A recent editorial from Cornell concludes that nobody really knows how much fish you can eat without harming yourself or the environment (The American Journal of Clinical Nutrition, May, 2014;99(5): 973-974). The editorial, by respected nutritionists Marion Nestle and Mal Nesheim, notes three dilemmas: “Eating more fish might raise methyl mercury 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.”
Fewer Heart Attacks
Several studies show that people who eat fish twice a week have fewer heart attacks than those who eat no fish. No good data show that you gain any more protection from heart attacks by eating fish more often than that. Fish are thought to reduce heart attack risk because they contain omega-3 fatty acids that help to prevent inflammation and clotting.
Methyl Mercury Content of Fish
On the other hand, some fish contain methyl mercury, a poison that may increase risk for some cancers. Since fish spend their entire lives accumulating mercury in their bodies, the fish that are largest and live the longest are the ones that have the most mercury.
The study cited in the Cornell editorial compared blood levels of methyl mercury in 10,673 people to the amounts and types of fish they consumed (Am J Clin Nutr 2014;99:1066–70). Those who ate large predatory, long-lived fish (tilefish, shark, swordfish, king mackerel and large albacore (white) tuna) had the highest blood levels of mercury. Those who ate short-lived fish such as salmon, anchovies, sardines, small tuna and trout had lower blood levels of mercury. No significant amount was found in those who reported eating only shrimp or crab. Small amounts of mercury were linked to both farmed and wild-caught salmon.
Why Farmed Salmon Might Contain Mercury
Salmon in the wild contain low levels of methyl mercury because they rarely live more than four or five years and do not live long enough to accumulate much methyl mercury in their bodies. Farmed catfish and tilapia contain virtually no mercury because they can be fed only corn and other grains. However, farmed salmon will not thrive on a grain diet and must be fed fish meal. The fish meal may contain mercury, depending on whether or not its fish source contains mercury.
Current Recommendations
The 2010 Dietary Guidelines recommend that North Americans eat a half pound of seafood per week. Actual consumption averages 3.5 ounces per week. The authors of the Cornell editorial recommend new studies to shape the 2015 Dietary Guidelines on seafood consumption.
Dr. Gabe Mirkin is a Villager. Learn more at www.drmirkin.com
