There's no question that you are getting mixed messages on tuna, a saltwater fish that can range anywhere in size from around four pounds to upwards of 1,500 pounds, depending on the species. While U.S. dietary guidelines recommend eating two servings of fish per week, fish also can contain trace amounts of mercury, which is not healthy.
Healthy Nutrition From Tuna
Tuna, for its part, is a source of high-quality protein with almost no fat. It contains all essential amino acids required by the body for growth and maintenance of lean muscle tissue. Canned tuna can be a good source of heart-healthy omega-3 fatty acids, with 150 milligrams or more per four-ounce serving.
Mercury Concerns and Tuna
At the same time, research shows that tuna contains mercury, which accumulates in larger fish that are higher on the food chain. For most people, the fish doesn't contain enough mercury to be a concern, but there are certain groups of people where it may pose an issue—specifically, pregnant women, nursing women, babies, and young children. That's because mercury can be especially toxic to a developing child's nervous system. The risk is dose-dependent, meaning that babies and children exposed to more mercury are more at risk for problems. Mercury can pass between a mother and her unborn baby.
If you're pregnant or nursing, you should limit your consumption of tuna, and if you have a baby or young child, you should limit that child's consumption.
How Much Tuna is Too Much?
According to 2017 guidelines from the U.S. Food and Drug Administration (FDA), women who are pregnant or nursing, planning to become pregnant, or babies and young children should completely avoid seven types of fish that are extremely high in mercury: shark, swordfish, king mackerel, marlin, orange roughy, tilefish (from the Gulf of Mexico) and bigeye tuna. Note that other types of tuna are not on this list.
The women and children who are at risk can eat up to 12 ounces (two servings) per week of fish and shellfish known to be lower in mercury content. These include canned light tuna, shrimp, salmon, pollock, and catfish. They should, however, limit themselves to 6 ounces (one serving) per week of Albacore/white tuna (canned, fresh, or frozen) and yellowfin tuna.
Canned tuna has less mercury than fresh or frozen tuna steaks. This is because smaller fish—which accumulate less mercury—are canned, while larger fish—which accumulate more mercury—are used for tuna steaks. Also, light canned tuna has less mercury than white canned tuna (also known as Albacore tuna).
The bottom line is, tuna (like most things) is good in moderation and not good in excess. If you enjoy tuna, you can include it as a healthy food in your diet. Just make sure not to overdo it, especially if you fall into one of the at-risk groups.