In restaurants across the US, one in three food items labeled “gluten-free” were found to have traces of the protein despite being advertised otherwise, according to a preliminary study presented earlier this week at the annual American College of Gastroenterology scientific meeting. And if you have a hankering for gluten-free pasta, pizzas, or other dinner items, chances are they're more likely than other foods and meals to be contaminated with gluten – the protein found in wheat, rye, barley, and other grains.

Don’t go berating your favorite pizzeria just yet. The authors note that the issue is probably one of accidental cross-contamination, such as a chef who might use the same cutting board or utensils for both gluten and gluten-free options.

"The fact that gluten was so often found in pizza suggests that sharing an oven with gluten-containing pizza is a prime setting for cross-contamination," explained study author Benjamin Lebwohl, from Columbia University's Celiac Disease Center, in a statement. "Gluten-free pasta can be contaminated if prepared in a pot of water that was used to prepare gluten-containing pasta."

In order to be considered gluten-free, food must contain less than 20 gluten parts per million. To test if restaurants were doing their due diligence, researchers used a portable gluten-free sensor to perform more than 5,600 tests over the course of 1.5 years. They found 27 percent of breakfast food items contained levels of gluten above the limit, as did 34 percent of dinner items. The authors note these percentages could be due to the amount of cross-contamination that increases throughout the day as food items are used more.

The findings are disconcerting considering even a small amount of gluten can cause damage to the intestinal lining in people with the genetically predisposed autoimmune disorder. Estimates suggest that one in 100 people around the world have celiac disease and as many as 2.5 million Americans are undiagnosed. When a person with gluten-intolerance eats grains that contain this protein, their immune system attacks their small intestine, damaging the lining that helps to absorb nutrients. Symptoms are uncomfortable and can include abdominal pain, heartburn, bloating, and constipation, among other digestive issues.

The US Food and Drug Administration regulates packaged goods that bear the gluten-free label, but there is no such federal oversight of gluten-free options in restaurants, according to the researchers, which is something they say is particularly worrying – and not just for those with celiac disease.

"There are also people who don't have celiac disease but have symptoms triggered by gluten," said Lebwohl. People who don’t have celiac disease can still have a sensitivity to gluten. Because of this, the researchers say restaurants need more education to take greater precautions when preparing food.