Tuna is one of the fish that contains the highest levels of mercury due to its long feeding process, which leads to the accumulation of mercury in its body.
Tuna is an excellent source of nutrients, providing high-quality protein, heart and brain-healthy fatty acids, along with essential vitamins and minerals like vitamin D, B12, and selenium. However, this nutrient-rich fish has a significant drawback: it is loaded with mercury, a heavy metal that is harmful to the nervous system, according to IFL Science.
Yellowfin tuna is one of the types of tuna processed for canning. (Photo: Thahaali).
Compared to most other seafood, tuna meat has a relatively high mercury content. The U.S. Food and Drug Administration (FDA) estimates that tuna contains an average of 0.126 to 0.689 parts per million of mercury. Some types of tuna even have higher concentrations. For instance, albacore tuna, also known as longfin tuna, typically has mercury levels about three times higher than skipjack or yellowfin tuna.
Most fish with significant mercury levels are apex predators such as sharks, swordfish, marlin, and king mackerel. This is due to biomagnification. Inorganic mercury enters the natural environment through industrial pollution and is then transformed into organic form, methylmercury, by microorganisms in the marine ecosystem. Small amounts of methylmercury can also form naturally by bacteria in water and sediments.
This chemical is consumed by small aquatic organisms, which in turn become food for larger predators in the food chain. Because mercury is absorbed faster than it is metabolized or excreted, significant amounts of the metal accumulate at each level of the food chain, with apex predators consuming the most. The presence of mercury in the environment is so prevalent that even tuna living in the open ocean contain this chemical.
A 2024 study found that mercury levels in tuna have remained unchanged even though global mercury emissions have decreased since the 1970s. Researchers speculate that the stable mercury levels in tuna may result from upward shifts of mercury from deeper ocean layers to surface waters, where tropical tuna typically feed. It is likely that this mercury was released into the environment many years or even decades ago.
The mercury levels in food are very low but can accumulate gradually, especially if consumed in large quantities over a long period. According to the Environmental Protection Agency (EPA), symptoms of methylmercury poisoning can include issues with peripheral vision, a tingling sensation often in the hands, feet, and around the mouth, lack of motor coordination, and impaired ability to speak, hear, and walk, along with muscle weakness.
However, you would need to consume a significant amount of tuna over a long duration before needing to worry about mercury poisoning. Some scientists estimate that a healthy adult can safely consume about 95 grams of tuna per week before facing a serious risk of mercury toxicity.
