Advice for fish consumption: challenging dilemmas

The 2010 Dietary Guidelines for Americans (1) advise Americans to consume 8 ounces (227 g) of seafood per week to reach an average intake of 250 mg/d of the omega-3 fatty acids EPA and DHA. This recommendation represents a substantial increase over current consumption amounts of ;3.5 oz/wk. It is based on ‘‘moderate, consistent evidence’’ that the health benefits of increased seafood consumption outweigh the risks associated with methylmercury, a toxic contaminant of large predatory food fish (tilefish, shark, swordfish, king mackerel) and, to a lesser extent, albacore (white) tuna. To avoid this toxin, the guidelines advise eating seafood typically found to be low in methylmercury, such as salmon, anchovies, sardines, and trout. Such advice, however, leads to at least 3 dilemmas. Eating more fish might raise methylmercury intake above safe amounts. Pressures to consume more fish might place impossible demands on an already threatened seafood supply. And the obvious solution— fish farming—raises concerns about what farmed fish are fed and how farmed fish affects the environment. In this issue of the Journal, Nielsen et al (2) address the first of these dilemmas. From analyses of blood samples collected in the 2007–2010 NHANES, they related blood mercury concentrations to the type and frequency (but not the amount) of seafood consumption by US adults. Of adults sampled, 4.6% showed blood concentrations 5.8 lg mercury/L, the Environmental Protection Agency’s cutoff for the dose without an appreciable lifetime risk of deleterious effects. NHANES analyzed the blood samples for total mercury, although most of it can be assumed to be methylmercury, the most toxic form. Much as expected, the risk of blood mercury concentrations exceeding the reference dose was greatest for adults who most frequently consumed shark and swordfish. It was less robust for salmon and tuna consumption frequency and did not increase with shrimp and crab consumption. The relation with frequency of salmon consumption is the one unexpected result. Typically, salmon are reported to have very low amounts of mercury contamination. The authors offer no explanation, but one possibility is that the salmon were farmed and consumed feed that was higher in mercury than would be encountered in the wild, thereby creating another potential dilemma. Because large predatory fish are usually high in methylmercury, and the developing brain of the fetus is most sensitive to this toxin, a joint Environmental Protection Agency/Food and Drug Administration advisory cautions pregnant women not to eat the 4 big predatory fish and to limit consumption of albacore tuna (3). The results reported by Nielsen et al are consistent with advice to pregnant women to avoid these fish. But because Food and Drug Administration monitoring data show a wide range of mercury amounts in other fish species, it is possible that advising greater consumption of a variety of fish could result in elevated blood mercury concentrations (4), particularly because pregnant women are advised to consume from at least 8 and up to 12 oz/wk from seafood choices that are low in methylmercury. Although the data reported by Nielsen et al indicate that 40% of US adults consume seafood 5 times/mo, per capita consumption amounts remain at approximately half the amounts suggested by the Dietary Guidelines (5). The dietary recommendations are based on fish as a good source of protein and omega-3 fatty acids but also on clinical studies associating fish consumption with a reduced risk of coronary artery disease in adults and with improved neurologic outcomes in infants and young children. It is not unreasonable to assume that following advice to nearly double seafood consumption would lead to a larger proportion of adults with blood mercury concentrations exceeding the reference dose (6). Karagas et al (7) reviewed the evidence for the health effects of mercury exposures at amounts similar to the EPA reference dose. In adults, the data relating these low amounts of methylmercury to cardiovascular effects were inconsistent. The investigators also found mixed results on whether prenatal exposure to low amounts of mercury affects fetal growth, although the data were more consistent in showing negative effects on early childhood neurocognition and behavior. In general, studies of cognition in children born to mothers exposed to methylmercury are complicated by the potentially beneficial effects of the higher amounts of omega-3 fatty acids obtained from greater seafood consumption (8). The second dilemma has to do with the environmental and social costs of dietary advice to eat more fish. Meeting the current guidelines requires more than a doubling of per capita fish intake in the United States to more than double the average intake of slightly more than 100 mg EPA and DHA fatty acids/d (5). For years, ecologists have been warning that exploitation of ocean fisheries at the present rate is not sustainable without widespread conservation efforts (9). Already, many ocean fisheries are fully exploited or are in decline. Pressures to increase consumption in