Nutritional advice is usually straightforward. There are things we’re supposed to eat more of: vegetables, fruit, whole grains. There are things we’re supposed to eat less of: trans fats, added sugars, anything coated with salty orange dust.
And then there’s fish.
We’re supposed to eat more of it because it has healthful omega-3 fats. But we’re supposed to eat less of it because it’s full of environmental contaminants. Balancing the risks and benefits is hard, even for the doctors and scientists in the field. Absent sound advice, it’s all but impossible for those of us trying simply to decide what’s for dinner.
Consider salmon. According to the joint advisory issued by the Food and Drug Administration and the Environmental Protection Agency, salmon is low in mercury and safe even for pregnant women. Yet the Environmental Defense Fund, an advocacy group, suggests that all adults - not just pregnant women - limit wild salmon (except for Alaskan) to one serving per month and farmed salmon to no more than two, because of PCB contamination.
This kind of disparity raises two questions: What do we know about fish, and what do we know about the advice we’re getting about fish?
We certainly know that there are benefits from eating fish and risks from eating contaminants. The advantages are mostly attributed to long-chain omega-3s, polyunsaturated fats that are found in almost all fish, and almost exclusively in fish. The best-established benefits are reduction of heart disease risk and, in the case of pregnant women, improved neurodevelopment in fetuses and young children. But there’s also a slate of less well-established benefits, ranging from reduced stroke risk to mood elevation.
The risks come from mercury and PCBs, both byproducts of industrial processes. (The United States banned PCBs in 1979, and environmental levels are persistent but gradually decreasing.) These contaminants accumulate in fish tissue; they become more concentrated as you go up the food chain as the bigger fish eat smaller, contaminated fish. Fish are the only predators we eat regularly, and long-lived predator species such as swordfish and tilefish generally have the highest contaminant levels.
Fish are also contaminated with dioxins, which are a byproduct of incineration. But that concern isn’t fish-specific because dioxins are found up and down the food chain, and we get more dioxins from meat and dairy than from fish.
Risk vs. benefit
In recent years, several scientific organizations have tried to weigh the risks of eating fish against the benefits. Notably, two groups - the Institute of Medicine (IOM) in 2007 and the Food and Agriculture Organization of the United Nations working with the World Health Organization (FAO/WHO) in 2010 - convened panels of nutritional, toxicological and epidemiological experts to review the data and do the math.
Both groups issued guidelines for pregnant women and other adults, and their recommendations are remarkably similar.
For pregnant women, their advice is in line with that of the well-known FDA/EPA joint advisory issued in 2004: Eat up to 12 ounces of fish per week, focusing on low-mercury fish and avoiding the four highest-mercury fish (swordfish, shark, tilefish and king mackerel).
For other adults, the advice could be summarized as “eat fish.”
If you eat more than two servings of fish per week, both his group and the IOM advise that you don’t always eat the same type, so you’re not eating one contaminant over and over.
So why aren’t we all just eating fish and not worrying about it?
Because of warnings such as those from the Environmental Defense Fund. Its guide to fish consumption lists 21 species of which adults should eat no more than one serving per month, and that’s “assuming no other contaminated fish is consumed.”
So one serving of flounder, perch or blackfin tuna would be your entire monthly allowance, and your choices for the rest of the month would be limited to the least-contaminated fish, such as halibut, sole or yellowfin tuna.
There is another problem with risk-only advisories: the issues with the FDA/EPA safe thresholds for mercury and PCBs. For mercury, that threshold is 0.1 micrograms per kilogram of body weight per day. For a 150-pound person, that would be fewer than seven micrograms a day. Swordfish, one of the most contaminated fish, can have 100 micrograms - two weeks’ allowance - in a 3.5-ounce serving.
For PCBs, it’s a more complex calculation of cancer risk called a cancer slope factor.While we’re waiting for advisories to improve, the best advice is simply to eat fish; the data show that the benefits outweigh the risks. If you’re pregnant, follow the FDA/EPA guidelines. If you’re not, mix up your species, and be sure to include smaller, bottom-of-the-food-chain fish. If you’re an angler, check with local authorities for contaminant levels where you fish, and limit your consumption if those levels are high.