Fish is an important part of our diets. Nutritionists often recommend two to three servings a week. To get that, many of us reach for canned tuna. But there are some serious health concerns about mercury in tuna. In a new investigation, Consumer Reports tested popular brands of canned tuna to find out how worried we should be.

It’s cheap, convenient, full of protein and omega-3 fatty acids, and a classic staple in our lunches: canned tuna. But the nutritional powerhouse from the sea comes with a warning.

Tuna, just like other fish, can contain mercury. And if you eat a lot of fish, you can expose yourself to this potentially dangerous heavy metal.

High amounts of mercury can lead to problems with fine motor coordination, speech, sleep, and walking. It’s also concerning for pregnant people because the developing brain and nervous system of a fetus are vulnerable.

Because so many people eat tuna and because of its potential risks, CR tested five popular brands of canned tuna for mercury.

The results: Popular albacore tuna had the highest mercury levels, while light varieties had relatively low mercury, on average, though results varied from can to can.

There were some cans CR tested that had high amounts of mercury. And because you can’t tell which can you purchase has high amounts of mercury, you may be exposing yourself to dangerous levels of this heavy metal.

For that reason, CR recommends that pregnant people completely avoid tuna. 

In response to CR’s findings, the National Fisheries Institute, a trade association that represents manufacturers of canned tuna, said that the mercury levels were well below the limit the Food and Drug Administration allows in canned tuna, and that these products are safe to consume.

If you’re concerned about limiting the amount of mercury you consume, and if you’ve eaten no other fish during the week, Consumer Reports says up to 12 ounces a week of Bumble Bee Chunk Light, Chicken of the Sea Chunk Light, Safe Catch Wild Elite, and StarKist Chunk Light tunas are the safer choices among the products it tested.

There are other types of seafood that are nutritious and have naturally lower levels of mercury, including oysters, salmon, and sardines.