Fast-casual restaurants chains Chipotle and Sweetgreen have sought to cash in on the latest trends in nutrition. But how much do customers really care about promises of gluten-free, organic and GMO-free food?

According to a recent survey by Zagat, not as much as you might think. Of the three, customers are most likely to prioritize GMO-free food, with 19 percent of respondents saying it is "very important" that a fast-casual restaurant is GMO-free. Meanwhile, only 5 percent of respondents say it's very important to have gluten-free options, compared to 81 percent saying it's not important or they have no opinion.

Food and ingredient quality is still almost unanimously customers' most valued quality in a fast-casual restaurant, suggesting that customers' personal assessments of the food, not trendy labels, keeps them coming back. The two other most important factors: value and cleanliness.

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Zagat surveyed more than 6,600 people across the U.S. for the publication's first report on fast-casual dining.

Other interesting findings from the report include how often people eat at fast casuals (two-thirds either go once or twice a week or a few times a month) and when they go most often, with 60 percent tending towards lunch. Only 2 percent most frequently visit fast-casual restaurants for breakfast – an area with huge potential for growth, as at some fast-food chains, such as McDonald's, breakfast makes up a fourth of sales.

How fast-casual customers conceptualize healthy and high-quality food will also continue to play a major role in how fast-casuals define themselves moving forward. The rise of gluten-free lifestyles already has: from 2010 to 2012, the number of gluten-free options increased tenfold on full-service menus. Chains from Pizza Hut to Panera have added or labeled gluten-free options in just the last year.

However, now the health-conscious crowd may be shifting its focus to GMOs: in April, Chipotle became the first national chain to go GMO free. Still, some customers want even more. On Monday, a lawsuit was filed against the chain claiming that some menu items continue to contain GMOs, such as soft drinks and meat products are made from animals fed GMOs.

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