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Canned tuna is an American grocery staple. This saltwater fish, which is common in diets all over the world, is an affordable, tasty food that’s full of protein, healthy fats, and vitamin B. Plus, it’s super easy to prepare — some people enjoy it straight out of the can, and the tuna melt sandwich is a popular item on restaurant and deli menus.
However, tuna can contain high amounts of mercury, a metallic element that can pose long-term dangers.
Tuna in the wild don’t produce mercury as part of their biological processes. The element instead makes its way into fish through industrial processes, such as coal burning, that emit mercury into the ocean. Via the water cycle, mercury that these processes emit can also make its way into the ocean.
Because tuna feed on smaller fish that absorb this oceanic mercury quite strongly, tuna frequently contains substantially more mercury than other common fish including salmon, scallops, and tilapia. Mercury, unlike other substances that are introduced into the bloodstream of living creatures, is not easily excreted, so not only do tuna regularly ingest mercury, but they also build it up in large amounts over time.
Given their capacity for storing high quantities of mercury, tuna can quickly pose problems for the human diet. In three ounces of tuna (a standard can of tuna has five ounces of tuna, with a serving size of two ounces), the mercury content can range from 10 to nearly 60 micrograms (mcg). The mercury content of canned light tuna lies on the lowest end of this range, whereas the common canned albacore tuna variety includes roughly 30 mcg of mercury per three ounces. The average person, depending on their weight, should not ingest more than four to nine mcg of mercury per day. And because, as with fish, mercury is difficult for people to excrete, high mercury intake can be challenging to combat.
Although mercury is toxic, overexposure to it is unlikely to result in death. Its effects are instead more gradual, interfering with everyday processes and skills. High levels of mercury exposure are linked with brain cell death, which can lead to reductions in memory, the ability to focus, and basic motor skills. Workplace studies have also shown that mercury can be linked to anxiety, depression, and more challenges in processing information.
Some studies have connected high levels of mercury with increased heart disease. Mercury is known to play a role in fat oxidation, a biological process that can increase the chance of heart disease. On the other hand, some scientists have suggested that the high content of healthy fats in tuna means that the net effect of eating tuna is overall positive despite mercury concerns.
Tuna consumption is not inherently unhealthy, but the mercury levels in the fish can present cause for concern if too much is eaten. The occasional tuna melt or a small daily chunk of tuna from a can is unlikely to cause serious harm to a person, but regular consumption of high amounts of tuna may be ill-advised.