Despite concerns about the safety of consuming mercury found in fish, it is more beneficial for healthy adults to keep fish in their diets rather than limit it, according to a long-term study at the University of Wisconsin-Milwaukee (UWM).
John Dellinger, professor of clinical laboratory sciences in UWM's College of Health Sciences, has recently completed a 12-year study of the health effects of consuming a large amount of fish. He conducted the study among adult members of the Ojibwe Tribe of the northern Great Lakes region.
Fish accumulate environmental toxins, such as mercury, in their bodies, and when people eat the fish, they also take in mercury, a chemical that causes birth defects.
The goal of the study was to find out if Native populations, which eat more lake fish than the general population, were suffering ill health affects because of it. The answer was surprising, even to Dellinger: Except for children and pregnant or nursing mothers, he says, the benefits of consuming more fish appear to outweigh the risk of slightly higher mercury levels in the body.
His study involved 822 adult volunteers from eight Ojibwa reservations. The study participants reported lower rates of diabetes, heart disease and even allergies, compared to the general population.
"There is a lot of nutritional value in fish," Dellinger says. "It contains high protein, vitamins and the fatty acid Omega 3."
Mercury is a byproduct of coal-burning power plants and heavy industry, but it also occurs in nature (in the fallout of volcanic eruptions, for example). It causes neuro-developmental damage in human embryos, fetuses and children. Some mercury can be found in almost all fish, however, especially larger ones that prey on the smaller.
"Mercury is harmful to people in large doses, particularly to a developing fetus," he says. "That much we know."
So in what amount does mercury begin to cause illness? Dellinger says that is still unknown, but the question is currently under investigation at UWM's Marine and Freshwater Biomedical Science Center (MFBSC).
To Dellinger, it's a question of dosage. The concentration of mercury in the fish consumed by the Ojibwe study participants was usually less than .5 part per million. The U.S. government limit is 1 part per million of mercury in fish, though the American Medical Association recommends that people follow the strictest guidelines for their area.
Wisconsin health officials post fish consumption advisories when the mercury level exceeds .05 parts per million, which is also recommended by the Environmental Protection Agency (EPA).
Dellinger says that level would result in people removing fish from their diets because it is difficult to find any fish with such a low level of mercury.
It's vital information as tribal communities make decisions about their diets, he says. And yet, the AMA policy in 2004 makes no mention of the offsetting benefits of consuming fish.
"In this study, we are defining the dosage in a more realistic scenario for human exposure," he says, "so we can balance the good news with the bad."
Participants in the study had mercury levels twice as high on average compared with adults who don't eat as much fish, but that by itself doesn't mean they are being harmed, says Dellinger. "Mercury presence at this level is only an indication that the person has been eating fish," he says.
Other findings in the study include recommendations, called the three S's, for controlling the amount of mercury children and pregnant women are exposed to, such as:
- Size. Smaller fish contain lower levels of mercury.
- Source. The level of mercury found in fish is different for every lake. Dellinger's study included GIS mapping of lakes in Northern Wisconsin, showing where the higher levels are found in certain types of fish.
- Species. The kind of fish matters, with bass, pike, walleye, or perch being preferable to salmon or trout.
Another factor Dellinger uncovered is that wild rice - also a dietary staple of Native tribes - may help negate the health threats of some mercury exposure.
He and Michael Carvan, assistant scientist at the MFBSC, have been experimenting with the nutrient selenium, found in wild rice. The two are investigating the resulting health effects of a combination of both mercury and selenium.
And since humans get their mercury from a food source, the scientists are providing mercury to their laboratory models - which happen to be zebrafish - the same way, feeding them a diet of mercury-containing trout and walleye.
Feeding fish to other fish? It may seem incongruous, but Dellinger conducts much of his human health research using zebrafish in lab experiments.
"The relationship between the brain and the heart is controlled by the autonomic nervous system," he says. "That neuro-pathway in humans is conserved [similar] in fish. If I get normal results in fish, I know it will be normal in humans as well."