We thought our readers might be interested to read the following report by renowned seafood expert Joyce Nettleton, DSc, whom we've known for several years.

We've reported on this topic before, but Dr. Nettleton puts it all together very clearly and succinctly in one place. It's a keeper!

Seafood: Weighing the Benefits and Risks

By Joyce A. Nettleton, DSc, January 2008

Few foods have confused consumers as much as seafood. Even though the health benefits associated with eating fish and shellfish continue to expand, fears of seafood contaminants sometimes overshadow the good associated with eating more fish. Are we being irrational? Risk experts say that people exaggerate the chance of rare but nasty events occurring, following their gut instincts rather than logic. Faced with conflicting information, consumers say, “Why take the chance when I don't have to?” This paper takes a close look at what we might gain or lose by eating fish and shellfish more often.

US Seafood Consumption

Compared with meat and poultry, Americans eat little seafood, about 14 g (½ oz) a day versus 210 g (7 ½ oz) a day for meat and poultry. Most people eat fewer than 2 servings of fish/week, the amount currently recommended. Low seafood consumption means that the intakes of long-chain omega-3 fatty acids—mainly EPA and DHA (eicosapentaenoic and docosahexaenoic acids)—are also low, about 100 mg/day.

These fatty acids occur almost exclusively in seafood and cannot be substituted by the omega-3 fatty acid found in plants. That is because only tiny amounts of the plant omega-3 are converted to the active long-chain forms. In contrast with Americans, Japanese adults consume 2 to 13 times more seafood, eating fish once to 8 times a week.

Low US seafood consumption is of particular concern for pregnant and nursing women. They obtain about 100 mg of DHA/day, half what experts believe they need for themselves and their infants—200 mg of DHA daily. The result of not eating much fish appears in the low DHA content of US women's breast milk (chart). The question is, do these low intakes matter? Much evidence says they do.

Special Need for DHA in Pregnancy and Lactation

Just as vitamins and minerals are needed for fetal development during pregnancy, so long-chain essential fatty acids are key nutrients for the developing nervous and vascular systems. Sufficient arachidonic acid, a long-chain omega-6, is provided in the usual diet, but adequate DHA depends on dietary intake and tissue stores of the preformed fatty acid. Vegetarians and women who do not eat fish have lower levels of DHA than women who eat fish or other foods with preformed DHA, such as omega-3-enriched eggs.

Their infants also have less DHA. Higher maternal DHA translates into higher DHA in the infant. DHA is needed for the infant's brain and eye development, brain structure and function, cognitive development, visual acuity, more mature sleep patterns and for healthy immune system maturation. DHA also contributes to slightly longer gestation with lower chance of complications or preterm delivery.

Mothers who consume more DHA during pregnancy lose less of their own DHA and may be less prone to postpartum depression. For these and other reasons, most US women need to boost their DHA intake during pregnancy and lactation.

Diverse Health Benefits From Seafood's Long-Chain Omega-3s

The long-chain omega-3s found in seafood benefit everyone's health. Evidence continues to accumulate revealing how seafood omega-3s may improve our health. The benefits include:

Heart health

  • Lower chance of dying from coronary heart disease and sudden cardiac death (20 to 50% less likely);
  • Improved electrical properties of the heart, including more stable rhythms, lower heart rate and better heart rate variability (adaptability);
  • Less chance of a first heart attack and other nonfatal cardiac events;
  • Improved blood lipid patterns with lower blood triglycerides (fats) and higher HDL or “good” cholesterol levels; however, seafood omega-3s have little effect on LDL or “bad” cholesterol;
  • Less unwanted blood clotting that could lead to a heart attack or stroke;
  • Reduced inflammation, an underlying contributor to heart disease and other disorders;
  • Healthier blood vessel function and blood flow;
  • Slower progression of atherosclerosis and clogged arteries.

Brain function

  • Sharper brain function including neurotransmission (communication between brain cells), protection of neurons from injury and disease, rapid responses to hormones and regulatory substances, and improved brain cell repair and regeneration;
  • Healthy cognition and memory, especially in aging; seafood omega-3s may lower the risk of developing Alzheimer's disease, other dementias and possibly Parkinson's disease;
  • Proper brain growth and development in fetal and infant life; insufficient intake of DHA and EPA is associated with lower brain DHA content and a greater chance of childhood behavioral disorders such as dyslexia, attention deficit hyperactivity disorder and conditions affecting movement and coordination;
  • Mental health—Reduced risk and severity of several mental disorders, including depression, bipolar disorder, and mood disorders such as anxiety, hostility and aggression.

Visual function

  • Long-chain omega-3s are vital for healthy visual and retinal function; they may lower the chance of developing age-related macular degeneration and possibly cataract, dry eye, glaucoma and other visual disorders.
  • The retina has the body's highest concentration of DHA, which aids in converting light to visual signals and in dim-light and night vision. DHA promotes healthy visual development in early infancy and protects against loss of blood vessels in visual damage.

Immune function

  • Seafood omega-3s promote immune system maturation in infancy and may lower the chance of childhood allergies. Increased omega-3 consumption may ease the symptoms of inflammatory conditions, such as rheumatoid arthritis, asthma, certain allergies and digestive disorders. Omega-3s tone down overactive immune responses making symptoms less severe, but they do not cure the conditions.
  • High levels of long-chain omega-3s have been used to treat rheumatoid arthritis, without the adverse side effects of some anti-inflammatory drugs. Some evidence suggests that increased intake of long-chain omega-3s during pregnancy reduces the infant's chance of developing allergies, such as eczema, allergic rhinitis and asthma.

Clinical conditions

  • Long-chain omega-3s may reduce the chance of type 2 diabetes and improve the welfare of patients needing total parenteral nutrition; they may be helpful in post-surgery and trauma, inflammatory bowel and Crohn's disease, possibly some forms of cancer and most recently bone health.

What Is the Potential Downside of Eating Seafood?

Most seafood contains detectable levels of contaminants because these are part of the environment and food chain. Of greatest concern is methylmercury, a heavy metal readily absorbed and potentially toxic. Inorganic mercury from volcanoes, the weathering of soils and rocks, coal-fired power plants, mining and various industrial processes is converted by aquatic microorganisms to methylmercury, the form found in fish.

The concern about methylmercury (mercury) is three-fold: it accumulates through the food chain and is most concentrated in large predator and long-lived fish; seafood is the major source of mercury in humans; and mercury is potentially toxic to the developing nervous system of the fetus and infant. Other undesirable effects in adults are much less certain16.

In contrast to the potential harm from mercury, the great majority of scientific evidence suggests that eating fish does much more for your health than against it. Seafood consumption has net health benefits in cardiovascular, neurologic, immune, behavioral and mental health outcomes. Moreover, seafood carries a protective factor against mercury toxicity.

Who Could be at Risk from Mercury?

Because mercury can affect the developing nervous system, pregnant and nursing mothers exposed to large amounts of mercury could jeopardize the brain development of the fetus and infant. Most other adults are at very low risk from mercury. Detrimental effects of mercury have been observed in accidental poisonings in Japan and Iraq, but there is no clinical evidence that women who consume fish, even large amounts of fish, as women in Japan and Iceland do, harm their infants.

In fact, the opposite seems to be the case. A large study in the United Kingdom reported that children whose mothers had the greatest fish consumption—more than the US Food and Drug Administration (FDA) recommends—had higher neurodevelop-mental scores than children whose mothers consumed less fish or none. Children whose mothers avoided fish had suboptimal scores.

Other studies suggest that children whose mothers have high fish intakes during their pregnancy are less likely to develop eczema and asthma. Thus, several lines of evidence suggest that avoiding fish consumption during pregnancy may be detrimental to the child's health and development.

Worries about the effects on childhood development of mothers' eating mercury-contaminated seafood have come mainly from studies of mothers and children in the Faroe Islands. There, women eat fish and pilot whale during pregnancy. Researchers reported that prenatal mercury exposure was significantly linked to deficits in various neurodevelopmental tests when the children were 14 years of age. In contrast, a different study of mothers and children in the Seychelles Islands, where fish consumption is 10 times greater than in the U.S., researchers found no evidence of adverse effects of maternal fish and mercury intake on child development through 11 years of follow-up. Why are the findings in these two large studies different?

There is a catch. In the Faroe Islands, pregnant women are exposed to mercury from consuming pilot whale, which is highly contaminated with mercury and organic pollutants. Fish consumption contributes relatively little mercury. When the contribution of pilot whale mercury is taken into account, the effects of fish consumption were all beneficial, whereas those including pilot whale were negative.

Thus, much of the fear about the developmental effects of mercury is based on eating contaminated pilot whale, not fish consumption. As Myers and Davidson commented, “There has never been even one child with prenatal mercury poisoning from [mothers] consuming fish documented outside Japan.”

The Mercury Shield in Fish: Selenium

The potentially toxic effect of mercury appears to be abolished by the selenium fish contains. That selenium protects against the toxicity of heavy metals has been known for 40 years, although how it works has only recently been described.

Selenium, a potent antioxidant, is incorporated into proteins capable of binding heavy metals. One of these proteins, SeP, is especially rich in metal binding sites. A recent study reported that when people were exposed to high levels of mercury, SeP bound the mercury and binding increased with greater mercury exposure.

Further, the selenium proteins protected against harmful oxygen products resulting from the increased mercury. Thus, when mercury exposure is high, more selenium is retained which binds the mercury, preventing its toxic effects. The brain selectively takes up and retains selenoproteins, even when dietary selenium is low, creating a cache to guard against the neurotoxic effects of mercury. Feeding sufficient selenium completely protected against the detrimental effects of a high mercury diet on growth30.

It so happens that seafood is the richest source of selenium in the diet. Amounts of selenium in fish nearly always exceed those of mercury, even in large pelagic fish that are more likely to carry higher levels of mercury.

A recent study of 15 species of pelagic fish reported that only mako shark had more mercury than selenium. In the Faroe Islands, pilot whale also has considerably more mercury than selenium, making it more harmful than if selenium were more abundant than mercury. Freshwater fish also vary widely in their selenium and mercury levels, so fish with high mercury and low selenium would be expected to pose a greater health risk than ocean fish.

The relative amounts of selenium and mercury may be the most reliable way to assess the potential risk from mercury in fish.

Choosing Fish Wisely

Smart consumers will include seafood on the menu regularly, at least 2 to 3 times a week so they obtain the many health benefits of long-chain omega-3s. Many, if not most, of these benefits cannot be obtained from plant sources of omega-3s (e.g., flax, walnuts, canola and soybean oils) because the body converts the omega-3 in plants to the long-chain omega-3s very inefficiently. Conversion to DHA is almost negligible, so pregnant and nursing women would be especially short-changed by relying solely on plants for their omega-3s.

All seafoods have some long-chain omega-3s, but the fattier species, such as salmon, rainbow trout, light tuna, mackerel, sardines, black cod and herring have the greatest amounts. Most fish are low in mercury, but large long-lived fish, such as shark, marlin, swordfish and very large halibut tend to have higher levels. With the exception of shark and possibly swordfish, even these large fish may pose minimal risk from mercury because of their abundant selenium.

However, the FDA's advice to pregnant and nursing women to avoid eating shark, swordfish, tilefish (golden bass or golden snapper) and king mackerel is safest. These species are more likely to have higher concentrations of mercury than is desirable.

  • US Dept. Agriculture, Economic Research Service. Food availability spreadsheets: Red meat, poultry and fish. Accessible at: http://www.ers.usda.gov/Data/FoodConsumption/FoodAvailSpreadsheets.htm
  • Kris-Etherton PM, Harris WS, Appel LJ for the Nutrition Committee. Fish consumption, fish oil, omega-3 fatty acids, and cardiovascular disease. Circulation 2002;106:2747-2757.
  • Dept. Health and Human Services and Department of Agriculture. The Report of the Dietary Guidelines Advisory Committee on Dietary Guidelines for Americans, 2005. Executive Summary. Accessible at: http://www.health.gov/dietaryguidelines/dga2005/report/HTML/A_ExecSummary.htm.
  • Institute of Medicine. Seafood Choices: Balancing Benefits and Risks. Nesheim MC, Yaktine AL (editors). 2007. National Academies Press. Washington, D.C. pp. 50-51.
  • Plourde M, Cunnane SC. Extremely limited synthesis of long chain polyunsturataes in adults: implications for their dietary essentiality and use as supplements. Appl Physiol Nutr Metab 2007;32:619-634.
  • Iso H, Kobayashi M, Ishihara J, Sasaki S, Okada K, Kokubo Y, Tsugane S: JPHC Study Group. Intake of fish and n3 fatty acids and risk of coronary heart disease among Japanese: the Japan Public Health Center-Based (JPHC) Study Cohort I. Circulation 2006;113:195-202.
  • Koletzko B, Cetin I, Brenna JT for the Perinatal Lipid Intake Working Group. Dietary fat intakes for pregnant and lactating women. Br J Nutr 2007;98:873-787.
  • Bourre JM. Dietary omega-3 fatty acids for women. Biomed Pharmacother 2007;61:105-112.
  • Innis SM. Fatty acids and early human development. Early Hum Dev 2007;83:761-766.
  • Reddy S, Sanders TA, Obeid O. The influence of maternal vegetarian diet on essential fatty acid status of the newborn. Eur J Clin Nutr 1994;48:358-368.
  • Hornstra G. Essential fatty acids in mothers and their neonates. Am J Clin Nutr 2000;71(5 Suppl):1262S-1269S.
  • Field CJ, Clandinin MT, Van Aerde JE. Polyunsaturated fatty acids and T-cell function: implications for the neonate. Lipids 2001;36:1025-1032.
  • Olsen SF, Osterdal ML, Salvig JD, Kesmodel U, Henriksen TB, Hedegaard M, Secher NJ. Duration of pregnancy in relation to seafood intake during early and mid pregnancy: prospective cohort. Eur J  Epidemiol 2006;21:749-758.
  • Environmental Protection Agency. Mercury: Basic information. 2007. Accessible at: http://www.epa.gov/mercury/about.htm.
  • Center for Food, Nutrition, and Agriculture Policy. University of Maryland. Real Mercury Facts. Accessible at: http://agresearch.umd.edu/CFNAP/realmercuryfacts/index.htm.
  • Mozaffarian D, Rimm E. Fish intake, contaminants, and human health. Evaluating the risks and benefits. JAMA 2006;296:1885-1899.
  • Hibbeln JR, Davis JM, Steer C, Emmett P, Rogers I, Williams C, Golding J. Maternal seafood consumption in pregnancy and neurodevelopmental outcomes in childhood (ALSPAC study): an observational cohort study. Lancet 2007;369:578-585.
  • Romieu I, Torrent M, Garcia-Esteban R, Ferrer C, Ribas-Fito N, Anto JM, Sunyer J. Maternal fish intake during pregnancy and atopy and asthma in infancy. Clin Exp Allergy 2007;37:518-525.
  • Salam MT, Li Y-F, Langholz B, Gilliland FD. Maternal fish consumption during pregnancy and risk of early childhood asthma. J Asthma 2005;42:513-518.
  • Debes F, Budtz-Jorgensen E, Weihe P, White RF, Grandjean P. Impact of prenatal methylmercury exposure on neurobehavioral function at age 14 years. Neurotoxicol Teratol 2006;28:536-547.
  • Myers GJ, Davidson PW, Cox C, Shamlaye CF, Palumbo D, Cernichiari E, Sloane-Reeves J, Wilding, GE, Kost J, Huang L-S, Clarkson TW. Prenatal methylmercury exposure from ocean fish consumption in the Seychelles child development study. Lancet 2003;361:1686-1692.
  • Davidson PW, Myers GJ, Cox C, Wilding GE, Shamlaye CF, Huang LS, Cernichiari E, Sloane-Reeves J, Palumbo D, Clarkson TW. Prenatal methyl mercury exposure from fish consumption and child development: a review of evidence and perspectives from the Seychelles Child Development Study. Neurotoxicology 2006;27:1106-1109.
  • Budtz-Jorgensen E, Grandjean P, Weihe P. Separation of risks and benefits of seafood intake. Environ Health Perspect 2007;115:323-327.
  • Myers GJ, Davidson PW. Fish consumption benefits children's development. Lancet 2007;369:537-538.
  • Kosta L, Byrne A, Zelenko V. Correlation between selenium and mercury in man following exposure to inorganic mercury. Nature 1975;254:238-239.
  • Parizek J, Ostadalova I. The protective effect of small amounts of selenite in sublimate intoxication. Experiential 1967;23:142-143.
  • Chen C, Yu H, Zhao J, Li B, Qu L, Liu S, Zhang P, Chai Z. The roles of serum selenium and selenoproteins on mercury toxicity in environmental and occupational exposure. Environ Health Perspect 2006;114:297-301.
  • Burk RF, Hill KE, Read R, Bellew T. Response of rat selenoprotein P to selenium administration and fate of its selenium. Am J Physiol 1991;261(1 Pt 1):E26-E30.
  • Nakayama A, Hill KE, Austin LM, Motley AK, Burk RF. All regions of mouse brain are dependent on selenoprotein P for maintenance of selenium. J Nutr 2007;137:690-693.
  • Ralston NVC, Blackwell JL, Raymond LJ. Importance of molar ratios in selenium-dependent protection against methylmercury toxicity. Biol Trace Elem Res 2007;119:255-268.
  • Kaneko JJ, Ralson NC. Selenium and mercury in pelagic fish in the central north Pacific near Hawaii. Biol Trace Elem Res 2007;119:242-254.