Gluten-free diets continue to grow in popularity, but most consumers are unaware of where gluten hides even as they avoid naturally gluten-free foods like rice.

Gluten-free foods, though consumed by about one in five Americans, are seriously misunderstood.

That’s according to a new survey conducted by NSF International, a nonprofit that does consumer research and certification programs for the food industry.

“The gluten-free market is growing in double digits every year, but there is almost a literacy barrier — how much do consumers actually understand when you say that something is gluten-free?” said Jaclyn Bowen, a general manager at Quality Assurance International and the director of NSF International’s Consumer Values Verified program.

About half of the survey respondents could correctly define gluten. According to a recent Gallup Poll, more than 20 percent of Americans include foods labeled as gluten free in their diet, with numbers slightly higher among those with less education and income.

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There are two major reasons why consumers avoid gluten.

Celiac disease and wheat-related allergies account for a small percentage of those who avoid gluten. For many of them, even a trace amount of gluten can trigger a serious reaction.

Though just 1 percent of the population has celiac disease, according to medical experts, nearly 10 percent of those surveyed by NSF International who avoided gluten said they did so because they had celiac disease.

Others avoid gluten for general health reasons and weight loss. But gluten-free food labeling can interfere with efforts for both groups.

More than a quarter of survey respondents believed that if a product did not contain wheat, it did not contain gluten. (Gluten is a sticky protein that occurs naturally in wheat, barley, and rye.)

Some consumers avoid too many foods, incorrectly identifying rice and potatoes as containing gluten. Others don’t avoid enough foods because they don’t know that beer, salad dressings, processed foods, and even dietary supplements often contain gluten.

Although the gluten-free craze started several years ago, it wasn’t until last year that the Food and Drug Administration (FDA) began enforcing a definition of gluten-free as containing less than 20 parts per million.

Manufacturers who label their products as gluten-free and don’t meet the definition can have their products recalled and potentially face legal action.

Still, the FDA inspects only a tiny fraction of the foods in stores and often relies on consumers to report problems. Manufacturers aren’t required to have foods labeled as gluten-free inspected by the government or a neutral third party before delivering them to grocery store shelves.

“Companies can label their products as gluten-free and not necessarily do any other diligence,” Bowen said.

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A strict gluten-free diet, like the one to which celiac patients must adhere, requires shoppers to do quite a bit of research. Gluten can be found in foods that contain not just barley or rye but also starch, maltodextrin, or anti-caking agents.

“It’s a matter of consumers recognizing that when they see these other types of ingredients, they will need to do a little more research,” Bowen said.

There are some active social media channels where those with celiac disease share information on unpleasant surprises in store-bought foods.

For instance, many patients with celiac disease rely on dietary supplements to ensure that they don’t miss out on the vitamins lots of us get through enriched flour, Bowen said.

But dietary supplements can contain gluten as an additive to prevent caking. Just 6 in 10 of the survey respondents knew to look for possible sources in dietary supplements. And just 1 in 4 knew that gluten can also hide in the spice mixes of packaged foods.

As for restaurant foods, that’s anyone’s guess. Although the FDA “encouraged the restaurant industry to move quickly to ensure that its use of gluten-free labeling was consistent with the federal definition,” the agency has no enforcement power over restaurants.

Just a third of those surveyed by NSF International knew that.

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The comedian Jimmy Kimmel poked fun of Los Angeles health fanatics who avoided gluten without knowing what it was. To some extent, consumers’ lack of knowledge about what’s in their food is comical.

But with consumers expected to spend $15 billion on gluten-free foods in 2016, perhaps they should know what they’re getting.

According to the Gallup Poll, people with less income and education are more likely to buy gluten-free foods. Gluten-free foods often cost more than twice as much as their conventional counterparts.

Nonwhites, who have lower rates of celiac disease, are also nearly twice as likely to buy gluten-free foods.

Research conducted by Dr. Joseph Murray, a celiac disease expert at the Mayo Clinic, supports the Gallup findings.

“There seems to be an increase of people avoiding gluten in non-Caucasian groups. It’s not because of celiac because that’s very rare,” Murray said. “They may think they’ll get a benefit in terms of weight loss, and some do it because they’ve heard [gluten] might be bad for them.”