Pregnant women on the hook for calculating risks, benefits of fish

Guidelines for maximizing omega-3 fats, minimizing mercury leave the math to the consumer

fish market

Fish is an excellent dietary choice for pregnant and nursing women, but only certain kinds of fish in certain amounts. 

Prezbob/Wikimedia Commons (CC BY-SA 3.0)

And we’re back for the latest edition of Don’t Eat That! This episode covers the FDA’s recent draft report that’s intended to clear the muddy waters on fish for pregnant and nursing women.

It’s a scaly subject: Fish is a wonderfood, packed with protein and brain-building omega-3 fats. But many fish are also packed with the toxic metal methylmercury. Most of our waterways are polluted with mercury from natural sources such as volcanoes and unnatural sources such as coal plants. Small fish feed on contaminated plants, and bigger fish feed on those smaller fish, until top predators — such as shark, swordfish and mackerel — are practically brimming with methylmercury, a form of the metal that’s readily absorbed into the body.

Methylmercury is at the heart of some tragic history. In the 1930s, the plastics manufacturing company Chisso began dumping waste, including methylmercury, into the Minamata Bay in Japan. By the 1950s, a generation of children suffered from brain damage and neurological impairments.

The simultaneous dangers and benefits of fish cast pregnant and nursing women into a catch-22: How do you eat enough fish to maximize the benefits, but avoid the toxins?

The government’s latest draft guidelines thread this hook ever so delicately, leaving most of the hard work of figuring out exactly what kinds of fish to eat, and how much of it, up to women.

Back in 2004, the FDA recommended that women who are pregnant, might become pregnant or are nursing avoid shark, swordfish, king mackerel and tilefish, and eat no more than 12 ounces of other fish a week. That recommendation may have scared some women away from eating fish at all. So for the first time, the updated recommendations include a minimum: Pregnant and breastfeeding women should eat at least 8 but no more than 12 ounces of fish low in methylmercury a week, the new recommendations say. The report provides a list of commonly consumed fish and their relative omega-3 fatty acid content and methylmercury baggage.

Salmon, you’ll see, comes with a stellar rating, packing in 1,200 to 2,400 milligrams of omega-3 fatty acids, but delivering only 2 micrograms of methylmercury per 4-ounce serving. Orange roughy, on the other hand, delivers just 42 milligrams of omega-3 fatty acids and is loaded down with 80 micrograms of methylmercury. That looks like a bad choice to me, even though it’s not on the “do not eat” list with the same offenders from the 2004 list: shark, tilefish from the Gulf of Mexico, swordfish and king mackerel.

Another fish that comes out looking bad is tuna. Canned white tuna, the more expensive variety also called albacore, delivers 1,000 milligrams of omega-3, but is packed with 40 micrograms of methylmercury. Mercury watchdog groups have been lobbying for albacore tuna to be included on the “do not eat” list for years. The new draft recommendations don’t go that far, but they do suggest that pregnant or breastfeeding women limit themselves to 6 ounces a week.

The guidelines don’t give specific limits of methylmercury women should stay under for any given week. Annoyingly, that calculation is left for women to do themselves. The maximum dose of methylmercury to be considered not likely to cause harm, called the RfD or reference dose, is 0.1 micrograms per kilogram of body weight per day.

For my weight, my limit is about 43 micrograms of methylmercury per week. Another look at that chart tells me that I can have catfish on Monday and sardines on Thursday and still be well below my methylmercury limit. But you know what I can’t have? A single serving of bluefin or albacore tuna steaks, marlin, lobster or orange roughy. I can have one serving of canned albacore, but that’ll nearly blow my entire weekly allowance.

And of course, these numbers provided by the FDA are estimates. Actual methylmercury concentrations may vary quite a bit. And for fish caught by your friend, all bets are off. To figure out if that bass or trout is safe to eat, your best bet is to try your luck with a map and calculate your own exposure.

If this advice feels overwhelming, that’s because it is. Pregnant women should really eat fish. But only the right kinds of fish. And only in certain amounts. Amid all of this bean-counting, it’s sad that the bigger picture has faded. People shouldn’t have to choose between the health benefits of fish and the harmful effects of mercury. Instead, we should be able to eat as much fish as we want, caught in waters free of pollutants.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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