More threads by David Baxter PhD

David Baxter PhD

Late Founder
Eating fish: There's a catch
October 26, 2005

The results are in, and there's no question about it: Fish is really, really good for you. Not only is it packed with healthful vitamins and minerals, it is also a major source of omega-3 fatty acids, which a veritable flood of recent studies shows lowers the chance of heart attack, makes babies smarter, wards off dementia and stroke in the elderly, and even seems to guard against dry-eye syndrome.

But there's a fly in the ointment. Actually, two flies.

Some fish are mercury-filled time bombs, according to a parade of reports from government agencies and environmental groups.

And people are emptying the oceans of some fish. Overfishing and habitat destruction has left some species as low as 1% of their original populations, according to federal data.

So what now? Do we eat fish at least twice as week, as many nutritionists say we should?

Or is it time to pack away the tartar sauce?

Not on your life, say five Harvard University studies published in the American Journal of Preventive Medicine. Their advice: Pick up that fish fork and start eating.

Overblown fears could cause consumers to lower their consumption and lose the "substantial nutritional benefits" fish offers, the researchers found.

Tests on fish oil supplements, which also provide omega-3 fatty acids, have uniformly found extremely low levels of methyl mercury. But nutritionists still urge people to eat fish rather than take pills because they consider it an excellent source of protein that is low in saturated fats.

Even so, seafood sales appear to be slipping. The Food and Drug Administration's advisory on mercury and fish consumption in March 2004 got lots of media attention. According to ACNielsen, volume sales of tuna sold in cans and envelopes fell 9% in the 12 months after the advisory, which singled out tuna. Information Resources Inc. shows that sales of refrigerated seafood in the USA fell 2.1% in 2004.

Worries that overfishing is depleting fish populations may be having an effect on consumption, too. A survey of 400 visitors to the Monterey Bay Aquarium in California who picked up a Seafood Watch wallet-size guide to non-endangered fish species found that 80% said they still buy less of certain types of seafood, even four months after their visit.

But there's a real risk that warnings about a few fish species directed only at a small portion of the population will get simplified in the public mind to the mantra "Don't eat fish," says David Acheson, chief medical officer of the Food and Drug Administration

From fish to foul
So how did fish, once so abundant, healthful and even virtuous -- remember that eating fish on Friday was a Christian penitential observance -- become the source of fear and guilt?

For mercury, the path goes back to recommendations first made in 2001 and updated in 2004; the FDA and Environmental Protection Agency said that some fish have so much mercury that eating them could be dangerous to the developing brain of a fetus.

Mercury is known to affect the migration of brain cells in fetuses and might prevent signal transmission pathways from developing properly. The researchers also fear damage to the autonomic nervous system, which tells blood vessels when to contract or relax and the heart how fast to beat.

The FDA says pregnant women, women who might become pregnant and young children shouldn't eat fish with high levels of mercury -- shark, swordfish, king mackerel or tilefish -- at all and shouldn't have more than two meals a week of any kind of fish.

And in a blow to the tuna industry, the FDA added that for those women and children, only one meal a week should be albacore, or white, tuna because its mercury levels are high enough that two servings are too much. Light tuna, which is darker than albacore and can be a mix of tuna types, is lower in mercury and is not subject to the recommendation.

There have been studies showing that consumption of large amounts of high-mercury fish can cause headaches and stomach pains. But the pitfalls of moderate consumption, if any, are much less clear.

The current government advice is to eat a balanced diet that includes a variety of fish, but there are no specific FDA recommendations on limits for older children and other adults. The American Heart Association says non-risk adults and kids can safely eat 14 ounces a week of fish with mercury levels that average 0.5 ppm.

Source of the problem
Some mercury in the environment -- exactly how much is a matter of great debate -- occurs naturally. But most scientists, as well as the EPA, say most environmental mercury comes from either digging it up for industrial uses, using it in mining or burning mercury-containing coal.

Since the 15th century, mercury in the environment has increased 200% to 500%, says Elsie Sunderland, an EPA scientist.

Today, one of the largest sources is burning coal. The EPA estimated in 1998 that U.S. coal-fired power plants emit about 50 tons of mercury a year into the atmosphere.

Once up in smoke, mercury can take any number of forms; in almost all of them, it wafts around the globe, slowly settling in a process called atmospheric transport and deposition.

A small portion that falls on water is eaten by bacteria and undergoes a chemical reaction that turns it into a very toxic form of the element called methyl mercury.

Inorganic mercury, the liquid kind found in old-style thermometers, is excreted rapidly by the body. But methyl mercury binds very tightly to muscle tissue and is much more toxic for that reason, Sunderland says.

That means with each step up the food chain, the amount of methyl mercury grows. The tiny plankton are eaten by smaller fish that are eaten by medium-size fish that are eaten by larger fish, and the amount of mercury is thus biologically amplified.

You'd have to drink 2,641 gallons of water to get the amount of mercury in a 3.5-ounce piece of shark or swordfish, Sunderland says.

The differences can be extreme. Government tests show that pollock, the fish used to make most fish sticks, has 0.06 parts per million of mercury. But in Mexican tilefish, which eat lots of other fish, it's 1.45 parts per million.

"Larger, older, predatory fish have the highest level," Acheson says. "Fish like salmon, which don't consume other fish, don't have the problem."

It's important to remember that even for those in the high-risk category, there are many species, such as cod, haddock or trout, that are healthful and very low in mercury, Acheson says.

But the government warnings are confusing and frightening enough that lots of people are walking past the fish counter, to the detriment of their health, the Harvard researchers say.

The next steps
So the question is: How can we eat the fish that's so good for us without damaging the environment? says George Leonard, science manager for the Seafood Watch program at Monterey Bay Aquarium.

Parts of the answer, such as mercury-emission controls and better management of fish stocks, are in the laps of governments.

But for the fish-loving citizen, there's good news, Leonard says. The fish that carry the highest health risks also are among the most endangered, he says. "Mercury bioaccumulates in fish at the top of the food chain. And those are typically fish we've overfished, like shark, swordfish and marlin."

So eating fish a little further down the food chain, which have lower mercury levels, is not only good for the planet, it's good for your health. Some of the best are catfish, Pacific cod, Pacific halibut, pollock, salmon, tilapia and trout, according to Seafood Watch.

The key seems to be making the messages simpler so that consumers aren't scared off, says Bill Hogarth of the National Oceanic and Atmospheric Administration. "The American public needs to be educated a little more to make sure that the American consumer is aware of the issues."
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