Dear Vanessa,

I eat a lot of fish for its health benefits, but I'm worried about getting too much mercury. How can I tell how much mercury is in the fish I'm eating? Is farmed fish better than wild-caught? Is there such a thing as mercury-free fish anymore?

— Worried in Woodland Hills, Calif.

Dear Worried,

The sad, short answer: You're right to be concerned, and, yes, you should expect any fish you eat to contain some mercury.

Eating seafood is, in fact, our main source of exposure to methylmercury (a highly toxic form of mercury). To mercury, add the dangers of ingesting chemicals and pesticides, in particular PCBs and DDT. While seafood consumption is our main source of mercury exposure, it's also our best source of omega 3s. So add the benefits of essential nutrients to the equation. (We’ll leave the heavy toll overfishing and irresponsible fish-farming has taken on oceans, marine life and local economies out of the equation for now).

The November 2005 Archives of Pathology and Laboratory Medicine summed up the issue: "Consumption of fish high in omega-3 fatty acids is advocated by the American Heart Association to decrease the risk of coronary artery disease. However, fish contain environmental toxins such as mercury, polychlorinated biphenyls (PCBs), and organochlorine (OC) pesticides, which may negate the beneficial cardiovascular effects of fish meals."

So? Do the benefits of eating fish — a low-fat, high-protein food and our best source of omega 3s — outweigh the risks of eating contaminated fish?

It depends.

Mercury consumed by children and pregnant or nursing women, for instance, carries greater negative impacts than it does for an adult male. The FDA and EPA set out some basic, perhaps too lax, guidelines for how much fish is presumably safe to eat, but they apply only to children and some women. The Environmental Defense Fund's Seafood Selector Health Alert List is thorough, and breaks down the maximum safe servings of a variety of fish for men, women, older kids (6-12) and younger kids (0-6).

It also depends on an individual's health and priorities. For instance, someone with heart disease will need to weigh the risk of cancer and other health dangers from contaminants with the heart-health benefits of omega 3s.

And it depends greatly on the amount and type of fish you eat. In general, remember that methylmercury, PCBs and dioxins accumulate over the fish's lifespan, and become concentrated as they move up the food chain (predators, larger and older fish are likely to have higher levels of mercury). In fact, top predators in a food chain may have levels a million times higher than what's found in the surrounding water. Even the EPA has found that there are no safe amounts of shark, swordfish, king mackerel or tilefish for children or women who may become pregnant, are pregnant or are nursing. The EDF has 10 fish on its list that it finds unsafe for anyone, in any amount.

Check out these sites that list the most- and least-contaminated types of fish to help you make informed choices. The Green Guide has a version designed for mobile devices, which is useful when you're standing at the fish counter or puzzling over menu choices. For health advisories specific to your area (local toxin warnings — good for those who catch their own fish or share with family and friends) go to the EPA's Fish Advisories page. And remember to look closely at other lists you may find: seafood guides can be based on the ecological impact that human consumption has on the marine environment, and not the impact that fish consumption can have on humans. Of course, what's good for the oceans is good for the humans, so get the best of the best and make sure your mercury-free fish is as fish-friendly as it is human-healthy.

Though fish populations are, on the whole, in rapid decline — and most contaminant levels on the rise — there are still some good choices that can put you on the winning side of the benefit vs. risks equation.

Hope this clarifies the issue for you a bit, at least more than it confuses.

Keep it green,


P.S. —  I don't mean to ignore your question about farm-raised vs. wild-caught. While related to issues of mercury content, the questions surrounding methods of raising or catching seafood is a whole other quandary -- one that I'll do my best to sort out for you, so check back.

Good to know:

Contaminants found in fish can last years in the human body. Some have a half-life of as much as a decade. In other words, half the amount of toxins a woman consumes when she's 20 will still be in her system when she's 30. Hence the warnings to "women of child-bearing age." Warnings for women who are already pregnant may be a little too little, a little too late.

1) Though we talk about mercury in seafood, it's really mercury in the form of methylmercury that's worrisome. Airborne mercury settles in water where microorganisms change it into methylmercury, a highly toxic form that builds up in the tissue of fish, shellfish and animals that eat fish.

2) The EPA's fish advisories, for instance, involve five primary contaminants: mercury, PCBs, chlordane, dioxins and DDT.

3) The omega 3 essential fatty acids are polyunsaturated. These desirable fats cannot be made by the human body, so they must be obtained from food. Omega 3s are required for normal development of the brain, eyes and nerve tissue in humans. Although omega 3 fats can be obtained from plant sources (such as flaxseed), the best sources are animal foods. This is because the omega 3s in plants are in the form of ALA and must be converted into EPA and DHA (the beneficial omega 3s). The conversion process is a complicated one, even for very healthy bodies, and only a small percentage of ALA will end up as EPA and DHA. Three of the top omega 3 sources are cold-water fish oil, grass-fed red meat and natural eggs. Most of the clinical studies on omega 3 benefits are based on fish oils from consuming fish and fish-oil supplements.

4) Why is eating fish healthy? Fish is a high-protein, low-fat food that provides a range of health benefits. White-fleshed fish, in particular, is lower in fat than any other source of animal protein, and oily fish are high in omega-3 fatty acids. Since the human body can’t make significant amounts of these essential nutrients, fish are an important part of the diet. Omega 3 fatty acids have been found to:

  • improve heart health (including reducing the risks of cardiovascular disease, lowering unhealthy triglyceride levels, playing a role in the regulation of blood clotting and vessel constriction, and controlling cardiac arrhythmia)
  • improve memory and brain function, even reducing depression and halting mental decline in older people, and may reduce the risk of developing Alzheimer's disease
  • help reduce the symptoms of inflammatory arthritis
  • be important for prenatal and postnatal neurological development
  • improve eye health
5) Mercury exposure at high levels can harm the brain, heart, kidneys, lungs and immune system of people of all ages, but one of the great dangers of mercury poisoning is its impact on neurological development, and is therefore particularly dangerous for fetuses and children because their brains and nervous systems are still growing. Cognitive thinking, memory, attention, language, and fine-motor and visual-spatial skills can all be affected.

6) The FDA tests fish for mercury, and the EPA determines mercury levels that it considers safe for women of childbearing age. Also, the FDA sets the tolerance cutoffs for commercial fish, and the EPA sets limits for recreationally caught fish.

7) A few sites with guides to making informed fish consumption choices:

8) For more on mercury in fish, see MNN's translation of government data on the subject, "Diagnosis mercury."

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Thumbnail photo: Taras Kalapun/Flickr