Trend watchers speculate that changing demographics, a younger generation’s penchant for gastronomic adventure and more people developing a taste for it are driving the popularity of spicy food in the United States.

Regardless of why spicy food is spreading, industry experts don’t deny that it’s happening.

“It seems to be the way the world is going,” said Subway executive chef Chris Martone.

About 80 percent of the quick-service sandwich chain’s nearly 40,000 units now carry pepper Jack cheese, he said.

“Our Buffalo chicken’s a great protein for us,” Martone said, noting that many customers double up on spiciness, combining pepper Jack with hot sauce or jalapeño peppers, or adding any or all of those items to Buffalo chicken sandwiches.

Some Subway units also have spicy regional favorites, such as giardiniera, the piquant pickle mixture that gives a Chicago delicacy, the Italian beef sandwich, its distinctive flavor.

Martone said suppliers manufacture giardiniera for Subway units in Illinois and Indiana, whose customers enjoy it. “You have to have it in Chicago,” he said.

In a 2011 consumer trend report, restaurant research firm Technomic found that hot or spicy foods appealed to 48 percent of American consumers, up by 2 percentage points from 2009.

Just Salad’s Jalapeño Popper saladBut it’s not those adventuresome youngsters who are driving the hunger for hot food. Older Millennials and Generation X — those ages 25-44 in 2011 — were most enamored with spicy food. Fifty-seven percent of surveyed consumers ages 25-34 ranked their preferred level of heat at seven or higher on a scale of one to 10, up from 48 percent in 2009. Fifty-five percent of those ages 35-44 reported the same, up from 42 percent in 2009.

Figures dipped slightly with other age groups, most dramatically with those ages 55 and older, which dropped from 44 percent to 38 percent.

On restaurant menus, descriptions of spicy food have also grown. Research firm Datassential tracks more than 120 terms for spicy food on chain and independent restaurant menus. Of those terms, more than 70 grew in popularity between 2008 and 2012, including Thai chile, roasted poblano and chipotle. The fastest-growing descriptors are Southeast Asian Sriracha, Chinese chili oil and North African harissa.

Habanero, a particularly fiery chile used in Yucatán and West African cooking, also made Datassential’s top 10 list. The pepper is gaining traction at U.S. chain restaurants. McDonald’s will roll out several new versions of its Quarter Pounder in June, including a Habanero Ranch variety. Charleys has introduced a Spicy Hawaiian Chicken Sandwich as a limited-time offer through July 14. It includes chicken breast, ham, Swiss cheese and Charleys’ signature Roasted Pineapple & Habanero sauce.

Datassential doesn’t track Korean descriptors, such as kimchi, as part of its spicy listings, but that cuisine’s flavors — once only found in ethnic enclaves — are spreading at chains. For instance, California Pizza Kitchen recently put a Spicy Korean Barbecue Pizza on its menu. It features Korean barbecue pork and a nontraditional kimchi made with Napa cabbage, cilantro, carrots and cucumber tossed with Sriracha vinaigrette.

California Tortilla, a 39-unit fast-casual chain based in Rockville, Md., also put Korean spice on its menu with a trio of three tacos, including Korean BBQ, topped with Sriracha, Monterey Jack cheese, cabbage slaw, chile sauce and green onion.

Its other two tacos are Carnitas Verde, with enchilada sauce, onion-cilantro relish and Monterey Jack cheese, and Havana Chicken. Although Cuban food is not typically spicy, California Tortilla’s interpretation is doused in the chain’s signature spicy Havana sauce, along with Monterey Jack cheese, pico de gallo and avocado.