Marren Miranti, a 17-year-old senior who lives near Houston, Texas, began working out after she started going to the gym with her mom. “I love lifting weights,” says Marren. “It’s really satisfying to set a goal, work hard, and see your progress.” As Marren got more into fitness, she started following bodybuilders and personal trainers on Instagram, something that seemed harmless—at first. “I like getting ideas for workouts or ways to eat healthy. But when I look at people who are super fit, it just makes me feel bad sometimes,” admits Marren, who says scrolling through her feed can leave her with a slightly sick feeling. “If I’m already not feeling very confident in that moment, seeing those photos can have the power to ruin my day, or at least part of it.”

Welcome to the world of “fitness inspiration,” also called “fitspiration” or “fitspo.” Search #fitspo on Instagram and you’ll find more than 46 million posts. Most of them claim to be motivational. Think personal trainers, athletes, models, or even just regular gym-goers, all excited to tell you what they eat and how much they work out, while posing in bikinis or workout wear designed to flaunt their ripped abs or sculpted calves.

So what’s the harm? “The problem is that most of these posts aren’t actually motivating us to be healthier,” explains Lexie Kite, co-director of Beauty Redefined, a nonprofit devoted to promoting positive body image online. “They are motivating us to lose weight and get ‘fit’ to match up to certain beauty ideals—often at the expense of our health.” Most of the people in fitspo posts are thin and extremely toned, without what Kite calls “normal signs of living” like enlarged pores or acne. And common fitspo slogans like “Fit Girls Look Good Naked” and “Unless You Puke, Faint, or Die, Keep Going” aren’t empowering. They’re really reinforcing the idea that how our bodies look is the most important thing about us. #Nottrue. 

Perhaps the most insidious thing about fitspo is that it bills itself as the backlash to another dangerous social media trend: thinspo, which encouraged a totally unhealthy obsession with weight. Instagram banned that hashtag in 2012. Many fans of fitspo argue that it’s the antithinspo. But fitspo is still about achieving a certain body type—just now, in addition to being super thin, you’re expected to be super toned as well.

And while the thinspo trend primarily targeted girls and women, fitspo is more equal opportunity, with nearly as many guys posting photos of their chiseled physiques as girls. “Boys are sold the lie that they need to take up more space in the world by having bigger muscles, just like girls are told to take up less space by being thin,” explains Kite. “Both are harmful ideals that can lead to unhealthy behaviors like purging, over-exercising, or the use of diet pills and steroids.” Ian Kirby, Marren's friend and a fellow fitness buff, says, “These people are on another level. It's easy to see this stuff and be like, man, what will it take for me to get there?” He steers clear of steroids and other extreme tactics, but he often sees them promoted online. “Sometimes you can just tell a guy is obviously ’roided up,” he says. “But if you didn’t realize that, it might give you unrealistic expectations.”

Of course, this doesn’t mean that simply wanting to be fit is dangerous. “If you want to run faster or get stronger, you have to push yourself—and that’s normal and healthy,” says David LaPorte, a psychology professor at Indiana University of Pennsylvania who studies eating disorders. “But when you start experimenting with lots of products designed to promote weight loss or muscle building, or if you’re regularly saying, ‘I can’t hang out with my friends or do other fun things because I have to get my workout in’—those can be signs that you’re developing a disordered relationship with food or exercise.” And while people develop eating disorders for many complicated reasons, LaPorte’s research shows that being exposed to fitspo-style images can play a role. In one study, 84 percent of college students who spent just 90 minutes looking at “thinspiration” online and kept food journals for a week cut their caloric intake by an average of 2,470 calories. “They basically stopped eating for the equivalent of a whole day as a result of that one short exposure,” LaPorte says.