. Some research has also linked them to depression.

Silent strokes are surprisingly common. A study published in June found that more than 10 percent of apparently healthy middle-aged adults had experienced at least one silent stroke. Other research suggests that almost 20 percent of older adults may be affected. The incidence increases with age.

A significant benefit
For the Neurology study, more than 2,000 adults aged 65 and older underwent two brain scans five years apart to see if they had any of the tell-tale small brain lesions of a past silent stroke. Participants were also asked to fill out questionnaires about their fish consumption.

The study found that eating fish high in omega-3 fatty acids (such as tuna, salmon, mackerel, herring, sardines and anchovies) three or more times a week lowered the risk of silent stroke by almost 26 percent. Even once-a-week consumption of these types of fish was beneficial, reducing the risk by 13 percent.

Regular fish eaters in the study also had fewer changes in the white matter in their brains. (White matter is composed mostly of nerve fibers, called axons, which carry information between brain cells.) Like silent strokes, white matter abnormalities are associated with cognitive impairment and a higher risk of future full-blown strokes.

Baked, not fried
Not all fish are alike, as far as your brain is concerned. The study found that fried fish did not lower the risk of silent strokes, perhaps because of something that happens to the fish when it is fried or because the types of fish best for frying (such as cod, catfish and snapper) are lean and thus low in omega-3 fatty acids. 

"The message here is not to just go out and eat more fish, because you may then go out and eat more fried fish," said Siscovick.

Nor is the message to start taking fish oil supplements, he added. There may very well be nutrients in addition to the omega-3 fatty acids that give fish its brainy benefits. So give up on the dietary shortcuts and focus instead on incorporating a moderate amount (one to three servings) of baked or broiled fish into your weekly meals.

The question of contaminants
Before you start hooking more fish into your diet, however, you need to consider its dark underside: environmental chemical contaminants.

Some fish rich in healthful omega-3 fatty acids, like tuna and mackerel, also tend to be loaded with methylmercury, polychlorinated biphenyls (PCBs), perfluorinated compounds (PFCs) and other industrial chemicals. These pollutants can accumulate in the body — where they can be toxic to the brain, particularly to the developing brains of fetuses and young children.

So choose your fish wisely.

"Eat smaller fish and fish that are lower on the food chain — fish that are more vegetarian," said Deborah Swackhamer, PhD, professor of environmental health sciences at the University of Minnesota. "The contaminants are carried up the food web and are retained at each level of that web. So the top predators have a lot more contaminants than those lower on the food chain."

In other words, it's better to catch and eat a small, young walleye than a large, old one.

Resources

For general fish-eating guidelines as well as consumption guidelines for fish caught in specific Minnesota's lakes and rivers, go to the Minnesota Department of Health's Web site.

The American Heart Association also has a handy chart that compares the methylmercury and omega-3 fatty acids levels in popular fish.

The Sierra Club also offers a simple fish-eating guide for women of childbearing age and small children.