CHICAGO — Chicagoans feeding dollars into park vending machines are buying granola bars and dried fruit instead of mini donuts and cheesy potato chips — and they’re not complaining, according to a study by Northwestern University researchers released last week.

It found that nearly 90 percent of Chicago park-goers said they were satisfied with the food in the new healthy-options-only vending machines.

What’s more, they spoke with their wallets: The healthier snacks boosted average monthly per-machine sales from $84 to $371 in a little over a year across the Chicago Park District.

The sales climb challenges vendors’ previous resistance to nutritious snacks — namely, the fear of losing revenue from tried-and-true items like candy and chips.

“What Chicago parks have done is prove a model where vendors can increase nutrition standards and see their revenue go up at the same time,” said Kate Bishop, nutrition policy associate with the Center for Science in the Public Interest.

“Word will catch on and vendors will be forced to switch to the model as what’s part of a larger movement of localities, states and the country,” she said.

The peer-reviewed study was published in the August 2014 issue of the Centers for Disease Control and Prevention’s monthly journal, “Preventing Chronic Disease.”

The Northwestern researchers surveyed, interviewed and observed vending machine consumers at 10 Chicago parks, selected for diversity based on the race, ethnicity and location of surrounding neighborhoods, since the start of the park district’s 2011 initiative. Of those surveyed, 88 percent reported enjoying the healthy snacks they tried, and 98 percent indicated they would purchase the snacks again.

The district signed a five-year contract with national vendor Compass Group USA, setting nutritional standards on all 98 park machines for calories, sugar, fat and sodium, researchers said. The contract also specified uniform prices for the snacks to eliminate price as a driving factor.

The food items themselves — such as baked potato chips, fiber granola bars, animal crackers, pretzels, nuts and dried fruits — were chosen by Colleen Lammel-Harmon, the district’s wellness manager and a registered dietitian. She even hand-picked a more nutritious candy bar option: dark chocolate with almonds.

One in five Chicago children entering kindergarten and almost one in three children entering the sixth grade are obese, according to data from the Consortium to Lower Obesity in Chicago Children. Almost 200,000 Chicago children are enrolled in one or more park district programs, many of which are targeted to kids.

It may seem doubtful that kids would willingly opt for a granola bar over a candy bar, but researchers speculate that consumers are buying the machines’ healthier snacks initially because that’s what is available.

“People are going to buy something, regardless, when they’re hungry,” said Northwestern University’s Maryann Mason, one of the study’s main researchers. “When you have a mix of healthy and less healthy items they might be reluctant to try something new, but in this case all the choices are healthier.”