In June, the Food and Drug Administration (FDA) and the Environmental Protection Agency issued an updated draft of advice on fish consumption for pregnant and breastfeeding women. The stated reason for the update was a recent survey indicating that 21% of pregnant women had eaten no fish during the preceding month and 75% had eaten <4 ounces a week and were thus missing nutrients deemed important for fetal growth and development. Presumably, many women learned of the recommendations that were drafted in 2001 and were reaffirmed in 2004 by these 2 agencies (warning of potential mercury contamination in fish and advising that consumption of fish low in mercury be limited to 12 ounces per week and consumption of albacore tuna limited to 6 ounces) and decided not to eat fish at all. Without specifying all the “numerous dietary and health benefits” ascribed to fish consumption, the updated draft referred to the 2010 Dietary Guidelines for Americans, which provided a table listing the milligrams of omega 3 fatty acids (eicosapentaenoic acid [EPA] and ducosahexaenoic acid [DHA]) and the micrograms of mercury in 4 ounce servings of a variety of fish ( Table ). The underlying message seemed to be that, rather than avoiding fish consumption during pregnancy and breastfeeding, women should eat fish with high levels of omega 3 fatty acids and low levels of mercury.

It seems safe to assume that, at least in part, this advice reflects data that indicate that DHA and EPA are important for normal fetal development. Omega 3 fatty acids are important components of membranes in all cells of the body and are essential for the maintenance of membrane fluidity and normal functioning of membrane-bound enzymes, receptors, and transporters. Omega 3 fatty acids are vitally important for normal development of the brain and retinal tissues (especially myelin and retinal photoreceptors) and for maintenance of normal neurotransmission and connectivity. Unique among the primates, only human fetuses develop a layer of body fat in the third trimester that serves as a reservoir of DHA and other fatty acids that are needed for brain development and metabolism; it is believed that regular access to a shore-based diet and the ability to store DHA in fetal body fat contributed significantly to the evolution of the human brain. Maternal ingestion of adequate quantities of fish (defined in many studies as at least 340 grams of oily fish each week) has been associated with better childhood IQ scores, fine motor coordination, and communication and social skills, and reduced incidence of postpartum depression. Omega 3 fatty acids are substrates for the synthesis of a variety of antiinflammatory and inflammation-resolving mediators and also favorably alter the production of thromboxane and prostaglandin E2, which may account for the longer gestations and higher birthweights that are associated with diets based largely on fish. A variety of literature touts omega 3 fatty acids for improving cardiovascular health by preventing fatal arrhythmias and reducing triglyceride and C-reactive protein levels. The Centers for Disease Control and Prevention monitors serum levels of DHA and EPA as indicators of a healthy diet, and the American Heart Association specifically includes high intake of marine omega 3 fatty acids in its definition of a heart healthy diet.

Some groups criticized the FDA advice because it did not specify how much omega 3 fatty acid to ingest each day or state which fish would safely provide that amount. Although the FDA did not clarify how to choose from among the fish on the list, it did specify which fish to avoid because of high mercury levels. Using data derived from mercury poisonings around the world, the Environmental Protection Agency has determined that the safe allowable methyl mercury intake during pregnancy is 0.1 μg/kg per day or roughly 6-8 μg per day, 42-64 μg per week. The FDA advised against eating shark, tilefish, swordfish, and king mackerel during pregnancy or while breastfeeding, because they have the highest mercury levels (100-220 μg per 4 ounce serving ). The FDA also specifically recommended limiting consumption of albacore tuna to 6 ounces per week, although other kinds of tuna (bluefin, skipjack, and yellowfin) and lobster also have moderately high mercury levels (30-60 μg per 4 ounce serving). Although farmed fish often contain less mercury than wild caught fish, they contain higher levels of polychlorinated biphenyls (PCBs) and dioxin. Fatty sport fish from PCB contaminated lakes (such as the Great Lakes) may have unacceptably high levels; antenatal PCB exposure has been associated with lower full scale and verbal IQ scores, greater impulsivity, and poorer concentration and memory in exposed children. Shrimp, the most commonly consumed seafood in the United States, usually contains no mercury and thus might be considered the ideal fish to eat during pregnancy, but a 4-ounce serving contains only 100 μg of DHA and EPA.

Although taking omega 3 fatty acid or fish oil supplements would seem to be a great way to reap the benefits of DHA and EPA without risking exposure to mercury and other toxins, the FDA draft also stated that “the entire package of nutrients that fish provide may be needed to fully benefit fetal and child development” and that women who avoid fish and instead take omega 3 fatty acid supplements may miss out on many other important nutrients that are required for overall health. These statements may have been a way of acknowledging that, although epidemiologic data indicate that eating a diet that includes lots of oily fish is associated with a wide variety of health benefits, randomized controlled trials of DHA and EPA or fish oil supplementation generally have been disappointing. Randomized trials of antenatal omega 3 fatty acid or fish oil supplements have found no sustained improvement in cognitive, language, or motor scores in children who were exposed as fetuses; no benefit for cardiovascular health (1 Cochrane review included 48 trials with almost 37,000 participants); no reduction in preeclampsia, intrauterine growth restriction, or antenatal or postpartum depression; and only modest effects on birthweight and length of gestation.

It could be that a diet replete with fish is only an indicator of generally beneficial health behaviors, all of which have a cumulative positive effect on outcome. It is also possible that the DHA and EPA found in dietary supplements do not have the same metabolic properties as the naturally occurring form. Recent evaluation of fish oil preparations has shown that the stability of such products varies widely, in part because toxic contaminants such as pesticides and mercury have to be removed, and that process also removes antioxidants that impart stability and thus have to be restored to keep the product from deteriorating. Importantly, dioxins and PCBs often are not eliminated entirely, and fish oil products, especially those made from farmed fish, may contain levels above the recommended limit. Highly concentrated fish oils have particularly low stability, deteriorate rapidly, and, on ingestion, induce consumption of antioxidants and formation of free radicals. In addition, bioavailability varies depending on the chemical form in which DHA and EPA are bound and whether the supplement is taken with food.

It also seems likely that we do not really know what the ideal dose of DHA and EPA should be. The daily requirement for DHA and EPA has not been established, and most of the published studies that used omega 3 fatty acid supplements used a wide range of doses, from 133 mg to >7.0 g per day. Although the FDA recommends that pregnant women eat at least 8 ounces and up to 12 ounces of fish per week, that amount could correspond to 200-2400 mg of EPA and DPA, depending on the fish chosen, and appears to be derived empirically. Ironically, in April of this year the FDA published a rule prohibiting manufacturers from stating that their products are “high” or “rich in” or an “excellent source of” DHA and EPA, because no reference levels for DHA or EPA intake have been established.

Which brings us back to the relatively vague FDA advice: pregnant and nursing women should eat fish because fish contains omega 3 fatty acids essential for fetal brain development and because it does not appear that dietary supplements have the same beneficial effects. However, they should not eat fish with high mercury levels and should be cautious about eating wild-caught local fish if they are unclear about the level of contaminants in the water. Choosing a fish with relatively high DHA and EPA levels sounds good (although it rules out shrimp), but the optimal daily dose of DHA and EPA is unknown. Looking at the list, salmon, pollack, and squid seem like good choices (600-900 μg of DHA and EPA, 2-11 μg of mercury per 4 ounce serving). After the Atlantic cod population plummeted in the 1990s because of over fishing, pollack became the main fish used for fast food fish sandwiches in the United States, so it might be that the most widely consumed prepared fish is also the best fish choice. Just tell your pregnant patients to go easy on the fries.