One quarter of all adults living in NYC and half of Asian New Yorkers have blood mercury levels equal to or higher than the level that is considered to be safe.1 Sounds scary, but what does it mean?
Methylmercury (MeHg) is a toxic substance that humans mainly intake from eating contaminated fish. MeHg levels tend to be increased in fish that are higher on the food chain (e.g. swordfish), as they consume lots of smaller fish (i.e. plankton) that have mercury in them and therefore accumulate more MeHg than a smaller, younger fish would. It’s important to be cautious and choose those fish that have lower MeHg levels to avoid the host of problems that excess mercury can cause. Symptoms of mercury poisoning include muscle weakness; mental changes (e.g. memory loss); impairments in coordinated movements like walking; and skin rashes. Some even suggest that consistently low levels of methylmercury can increase the risk for heart disease. Pregnant women are particularly advised against consuming fish with high MeHg levels because excess mercury has been shown to cause irreversible damage to the fetus’s central nervous system, leading to cerebral palsy, blindness, deafness, seizure disorders, and intellectual disabilities.
One of the big issues with answering the question of which fish is best to choose is that one type of seafood can be raised in different ways; the way the fish is raised will impact their mercury content and nutritional value. For instance, one review found that mercury concentrations of wild-raised seafood was higher than the farmed version; however, this has relationship has not always been shown, as some studies find that there is no significant difference in mercury levels of farmed versus wild salmon as well as farmed versus wild cod.
The idea of some fish being potentially harmful to you is a difficult concept because we are repeatedly told that fish is one of the best foods to eat, and we should be substituting it as the protein in our meal in place of other animal-based proteins. Fish such as albacore tuna, salmon, sardines and halibut are good sources of omega-3 fats, consumption of which have been shown to be protective against heart disease and are considered anti-inflammatory. Although consumed less frequently (most frequently in sardines), the bones of fish are a great source of non-dairy calcium. But now we know that certain fish can contain high levels of this toxic substance, mercury, which is not so good for our health.
So what do we do?
Limit intake of high mercury fish such as shark, swordfish, king mackerel, bigeye and ahi tuna, and tilefish to a few times yearly; if you’re pregnant, nursing or plan to become pregnant, avoid these all together. Instead, choose seafood that is lower in mercury including shrimp, oysters, canned light tuna, sardines, and wild salmon; aim for 2-3 4oz portions of these type of fish weekly. Because albacore (“white”) tuna has more mercury than canned light tuna, choose the latter as often you can. You can get your mercury levels tested with your MD, particularly before starting a family.