U.S.-caught albacore tuna is safest, but it's not in sushi.

RODALE NEWS, EMMAUS, PA—The day it was discovered that tuna may contain large amounts of mercury, tuna salad sandwiches became social pariahs. Experts advised women and children to limit tuna consumption, as mercury can build up in the blood and even cause brain damage. In kids, high mercury levels can impair neurological development. But a new study published in the journal Biological Letters finds that all tuna is not created equal. While most species contain dangerous levels of mercury, some have significantly lower amounts than others. Unfortunately, learning how to select the healthiest fish and avoid the unhealthiest can be difficult, thanks to strange, sometimes absent labeling on menus and packages.

THE DETAILS: The study authors worked with The New York Times newspaper to collect tuna samples from 54 sushi restaurants and 15 grocery stores in New York, New Jersey, and Colorado. In total, they collected 100 samples of tuna. Using DNA tests, they identified the exact species of tuna and, because mercury content is usually lower in tuna with a high fat content and vice versa, classified them as either lean (akami) or fatty (toro). Finally, each sample was tested for mercury content.

Turns out the highest levels of mercury were found in two species, bigeye and lean bluefin (bluefin akami), and the lowest levels were in yellowfin and fatty bluefin (bluefin toro). That said, every species tested was found to be above safe exposure levels set by the Environmental Protection Agency (EPA). The researchers also found higher mercury levels than have been reported by the Food and Drug Administration (FDA), and noted as well that tuna sold in restaurants contained higher levels than tuna bought in grocery stores. Most likely this is because sushi-grade tuna is leaner and comes from larger fish, note the researchers. Finally, 77 percent of the tuna that came from grocery stores was yellowfin, which has relatively low mercury levels.

WHAT IT MEANS: Mercury-contaminated fish remains our primary source of exposure to this brain-damaging heavy metal, according to the EPA. And predatory fish like tuna, swordfish, and sharks contain the highest levels among all fish. Fortunately, recent studies have shown that smarter fish choices have led to lower levels of mercury in women who are either pregnant or of childbearing age.