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Among health-conscious eaters, seaweed holds a reputation as a nutrient-rich superfood. Seaweed is stuffed with vitamins and protein, chock full of iron—and at least one kind tastes like bacon. If that’s not a superfood, we don’t know what is.

Seaweed, however, has the same problem as other foods, super or not: even if packed with nutrients, there’s no guarantee your body can access each and every nutritional goodie offered. And as new research shows, some seaweeds are more healthful than others—at least when it comes to iron.

In their lab at the University of Hawaii, nutritionists studied 13 varieties of popular edible seaweeds, and found that, while many of them are indeed rich in iron, only two types—nori and sea lettuce—provide more bioavailable iron than you’d get from eating the same amount of spinach. Popeye would be pleased.

But the researchers also found that several types of seaweed contain high levels of arsenic and other potentially harmful minerals, making them risky to consume in large quantities. In particular, hijiki, a popular type of seaweed that is often consumed for its iron content, tested high in arsenic, while a sample of red ogo contained excessive amounts of manganese. The prevalence of toxins varied according to where the seaweed had been grown. That means it may not be a healthy idea to double- or triple-down on eating seaweed as a “superfood.”

The research reinforces what food experts have been saying all along, says Joannie Dobbs, a clinical nutritionist who worked on the study. “Ideally we get our nutrients from a variety of foods, obtained from a variety of locations—and remember: there are no superfoods. These principles include seaweed.”

Dobbs says researchers started studying the bioavailability of iron in seaweed because many people—especially vegetarians and those who consume very little red meat—have iron deficiencies. So scientists set out to identify alternative, plant-based sources of iron.

By breaking down samples of the various seaweeds using digestive fluids, and then “feeding” the solution to human intestinal cells in the lab, the scientists tested iron absorption rates. Michael Dunn, a researcher who worked on the project, says he was surprised by the nutritional variability of the seaweed. He was also surprised to find that adding vitamin C to the mix further improved the intestinal cells’ absorption of the iron from nori and sea lettuce.

“Only some seaweeds can be considered good sources of bioavailable iron,” says Dunn. If you’re looking for the most iron-rich seaweed for your sushi or smoothie, then nori or sea lettuce are the ways to go. And for the servers out there, we apologize: soon you’ll need to know the seaweed’s species, as well as the chicken’s name.