Now, two top consumer protection agencies are at odds on whether that advice should be reconsidered to encourage all people to eat more fish, in order to promote healthy hearts.
The Food and Drug Administration has been circulating a draft report within the government that argues the health benefits of eating fish outweigh the potential ill effects of mercury. But the Environmental Protection Agency has fired off a memo to the White House calling the 270-page FDA study "scientifically flawed and inadequate" and an "oversimplification" lacking analytical rigor.
Environmental groups are crying foul. They say it's a sneak attempt to undercut important public health advice in the waning hours of a Bush administration that has treated science as a stepchild.
"The FDA was once a fearsome protector of the public health. Now it's nothing more than a patsy for polluters," Richard Wiles, executive director of the Environmental Working Group, said in a statement.
The food industry is praising the FDA's shift. One organization, the Center for Consumer Freedom, called it "long overdue and a huge public-health victory" that "just might be the best Christmas present health-conscious Americans could hope for."
The interagency feud spilled into the open Friday when the Environmental Working Group released copies of the dueling memos. The dispute was first reported by the Washington Post.
The FDA is embroiled in another controversy over the science of food safety. Recently, a panel of outside advisers challenged the agency on bisphenol A, or BPA, a chemical used to make plastic for food packaging and other consumers goods. The independent experts said that FDA's conclusion that low doses of BPA are safe was scientifically flawed.
The battle over mercury is now attracting the interest of Congress. "FDA should not change anything it cannot back up with the best science, because we know that mercury can cause brain and cardiovascular damage," said Sen. Barbara Boxer, D-Calif., who chairs the Environment and Public Works Committee. "FDA should not play politics with the health of our families."
At the FDA, officials sought to tamp down the controversy and dispel concerns that the agency is about to toss out the government's current mercury guidance.
"It would be a mistake to assume that this draft report represents the FDA's official position because a final determination on these matters has not been reached," said spokesman Michael Herndon. "Following the discussion among government agencies, FDA intends to seek public comment. This will all be done in a very public and transparent manner, and the FDA will make no final determination until all the relevant comments and scientific analysis has been carefully considered."
Mercury occurs naturally and is also released in the environment through pollution. Very high levels in the bloodstream can damage the nervous system of developing fetuses and young children, causing learning disabilities and other problems. Fish absorb mercury in the water and as they feed on plankton and other smaller fish. Some fish, like king mackerel and swordfish, accumulate higher levels of mercury.
Fish and shellfish are the biggest sources of human exposure to mercury. Fetuses and young children are the most susceptible to harm. About 8 percent of U.S. women of childbearing age have enough mercury in their blood to be at risk of having babies with subtle learning disabilities, the Centers for Disease Control and Prevention estimates.
Because of such concerns, the FDA and EPA have recommended that women of child-bearing age and young children not eat shark, swordfish, king mackerel or tilefish, which contain high levels of mercury. The agencies also advised that they eat no more than two meals a week of fish and shellfish that are lower in mercury, a total of 12 ounces. And since "white" albacore tuna has more mercury than chunk light tuna, they recommended no more than six ounces a week.
The FDA's draft report said the latest studies seem to indicate that the risks may not be as dire as previously thought. The agency also sought to weigh the risk of mercury against the benefits of eating more fish.
Current research suggests "a beneficial impact on fetal neurodevelopment from the mother's consumption of fish, even though they contain methylmercury," the report said.
"The net effect is not necessarily adverse, and could in fact be beneficial," it added.
But the EPA said, "this FDA report bases its conclusions on models that use very limited inputs from studies that have significant problems for risk analysis."