A diet rich in fish: high-end consumers face more mercury risks. (Science Selections).

Fancy fish aficionados, take note: if your forays into piscatorial paradise are frequent, you could be ingesting dangerously high amounts of mercury. In this month's issue, Jane Hightower and Dan Moore of the California Pacific Medical Center in San Francisco report that people who often eat certain expensive commercially caught fish are at risk for excessive exposure to mercury [EHP 111:604-608].

Mercury can accumulate in the body if eaten at a greater rate than it is excreted. It can cause several well-documented adverse health effects, including impairment of the immune, cardiovascular, and reproductive systems.

The researchers evaluated all of the patients in a general internal medicine practice in San Francisco who came in for an office visit over a period of one year. A total of 720 mostly middle- to upper-income patients were asked to estimate how much they ate of several types of fish. Of these, 93 females and 30 males (including 7 children under age 12) warranted having their mercury levels measured, either because of their fish consumption habits or because they presented with symptoms consistent with mercury overexposure (such as fatigue, decreased memory, and joint pain).

Thirty-four subjects, including all the children, were excluded from further analysis because of various confounding factors. The researchers did note, however, that the mean mercury level for all the women in the survey was 10 times that found in a recent population survey by the Centers for Disease Control and Prevention. Some of the children had over 40 times the national mean.

For the 89 subjects whose blood results underwent statistical analysis, the findings were alarming. Ninety-two percent of these subjects had blood mercury concentrations above 5 micrograms per liter ([micro]g/L), the maximum level recommended by the U.S. Environmental Protection Agency and the National Academy of Sciences. Eighteen percent had concentrations above 20 [micro]g/L.

Those participants with higher blood mercury appeared to have eaten more of the large predator species, particularly swordfish and ahi. These types of fish (which are consumed as steaks, sushi, or sashimi) are often more expensive. However, the study subjects were relatively affluent and were not deterred by price. Thus the researchers feel they have identified a subpopulation at risk for mercury overexposure, with higher income and education the most apparent risk factors.

The good news to emerge from the study is that dangerously high blood mercury is reversible, although it can take several months to reduce levels to an acceptable point. The researchers followed 67 subjects over time after they stopped or greatly reduced their consumption of moderate- to high-mercury-content fish. Their blood mercury declined rapidly in the first three weeks, followed by a slower reduction over time. After 41 weeks, all but two patients, who continued to eat large predatory fish, had reduced their level to below 5 [micro]g/L.

Current wisdom holds that eating fish high in omega-3 fatty acids is a good way to prevent heart disease and enhance nutrition. Yet the mercury concentrations in some fish may be enough to counter these good effects. According to Hightower and Moore, clinicians should be alert for patients who may present with side effects caused by the contaminants that are present in many fish. They recommend that dietary histories encompassing fish consumption become part of a comprehensive health screening to identify people at risk for mercury accumulation.

Because adverse effects in children are believed to occur when the pregnant mother's blood mercury level is more than 15 [micro]g/L, prepregnancy screening is important for patients who eat fish frequently. The researchers also point out that more testing of fish for mercury content is needed, and they urge that information and advisories resulting from further testing be made readily available to consumers where fish is sold.