Tuna

Researchers have recently discovered that tuna contains the mineral selenium in an unusual form called selenoneine. This form of selenium plays an important role in the health of the fish by serving as an antioxidant and protecting the fish's red blood cells from free radical damage. Interestingly, it is also able to bind together with mercury compounds in the fish's body (including methyl mercury, or MeHg) and lower their risk of mercury related problems. Because there are approximately 2­3 milligrams of selenoneine in a 4­ounce serving of tuna, we are likely to get some of this same antioxidant protection when we eat tuna. Equally interesting, perhaps tuna will turn out to be a fish that—even when contaminated with mercury—might pose less of a mercury risk than might otherwise be expected due to the presence of selenoneine. There is some evidence to show that lower concentrations of both selenoneine and selenium itself may be present in fresh tuna that lacks its characteristic reddish color and is more watery and softer in texture at the time of purchase. However, further research is needed to determine exactly how the selenium content of tuna is related both to its appearance and also to its potential mercury risk.

In a much discussed recent study about fish intake and cardiovascular disease, researchers have uncovered a very interesting trend involving tuna. Over 15,000 U.S. men and women participated in this 15+ year study, and their intake of fish—including canned tuna, darker oily fish like salmon and sardines, lighter non­oily fish like cod, and shellfish like shrimp and lobster—was compared with their likelihood of experiencing a specific type of cardiovascular problem called atrial fibrillation (AF). In AF, electrical impulses within the heart become irregular. In the broadest dietary context, total consumption of fish did not help these participants lower their risk of AF—even if fish were eaten more than twice per week. However, researchers did find risk of AF to be reduced when a specific combination of fish— namely, canned tuna and oily fish like salmon—was eaten more than twice per week. (The average serving size here was 3­4 ounces.) Interestingly, this specific fish combination was more closely related to decreased risk of AF than the total amount of omega­3 fatty acids provided by the fish. While we cannot be sure about the reasons for benefits from this specific combination of fish, the contribution of tuna to decreased risk of this cardiovascular related problem seems important and worth careful future investigation.

During the preparation of tuna for canning, whole fish are often steamed for a period of hours, and during this process, a watery liquid (called cooking juice) is created that frequently gets discarded as waste by tuna manufacturers. In recent studies, however, scientists have examined the nutrient composition of this cooking juice and determined that small protein fragments—called peptides—are present in the cooking juice and that they possess strong antioxidant properties. The antioxidant properties of these tuna peptides include the ability to protect cell membranes from oxygen related
damage (a process called lipid peroxidation). While the manufacturing of canned tuna is obviously a very different and much more lengthy process than the very short duration cooking methods that you would be using to steam, sear, or broil tuna in your own kitchen, it is still very likely that your at home cooking methods for preparing fresh tuna will result in creation of some of these same beneficial antioxidant peptides.