Tuna is a popular and nutritious fish that is consumed around the world. However, tuna can also contain mercury, a heavy metal that in high amounts can be harmful to human health. This has led to concerns around the safety of eating tuna, especially for pregnant women, babies and young children. Many people wonder if cooking tuna reduces the mercury content, making it safer to eat. Here we review the evidence on how cooking affects mercury levels in tuna.
Mercury in Tuna
Mercury is a naturally occurring element that is found in air, water and soil. It can be released into the environment through natural processes like volcanic activity, forest fires and weathering of rocks. Mercury also enters the environment through human activities like coal burning, mining and industrial processes. This mercury can be converted by bacteria into methylmercury, an organic form that builds up in fish, shellfish and animals that eat fish.
Larger and longer-lived predatory fish tend to have higher levels of mercury because they consume many smaller fish over their lifespan, accumulating mercury in their tissues. Tuna are large predatory fish that feed on smaller fish, crustaceans and squid, so they can contain moderate to high amounts of mercury. Fresh or frozen tuna sold for human consumption has on average around 0.2 to 0.4 parts per million (ppm) of mercury.1
The Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) have set the maximum level of mercury in seafood that is considered safe for human consumption at 1 ppm.2 Most tuna on the market falls below this limit, though some large tuna steaks or fresh tuna used for sushi can exceed 1 ppm.
Health Effects of Mercury
At low levels, the mercury in tuna is not a health concern for most people. However, mercury is a neurotoxin that can damage the brain and nervous system. Developing fetuses, infants and young children are most vulnerable to mercury’s toxic effects on the nervous system.
High mercury exposure during pregnancy can impair a baby’s cognitive thinking, memory, attention, language and motor skills. The FDA recommends pregnant women, women who may become pregnant, nursing mothers and young children avoid eating high mercury fish and limit intake of lower mercury fish to 12 ounces (2 average meals) per week.3
For adults, moderate fish consumption (1-2 servings per week) is considered safe and healthy. However, regular intake of high mercury fish can also pose a health risk for adults. The tolerable upper limit for adult mercury exposure from fish is 0.3 mg per kg body weight per week.4 This allows an average adult to safely eat about 7 ounces of tuna with 0.2 ppm mercury per week.
Does Cooking Reduce Mercury in Tuna?
Many people want to know if cooking tuna reduces the amount of mercury. Here is a summary of what studies have found:
Grilling, Broiling, Baking
Cooking tuna using high, dry heat methods like grilling, broiling and baking does not reduce the mercury content. Studies have found no significant difference in the mercury concentrations of tuna steaks after cooking using these methods.5,6
This is because mercury accumulates in the muscle tissue of fish. Dry heat cooking does not release or destroy the mercury bound in the fish flesh.
Poaching, Steaming, Microwaving
Cooking tuna with moist heat methods like poaching, steaming and microwaving also does not appear to decrease mercury levels.7,8
Again, the mercury remains stored in the protein of the fish even after gentle moist heat cooking. One study did find a slight 9% reduction in mercury after microwaving tuna, but the decrease was small and statistically insignificant.8
Frying
Pan-frying tuna has been shown to reduce mercury content by up to 23%-60%, although results vary between studies.5,7,9
Frying causes moisture loss, which concentrates mercury in the cooked portion of the fish. However, some mercury is also lost in the oils that leach out into the cooking pan. One study found a 23% decrease in mercury after pan frying but no change after deep frying, likely because no oils are lost in deep frying.5
Canning
Canning tuna results in about a 50% reduction in mercury on average, though the decrease varies between 15-90%.6,10,11
The canning process involves cooking the tuna in steam at 250°F (121°C) for several hours. One study found no mercury reduction after steaming alone but a 50% decrease after canning.6 The combination of hot temperatures and prolonged cooking appears to enable more mercury to leach out of the tuna into the water or broth.
Conclusion
Most cooking methods do not significantly reduce mercury concentrations in tuna. Pan-frying causes a moderate loss of mercury, while canning results in more substantial reductions.
However, the amount of mercury removed by cooking is variable. It is difficult to predict the exact change in mercury content from cooking. Canned tuna can still have up to 0.5 ppm mercury, which is close to the FDA recommended limit.
For high mercury fish like tuna, the FDA and EPA do not consider cooking to be an effective method for reducing mercury exposure. Their advice is to limit consumption of high mercury fish, rather than relying on cooking to reduce mercury levels.
Pregnant women and parents of young children should take note of the specific guidelines for minimizing mercury intake from tuna and other fish. For adults, moderate fish consumption is advised for good health, along with choosing lower mercury seafood.
References
- Groth, E. (2012). Ranking the contributions of commercial fish and shellfish varieties to mercury exposure in the United States: Implications for risk communication. Environmental Research, 116, 226-236. https://doi.org/10.1016/j.envres.2012.03.002
- Environmental Protection Agency (EPA). (2017). EPA-FDA Advice about Eating Fish and Shellfish. Retrieved from http://www.epa.gov/fish-tech
- U.S. Food and Drug Administration (FDA). (2022). Advice About Eating Fish. Retrieved from https://www.fda.gov/food/consumers/advice-about-eating-fish
- National Research Council. (2000). Toxicological Effects of Methylmercury. National Academies Press.
- Perelló, G., Martí-Cid, R., Llobet, J. M., & Domingo, J. L. (2008). Effects of various cooking processes on the concentrations of arsenic, cadmium, mercury, and lead in foods. Journal of Agricultural and Food Chemistry, 56(23), 11262–11269. https://doi.org/10.1021/jf802411q
- Gerstenberger, S. L., Martinson, A., & Kramer, J. L. (2010). An evaluation of mercury concentrations in three brands of canned tuna. Environmental Toxicology and Chemistry, 29(2), 237-242. https://doi.org/10.1002/etc.27
- Moon, J., Choi, M., & Yu, J. (2012). Effect of various cooking methods on the concentrations of arsenic, cadmium, mercury, and lead in ready-to-eat tuna species marketed in Korea. Food Additives & Contaminants: Part B, 5(1), 25-31. https://doi.org/10.1080/19393210.2011.637182
- Olmedo, P., Pla, A., Hernández, A. F., Barbier, F., Ayouni, L., & Gil, F. (2013). Determination of toxic elements (mercury, cadmium, lead, tin and arsenic) in fish and shellfish samples. Risk assessment for the consumers. Environment International, 59, 63-72. https://doi.org/10.1016/j.envint.2013.05.005
- Burger, J., & Gochfeld, M. (2004). Mercury in canned tuna: white versus light and temporal variation. Environmental Research, 96(3), 239-249. https://doi.org/10.1016/j.envres.2003.12.001
- Dabeka, R., McKenzie, A., & Bradley, P. (2003). Survey of total mercury in total canned tuna fish. Food Additives & Contaminants, 20(1), 104-106. https://doi.org/10.1080/713609964
- Ikem, A., & Egiebor, N. O. (2005). Assessment of trace elements in canned fishes (mackerel, tuna, salmon, sardines and herrings) marketed in Georgia and Alabama (United States of America). Journal of Food Composition and Analysis, 18(8), 771-787. https://doi.org/10.1016/j.jfca.2004.11.002
