What’s the hottest diet trend these days? Forget juicing and gluten-free – it’s all about throw-back, old-school diet foods that actually work like low-fat cottage cheese, grapefruit, and air-popped popcorn. In fact, new research shows that a lot of retro diet foods are more effective for weight loss than most of the fad diets that have come and gone.

Interestingly, Millennials - who weren’t even alive the last time these foods were “in” – are at the forefront of the shift because they don’t trust today’s trendy diets. And they’re smart not to – because most fad diets don’t actually work.

That’s from Dr. Brian Wansink, director of the Food and Brand Lab at Cornell University. He says diet trends start to die out when people realize they don’t work and then there's renewed interest in old, stand-by diets that have been proven to work.

Typically, retro diet foods fall into two camps: Low-calorie density foods – which have few calories per ounce - like Jell-O, grapefruit and cottage cheese. Then there are high-volume foods, like air-popped popcorn and salad which are packed with air or water, so you feel like you’re eating more than you are.

And along with old-school diet foods – people are also trying old-school fitness trends. Things like push-ups, jumping rope, and hula-hooping. They’re exercises you probably did in PE, way back when, and now they’re popping up everywhere from "The Dr. Oz Show" to your gym’s class schedule. Why? Because often, the things that stand the test of time work. So before you shell out for a trendy juice cleanse you may want to just hit up the produce section of the supermarket and go old-school.