Mercury poisoning can occur from consuming tuna, though it typically results from high and prolonged intake. Methylmercury, an organic form of mercury, accumulates in fish and can be harmful in elevated amounts. This article explores how mercury enters tuna, its health implications, and advice for safe consumption.
How Mercury Enters Tuna
Mercury is released into the environment from both natural processes, such as volcanic activity, and human activities, primarily coal burning and industrial processes. Once in aquatic environments, microorganisms convert inorganic mercury into methylmercury, which is the form that readily accumulates in marine life. This methylmercury is then absorbed by algae at the base of the food chain.
The process of bioaccumulation means that mercury builds up in an individual organism over time, as it absorbs methylmercury faster than it can excrete it. Biomagnification further increases mercury concentrations as it moves up the food chain; smaller organisms containing mercury are consumed by larger ones, leading to higher levels in predatory fish. Tuna, being a large, predatory fish, tends to have higher mercury levels because they consume many smaller, mercury-containing fish. Mercury levels vary among different tuna species, with bigeye tuna generally having the highest concentrations, followed by albacore and yellowfin. Canned light tuna, often from skipjack, typically contains the lowest mercury levels.
Signs of Mercury Exposure
Methylmercury poisoning primarily affects the nervous system. Symptoms often include numbness or tingling sensations, particularly in the hands, feet, and around the mouth. Individuals may also experience coordination problems, difficulty with walking, changes in vision, and impaired speech or hearing. Muscle weakness is another common sign of elevated mercury exposure.
Certain populations are more sensitive to the effects of mercury. Pregnant individuals are a particular concern because methylmercury can cross the placental barrier, potentially harming the developing fetus and leading to developmental issues, including brain damage. Young children are also susceptible, as their developing brains and nervous systems are highly sensitive to mercury. Additionally, mercury can accumulate in breast milk, posing a risk to nursing infants.
Eating Tuna Safely
Making informed choices about tuna consumption helps minimize mercury exposure while retaining the nutritional benefits of fish. Health organizations like the FDA and EPA provide guidelines for safe fish consumption. For general adults, the FDA recommends consuming 3–5 ounces of fish, two to three times per week, to obtain beneficial omega-3 fatty acids and other nutrients. These fatty acids contribute to heart health and support brain function.
To reduce mercury intake, selecting lower-mercury tuna varieties is advisable. Canned light tuna, primarily skipjack, is considered a “best choice” due to its lower mercury content and can be consumed two to three times per week. Albacore tuna, also known as white tuna, has higher mercury levels and is categorized as a “good choice,” with a recommendation of one serving per week for adults. Bigeye tuna, often used in sushi, contains the highest mercury levels and should be consumed infrequently or avoided. Diversifying fish consumption with other low-mercury options like salmon, cod, and sardines further helps manage intake.