Harvard School of Public Health Study on Contaminants in Fish and Seafood
Fish is a very important part of a healthy diet. Fish and other seafood are the major sources of healthful long-chain omega-3 fats and are also rich in other nutrients such as vitamin D and selenium, high in protein, and low in saturated fat. There is strong evidence that eating fish or taking fish oil is good for the heart and blood vessels. An analysis of 20 studies involving hundreds of thousands of participants indicates that eating approximately one to two 3-ounce servings of fatty fish a week—salmon, herring, mackerel, anchovies, or sardines—reduces the risk of dying from heart disease by 36 percent. (1)
Eating fish fights heart disease in several ways. The omega-3 fats in fish protect the heart against the development of erratic and potentially deadly cardiac rhythm disturbances. They also lower blood pressure and heart rate, improve blood vessel function, and, at higher doses, lower triglycerides and may ease inflammation. The strong and consistent evidence for benefits is such that the Dietary Guidelines for Americans, the American Heart Association, and others suggest that everyone eat fish twice a week. (2, 3)
Unfortunately, fewer than one in five Americans heeds that advice. About one-third of Americans eat seafood once a week, while nearly half eat fish only occasionally or not at all. (4) Although some people may simply not like fish, the generally low consumption is likely also caused by other factors, including perceptions about cost, access to stores that sell fish, and uncertainty about how to prepare or cook fish. Still others may avoid seafood because they worry that they—or their children—will be harmed by mercury, pesticide residues, or other possible toxins that are in some types of fish.
Should you forgo fish because of the contaminants they might carry? It’s a controversial topic that is often fueled more by emotion than by fact. Here’s what’s known about the benefits and risks of eating fish and other seafood:
- Known or likely benefits:In a comprehensive analysis of human studies, Harvard School of Public Health professors Dariush Mozaffarian and Eric Rimm calculated that eating about 2 grams per week of omega-3 fatty acids in fish, equal to about one or two servings of fatty fish a week, reduces the chances of dying from heart disease by more than one-third. (1) Both observational studies and controlled trials have also demonstrated that the omega-3 fats in fish are important for optimal development of a baby’s brain and nervous system, and that the children of women who consume lower amounts of fish or omega-3’s during pregnancy and breast-feeding have evidence of delayed brain development.
- Possible benefits:Eating fish once or twice a week may also reduce the risk of stroke, depression, Alzheimer’s disease, and other chronic conditions. (11)
- Possible risks: Numerous pollutants make their way into the foods we eat, from fruits and vegetables to eggs and meat. Fish are no exception. The contaminants of most concern today are mercury, polychlorinated biphenyls (PCBs), dioxins, and pesticide residues. Very high levels of mercury can damage nerves in adults and disrupt development of the brain and nervous system in a fetus or young child. The effect of the far lower levels of mercury currently found in fish are controversial. They have been linked to subtle changes in nervous system development and a possible increased risk of cardiovascular disease. The case for PCBs and dioxins isn’t so clear. A comprehensive report on the benefits and risks of eating fish compiled by the Institute of Medicine calls the risk of cancer from PCBs “overrated.” (5)
Avoiding fish is certainly one way to avoid mercury or PCBs. But is that the wisest choice, given the benefits of eating fish? Drs. Mozaffarian and Rimm put this in perspective in their analysis in the Journal of the American Medical Association. (1) First, reviewing data from the Environmental Protection Agency and elsewhere, they calculated that if 100,000 people ate farmed salmon twice a week for 70 years, the extra PCB intake could potentially cause 24 extra deaths from cancer—but would prevent at least 7,000 deaths from heart disease. Second, levels of PCBs and dioxins in fish are very low, similar to levels in meats, dairy products, and eggs. Third, more than 90 percent of the PCBs and dioxins in the U.S. food supply come from such non-seafood sources, including meats, dairy, eggs, and vegetables. So, given these limited health effects, low levels in fish, and major sources from other foods, the levels of PCBs and dioxins in fish should not influence your decision about which fish to eat (just as it does not influence your decision about whether or not to eat vegetables, meats, dairy products, or eggs, the major sources of PCBs and dioxins). One exception: if you eat local freshwater fish caught by friends or family, it makes sense to consult local advisories about the amounts of such fish you should eat.
Learn more about new study that finds no link between mercury exposure and heart disease, from Harvard School of Public Health and Brigham and Women’s Hospital.
At the levels commonly consumed from fish, there is also limited and conflicting evidence for effects of mercury in adults; thus, the Environmental Protection Agency, the Food and Drug Administration, the Institutes of Medicine report, and the analysis by Mozaffarian and Rimm all conclude that this evidence is insufficient to recommend limitations on fish intake in adults, given the established benefits of fish consumption for cardiovascular disease. In fact, the easiest way to avoid concern about contaminants is simply to eat a variety of fish and other seafood.
Except perhaps for a few fish species, the scale tips in favor of fish consumption for women who are pregnant. High intake of mercury appears to hamper a baby’s brain development. (6) But low intake of omega-3 fats from fish is at least as dangerous. In a study of almost 12,000 pregnant women, children born to those who ate less than two servings of fish a week didn’t do as well on tests of intelligence, behavior, and development as children born to mothers who ate fish at least twice a week. (7) A study conducted by Harvard researchers showed that visual recognition scores in six-month-olds were highest in those whose mothers ate at least two servings of fish a week during pregnancy but who also had low mercury levels. (8) Several other observational studies of fish intake during pregnancy, and randomized controlled trials of fish oil during pregnancy or breast feeding, have found similar benefits of mothers’ fish or fish oil intake for their babies’ brain development.
So, women should recognize that avoiding seafood altogether is likely to harm their babies’ brain development. The healthiest approach for women who are or may become pregnant, nursing mothers, and young children is to eat two servings per week of fish or other seafood, including up to one serving per week of white (albacore) canned tuna, and avoid the four fish species higher in mercury (shark, swordfish, tilefish, king mackerel). It is important that women recognize that the list of fish and seafood that they should eat is far larger than the few specific species to be avoided. Here’s what the Environmental Protection Agency and Food and Drug Administration recommend for women who are or may become pregnant, nursing mothers, and young children:
- Don’t eat shark, swordfish, king mackerel, or tilefish (sometimes called golden bass or golden snapper) because they contain high levels of mercury.
- Eat up to 12 ounces (two average meals) a week of a variety of fish and shellfish that are lower in mercury. Shrimp, canned light tuna, salmon, pollock, and catfish are low-mercury fish. Albacore (“white”) tuna has more mercury than canned light tuna. So limit your intake of albacore tuna to once a week. You can find a table of various fish, their omega-3 fatty acid content, and their average load of mercury and other contaminants online in the article by Mozaffarian and Rimm. (1)
- Check local advisories about the safety of fish caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but don’t consume any other fish during that week.
- So, these recommendations emphasize that women who are or may become pregnant, nursing mothers, and young children should eat fish, avoiding only four specific (and generally rarely consumed) fish species. Importantly, the latter limitation does not apply to the rest of the population, for whom the evidence supports simply choosing a variety of fish and seafood.
What If You Hate Fish?
Not all omega-3 fats come from fish. In fact, Americans also consume plant omega-3s in the form of alpha-linolenic acid (ALA), which is found in flax seeds, walnuts, and a few vegetable oils. In the human body, ALA is not converted to the marine omega-3s. EPA and DHA, to any large extent. So, the evidence does not support eating ALA as a replacement for seafood consumption. On the other hand, some data from observational studies like the Nurses’ Health Study suggest that getting extra ALA may reduce the chances of cardiovascular disease. (9) Another analysis, from the Health Professionals Follow-up Study, showed that higher intake of ALA may be particularly important for protection against heart disease in people who didn’t eat much fish. (10) Since these findings haven’t yet been replicated in randomized trials, the exact benefit of ALA is still a bit up in the air, but eating more foods rich in this good fat may also be good for health.
- Mozaffarian D, Rimm EB. Fish intake, contaminants, and human health: evaluating the risks and the benefits. JAMA.2006; 296:1885-99.
- U.S. Dept. of Health and Human Services USDA. Dietary Guidelines for Americans, 2005. Washington, D.C., 2005.
- Kris-Etherton PM, Harris WS, Appel LJ. Fish consumption, fish oil, omega-3 fatty acids, and cardiovascular disease. Circulation. 2002; 106:2747-57.
- Attitudes and Beliefs About Eating Fish: A National Opinion Survey Conducted for The Center for Food, Nutrition and Agriculture Policy.
- Seafood Choices: Balancing Risks and Benefits. Institute of Medicine: Washington, D.C., 2007.
- Grandjean P, Weihe P, White RF, et al. Cognitive deficit in 7-year-old children with prenatal exposure to methylmercury. Neurotoxicol Teratol.1997; 19:417-28.
- Hibbeln JR, Davis JM, Steer C, et al. Maternal seafood consumption in pregnancy and neurodevelopmental outcomes in childhood (ALSPAC study): an observational cohort study. Lancet.2007; 369:578-85.
- Oken E, Wright RO, Kleinman KP, et al. Maternal fish consumption, hair mercury, and infant cognition in a U.S. Cohort. Environ Health Perspect. 2005; 113:1376-80.
- Albert CM, Oh K, Whang W, et al. Dietary alpha-linolenic acid intake and risk of sudden cardiac death and coronary heart disease. Circulation.2005; 112:3232-8.
- Mozaffarian D, Ascherio A, Hu FB, et al. Interplay between different polyunsaturated fatty acids and risk of coronary heart disease in men. Circulation.2005; 111:157-64.
- Raji CA, Erikson KI, Lopez OL, Kuller LH, Gach HM, Thompson PM, Riverol M, Becker JT. Regular fish consumption and age-related brain gray matter loss. Am J. of Prev Med.2014; 47(4):444-51
The contents herein are not intended to offer personal medical advice. You should seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.