Some fish are high in mercury, a heavy metal that is easily absorbed by the body. Mercury is a toxin absorbed by fish through their gills while swimming and consuming mercury-rich plankton. Larger fish tend to contain more mercury than smaller fish. This is because large fish consume large amounts of smaller fish that feed on mercury-containing plankton.

In general, the larger and older the fish, the higher the mercury level.

Unfortunately, tuna is considered one of the most appropriate dietary sources13 of mercury in the world. “Because tuna is higher in the food chain, it tends to store more mercury in its fat,” nutritionist Dana Ellis Hannes, PhD, MPH, RD, tells mindbodygreen. Gunnes explains it mercury is a known neurological destroyer14 and adversely affects the brain, especially in developing fetuses and young children.

Keep that in mind mercury in raw tuna is more bioavailable13 to your body than the mercury found in cooked tuna, so people who frequently eat raw tuna products, such as tuna sashimi, are at increased risk of developing high mercury levels.

There are also concerns about the sustainability of fish higher up the food chain, which we will discuss in detail below.