Will Eating Fish Increase Your Risk of High Mercury Levels?
Nutrition is all about balance. While it is important to get enough of the good stuff, all nutrients and types of food have an upper limit for safe consumption. Even water can be lethal in high doses. A good example of a food for which it is important to get the balance right is seafood. Whereas many countries recommend consumption of fish one to three times per week for the omega-3 fatty acid content, fish also contain mercury and therefore it is important not to overdo it. A group of researchers (Nielsen and co-workers) recently looked at how fish consumption affected mercury levels in a representative sample of US adults. Briefly, they used data from 10,673 adults, who were categorized according to seafood consumption from a food frequency questionnaire. Blood mercury levels were measured. A cut-off of 5.8 µg/l blood mercury was used to indicate blood levels that are too high, as is also used by the Environmental Protection Agency (EPA).
The results of the survey showed that 83% of participants reported some seafood consumption in the previous 30 days, and 40% consumed seafood at least 5 times per month. 9% consumed only shellfish, 28% consumed only fish, whereas 43% of the total sample consumed both fish and shellfish.
People who did not consume any fish or seafood had the lowest blood mercury concentrations of 0.45 µg/l. For consumers of fish and/or shellfish, blood mercury concentrations increased as intake increased from 1-2 times per month, through 3-4 times per month, and up to 5 or more times per month. The category with the highest mean intakes was for people consuming both fish and seafood 5 or more times a month, who had blood levels of 1.8 µg/l. Unfortunately, despite the large proportion of subjects consuming fish 5 or more times per month, the authors had not created an additional category for subjects consuming fish above recommendations (equivalent to 9 or more times per week).
Around 5% of all adults in the survey had blood mercury levels above the 5.8 µg/l cut-off. Consumption of high-mercury fish such as swordfish or king mackerel resulted in a 460% probability of having elevated blood mercury concentrations. Consumption of tuna and salmon was also associated with a slightly higher risk of high mercury levels of around 14% for both types of fish.
These results are not too surprising given the mercury levels found in fish and shellfish (see table from the FDA). For example, tilefish is a high-mercury species and contains 1.45 ppm mercury. This is equivalent to 1.45 µg per gram fresh weight, or 145 µg for a 100 g portion. Using the EPA's consumption guidance of 0.1 µg consumed per kilogram bodyweight per day, a person weighing 70 kilograms would have a limit of 7.0 µg mercury per day to be within a safe level of consumption. This means that tilefish could be safely consumed once every three weeks if no other source of mercury was consumed. On the other hand, other commonly consumed fish such as fresh salmon contain only 0.022 µg per gram fresh weight, or 2.2 µg per 100 g portion, and could be consumed every day without exceeding safe limits. The increase in risk of excessive mercury consumption from salmon possibly reflect some confounding (people who consume salmon may be more likely to consume high-mercury fish) or differences in actual mercury levels in the fish consumed.
Even so, young children and women who could become pregnant are warned to be extra careful about avoiding fish that may contain high levels of mercury. An alternative to eating fish that avoids mercury while still meets omega-3 fatty acid intake recommendations is dietary supplementation. An analysis of mercury levels in fish oil used in dietary supplements found a mean mercury level of 0.5 µg/kg oil. To reach the maximum mercury intake level, a person weighing 70 kg would have to consume 14 kilograms of fish oil per day, a very unpleasant proposition. People normally consume fish oil supplements in the range of 0.25 to 5 grams per day therefore mercury intakes from fish oil are negligible.
The results of the study show that while higher levels of fish consumption result in higher mean mercury levels in the blood, and a greater chance of excessive levels, the majority of fish consumers have safe levels of mercury. For people who do not like to consume seafood, or have concerns about mercury in seafood, a supplement is a valid alternative.
Nielsen SJ, Kit BK, Aoki Y, Ogden CL. Seafood consumption and blood mercury concentrations in adults aged ≥20 y, 2007 - 2010. The American Journal of Clinical Nutrition. 2014. http://ajcn.nutrition.org/content/early/2014/02/12/ajcn.113.077081.abstract
Environmental Protection Agency. Integrated Risk Information System. Methylmercury (MeHg) (CASRN 22967-92-6). http://www.epa.gov/iris/subst/0073.htm
John TE, Rebry RM, Yan KL, Rice HB, Rowe WJ, NDNutrasource Diagnostics Inc. and Global Organization for EPA and DHA Omega-3. Measurement of environmental contaminants in a globally-representative sample of fish oil supplements. http://www.ifosprogram.com/media/10609/goed_manuscript_-_final__2_.pdf
US Food and Drug Administration. 2013. Mercury Levels in Commercial Fish and Shellfish (1990-2010). http://www.fda.gov/Food/FoodborneIllnessContaminants/Metals/ucm115644.htm
Modified February 20, 2014 to clarify wording on mercury intakes from fish oil. Thanks to Alan Titchenal from www.gotnutrients.net for the tip.