The Palo Alto Medical Research Foundation found that the adolescents bought an average of 1,038 calories at McDonald's and 955 calories at Subway, reports Mary MacVean at The Los Angeles Times.
The calorie difference is "not statistically significant," researchers told the LA Times. Teens are supposed to consume 850 calories per meal.
Subway's reputation as a healthy alternative helped it surpass McDonald's as the nation's top fast food chain.
The brand is known for its "eat fresh" marketing campaign. Customer Jared Fogle also famously lost weight eating the "Subway Diet."
But just because Subway has some healthy menu options doesn't mean that's what people are ordering, researchers said.
"Our study was not based on what people have the ability to pick, our study was based on what adolescents actually selected in a real-world setting," the researcher told MacVean.
The study asked 87 people aged 12 to 21 to go to McDonald's and Subway for a meal on different days in the afternoon.
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