The health benefits of eating fish, particularly fatty fish rich in omega-3 fatty acids, are well documented. Research suggests omega-3-fatty acids help protect against heart disease and perhaps also Alzheimer’s . But over the past few years, there has been increased concern about the high amounts of mercury found in some fish.

Mercury is found throughout our environment. It enters the air and then water through processes such as fossil fuel combustion, mining, and solid-waste incineration. Microorganisms in the water change the mercury into methylmercury, a highly toxic substance that builds up in fish and shellfish.

The main concern surrounding mercury is its affect on the developing nervous systems of unborn babies and young children. In 2004, the Environmental Protection Agency (EPA) and Food and Drug Administration (FDA) issued an advisory for women who are pregnant or nursing, women who may become pregnant, and young children to limit their consumption of fish and shellfish. But what about the rest of us?

A new study in the April 20, 2005 issue of the Journal of the American Medical Association looked at the associations between mercury and cognitive functioning in a group of US adults. Overall, they did not find strong evidence that blood mercury levels were associated with worse cognitive performance.

About the Study

This study included 474 randomly selected participants from the Baltimore Memory Study, an ongoing study involving 1,140 Baltimore residents between the ages of 50 and 70. The majority of participants classified their race as black (39%) or white (55%).

Participants completed food frequency questionnaires regarding their usual fish consumption and took neurobehavioral tests (which measure cognitive function) administered by trained technicians with no knowledge of the participants’ blood mercury levels or dietary history. The researchers measured total mercury levels in blood samples from the participants and then examined their associations with neurobehavioral test scores.

The Findings

The median blood mercury level was 2.1 micrograms per liter, which according to the researchers, is consistent with the mercury levels found in populations that do not eat a lot of fish. (The average amount of fish consumed in this study was 11.5 ounces per week.)

After adjusting for other factors that may have influenced cognitive functioning (e.g., age, educational achievement, fish consumption, alcohol consumption), increasing blood mercury was linked with worse performance on a test of visual memory, but better performance on a test of manual dexterity. The researchers concluded that overall their study provided no compelling evidence that blood mercury levels are adversely associated with neurobehavioral scores in these adults.

One limitation of this study is that it looked at individuals at one point in time only; blood mercury levels only reflect exposure over the past 5–6 months, which may not be representative of usual or past blood mercury levels.

How Does This Affect You?

This study suggests that blood mercury levels may not be an issue of concern in older adults who do not eat high amounts of fish. However, more studies, especially ones with more detailed assessments of blood mercury levels, are needed to confirm these results.

The American Heart Association recommends eating fish at least two times per week. Based on this study, there is no reason why older adults should not follow this recommendation. Although more research is needed, it seems safe to assume that in this population, and probably for younger men and most women as well, the protective benefits of omega-3 fatty acids outweigh the health risks posed by mercury contamination.

Those who eat a lot of fish, or just want to be on the safe side, should choose fish that is low in mercury content. Additionally, women of childbearing age and young children should still heed the government’s advice and limit their fish intake to 12 ounces per week. Fortunately, it is easy to maximize the benefits while minimizing the risks when it comes to fish—many are high in omega-3 fatty acids while relatively low in mercury. Among the best choices are salmon, herring, sardines, and canned light tuna.