Tuna is a widely eaten species of fish. However, canned tuna is often the most common source of mercury in the diet.
The term tuna encompasses several species of fish, including skipjack, albacore, yellowfin, and bigeye. Skipjack is the most commonly consumed species.
Mercury is a chemical often used in thermometers, thermostats and automotive light switches, as well as being put to use in industrial facilities, such as power plants, cement plants, and certain chemical manufacturers.
When released into the environment, mercury can become a public health issue when it settles into our oceans and waterways.
Natural bacteria absorb mercury and convert it into methylmercury, introducing it into the food chain. Small fish consume or absorb the methylmercury and are eaten by larger fish. Instead of breaking down or dissolving, however, mercury builds up at every level of the food chain.
Large fish, such as tuna, can have mercury concentrations in their bodies that are 10,000 times higher than those of their surrounding habitat.
However, tuna is still safe to eat in certain amounts. This article explains how much to eat without affecting health and clarifies the risks of consuming too much.
Mercury is odorless and invisible to humans. Once in the body, however, it can act as a neurotoxin and interfere with the brain and nervous system.
Exposure to mercury can be especially harmful to small children and women who are pregnant.
While the brain of a child develops, it rapidly absorbs nutrients. Mercury can affect that absorption, causing learning disabilities and developmental delays. In infants and fetuses, high doses can lead to cognitive difficulties, cerebral palsy, deafness, and blindness.
In adults, mercury poisoning can affect fertility and blood pressure regulation.
Mercury poisoning can also cause the following symptoms:
- memory loss
- vision loss
- numbness of extremities
There are two main types of canned tuna: White albacore and chunk light.
Chunk light is made mostly from skipjack tuna, a smaller species of tuna. Albacore tuna is a larger species and contains higher levels of mercury.
Canned white albacore tuna typically contains about 0.32 parts per million of mercury. Canned light tuna contains about 0.12 parts per million of mercury.
The following table contains the recommended amount of canned tuna that an individual should eat according to their body weight:
|Body weight in pounds (lb)||Recommended interval between servings of white albacore tuna||Recommended interval between servings of chunk light tuna|
|20||10 weeks||3 weeks|
|30||6 weeks||2 weeks|
|40||5 weeks||11 days|
|50||4 weeks||9 days|
|60||3 weeks||7 days|
|70||3 weeks||6 days|
|80||2 weeks||6 days|
|90||2 weeks||5 days|
|100||2 weeks||5 days|
|110||12 days||4 days|
|120||11 days||4 days|
|130||10 days||4 days|
|140||10 days||3 days|
|Over 150||9 days||3 days|
The figures in the table above are taken from U.S. Food and Drug Administration (FDA) test results for mercury and fish, and the Environmental Protection Agency's (EPA) determination of safe mercury levels.
The FDA recommends avoiding fresh albacore tuna and tuna steak during pregnancy. It is only safe to eat up to one serving of less than 170 g per week. Canned tuna, however, is safe to eat during pregnancy.
If you are still looking for the boost in protein and omega-3 fatty acids that fish can provide, try replacing albacore tuna and any large, predatory fish in the diet with salmon, herring, sardine, or anchovy. These contain less mercury, as they are lower down the food chain.
In humans, if mercury levels in the blood are found to be high, they can take up to 6 months or longer to reduce to a safe level.