For years, we doctors have been advising our patients to eat more fish because of the vast amount of research showing that fish is good for the heart. In general, that’s true. But it’s also true that not all fish for sale in the supermarket are equally heart-healthy, and, moreover, how you prepare the fish you eat can also make a big difference. Eating fish is a perennial, but imperfect, prescription!
Myth: All fish are good for you.
Fact: There a catch, as fishermen say. Wild varieties of fish like salmon, sardines, (canned) tuna, cod, and other fish with lower mercury levels are heart-smart.
Limit your intake of species with the highest content of mercury, such as shark, swordfish, king mackerel, and tilefish, as well as fresh and frozen tuna (canned tuna is okay). Stay away from farmed and fresh-water fish because of contaminants. And bake or broil your fish. Not fry it.
Before I get into the details, I would like to share a short story.
From about 1975 to 1990 my family and I pretty much followed a vegetarian-plus-fish type diet. As an avid fisherman, I contributed regularly to the dinner table with fish pulled out of New England streams and the Long Island Sound.
One summer day in 1990, I barbecued some fresh swordfish purchased at a local seafood shop. The next day I woke up with muscle pain, fever, brain fog, tremors, and a strange metallic taste in my mouth. I wasn’t myself for a couple of days. I figured the fish had made me sick but didn’t think the problem was typical bacteria-related food poisoning because I didn’t have the usual symptoms of nausea or diarrhea.
The mystery motivated me − an inquisitive doctor who fishes a lot − to do some sleuthing into seafood toxicity. From my research I learned that fish flesh was a source of toxic mercury. Little was known about the connection back then. I couldn’t say for sure if mercury in the swordfish meat had sickened me, but I decided to get my two teenage sons and myself tested for the mercury level in our bodies. We were all regular fish eaters.
There’s More in Fish besides Flesh, Fins, Oil, and Bones
The test results from hair and urine samples showed we were all over the top for mercury in our bodies. That was incentive enough to start backing off fish and adding more meat to our diet. We stopped eating fresh water fish altogether because of the contamination in streams and became very selective about ocean fish. We eliminated such high-mercury level fish as swordfish, tuna, mackerel, mako shark, tilefish, and orange roughy. We gave up shellfish as well, with the exception of scallops.
Fast forward to fall of 2006 and dinner out on Long Island with the family at a well-known seafood restaurant. Step, my oldest son was with us. He was having health problems at the time, but was feeling really good this particular evening and decided to order a big serving of lobster. He hadn’t eaten lobster in years.
“Don’t do it,” I warned him. “You may not be able to handle any bad stuff if the lobster is contaminated.”
But he decided to do it anyway. He ate everything except the meat from the big claw, intending to save that part for lunch the next day. But the next day he was disoriented, shaky, and weak. He had no nausea or diarrhea, so I suspected the problem likely wasn’t bacterial.
“I still have the claw left and I’ve decided to have it analyzed to see if I can find out what made me sick,” he said.
So Step contacted a nearby laboratory to test the lobster. The analysis cost him $450 and for his money he learned that the lobster was very high in mercury, and also contained lesser, but still troubling levels of cadmium and lead, all considered toxic metals. It also had minute levels of PCBs, common chemical contaminants from industry that are found in the environment.
My purpose in sharing these anecdotes with you is to put an underline and exclamation point on my concern about the dangers of toxic fish in today’s world. Yes, fish is a wonderful source of protein and heart-healthy fish oil (omega 3 fatty acids). But unfortunately, man has thoroughly polluted the planet and tainted this timeless supply of nourishment. It’s another sad sign of our times that fish have become our primary source of environmental exposure to mercury.
The Mercury-Selenium Connection
Nearly all fish contain trace amounts of mercury, some more than others. They absorb it from water as it passes over their gills and as they feed on other marine organisms. Larger predator fish get a higher dose from their prey. Mercury binds tightly to proteins in fish tissue, including muscle. The toxic mineral builds up in a process called bioaccumulation. Cooking does not appreciably reduce the mercury content.
Researchers still don’t know the precise impact of toxicity on individual human health, and some say the levels may not be as toxic to cardiovascular health as many people previously thought.
According to a 2011 Harvard study, the mercury level contained in the amount of dietary fish eaten by Americans isn’t as much of a contributor to coronary heart disease, stroke, or overall cardiovascular disease in adults as had been previously thought. The report, published in the New England Journal of Medicine, was based on questionnaires, lifestyle habits, and mineral assays taken from toenail clippings (that’s right, toenail clippings!) of more than three thousand individuals with cardiovascular disease and a similarly-sized control group without any heart issues. There was no increased risk of heart disease for those with the highest intake of mercury compared to those with the lowest. Toe clippings are used for assessing mercury because they provide more accurate evidence of long-term mercury exposure compared to hair samples.
Fish also contain a high level of selenium, an antioxidant mineral that specifically binds and neutralizes mercury − a little known fact of marine biology. Most fish, in fact, contain more selenium than mercury and some researchers believe this balance offsets the presence of mercury.
Bake or Broil, but Don’t Fry Your Fish
There is another twist to the fish tale. How to eat your fish? We doctors should probably hand out recipes along with our advice. According to another 2011 study, how fish is cooked can make a big difference.
That conclusion emerged from analysis of medical and lifestyle information available through the famous Women’s Health Initiative Observational Study. The multi-year, ongoing database includes results from food frequency questionnaires for nearly 100,000 women aged 50-79. The researchers probed the database to learn whether fish, and their omega-3 fatty acid content, might influence the risk for postmenopausal women to develop heart failure (HF). One in five adults develop HF over the course of a lifetime, and elderly, postmenopausal women are particularly at risk.
The results of the inquiry showed that a higher intake of baked or broiled fish was associated with a lower risk of HF while more consumption of fried fish could actually raise the risk. Specifically, they determined that consumption of more than 5 servings of baked/broiled fish per week was independently associated with a 30 percent lower risk, while eating more than 1 serving of fried fish per week had an increased risk of 48 percent.
While statistics like these can’t be taken literally, they do point in a direction that makes good sense to me, and support some basic, well-established facts. We know that the marine omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)), found in high amounts in fish, decrease inflammation, and improve heart function and benefit the sensitive endothelial lining of blood vessels. These fatty acids are less liable to be damaged by baking and broiling than by the high heat of frying. Frying, or course, frequently involves oils that can generate trans fatty acids known to increase inflammation and oxidative stress, and negatively affect endothelial function.
The Sinatra Solution
The Harvard mercury study and the selenium counterbalance idea are encouraging, but I am still wary about any mercury in the diet. For now, I recommend limiting intake of those species with the highest content of mercury such as shark, swordfish, king mackerel, and tilefish. If you like fish, as I do, opt for salmon, sardines, tuna, cod and other varieties with lower mercury levels.
Here are some general guidelines for playing it safe with fish:
- Eat baked and broiled fish as often as possible. Avoid fried fish.
- Pregnant women, breastfeeding mothers, and young children should avoid fish with a mercury content more than 1 part per million (ppm) such as shark, swordfish, tilefish, and king mackerel. The FDA advises pregnant women to avoid fish altogether so as to prevent possible harm to a sensitive fetus.
- Avoid fresh water fish. Mercury levels in freshwater fish vary. In general, bass, pike, muskellunge, and walleye are higher in mercury.
- Avoid ocean fish with higher levels of mercury.
- Avoid farm-raised fish because they are loaded with contaminants. Opt instead for wild caught fish.
- Beware of fresh and frozen tuna − the mercury content ranges from 0.5 ppm and 1.5 ppm). Canned tuna consists of smaller species with lower mercury levels.
- Be sure your daily supplement program includes selenium (200-300 mcg) and magnesium (400-600 mg). Both protect your body from mercury. Many people are deficient in these important minerals.
- Test the mercury level in your body through urine, blood, and hair analyses offered by alternative doctors, including naturopaths and physicians belonging to the American College for Advancement in Medicine (www.acam.org).
- Belin RJ, Greenland P, Martin L, et al. Fish Intake and the Risk of Incident Heart Failure: The Women’s Health Initiative Circulation: Heart Failure, 2011. [Published online ahead of print.]
- Mozaffarian D, et al. Mercury exposure and risk of cardiovascular disease in two U.S. Cohorts. N Engl J Med, 2011;364:1116-25.
- Ralston NV, Raymond LJ. Dietary selenium’s protective effects against methylmercury toxicity. Toxicology, 2010;278(1):112-23
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