Quick answer? They're strength training. Weightlifting. Trading tons of cardio for dumbbells and kettlebells and barbells. Here's how it works.
Strength training builds muscles; the more muscles you have, the more food you need to eat; and more muscle means a faster metabolism. In short, strength training means you can eat more, and that food won't get converted to fat. "Muscle uses more energy and thus burns more calories than fat," said Nicole Aurigemma, physiologist at the Penn State Muscle Biology Lab. She told POPSUGAR that this also means you'll have "a slightly higher resting metabolic rate, meaning that your metabolism is increasing with training."
You don't want to drastically increase your food right away, but rather start slow. Add little by little as you start your strength and resistance training regimen. You may end up needing a significant amount more calories to fuel your body (like Emma, above, for instance, who increased her intake by 1,000 calories a day).
We conferred with personal trainer and injury prevention specialist Liz Letchford, MS, ATC, PhD candidate about the process of strength training coupled with adding calories to the body in a healthy way that doesn't contribute to fat gain. She noted that many women limit their calories so much (or for so long) that they hit a plateau. "If the body has stopped responding to a calorie deficit [read: you're dieting but you're not losing weight], this can be attributed to two major things: hormones or survival," she told POPSUGAR. "Dieting for extended periods of time puts the body into a state of extreme stress, so it slows down all processes related to metabolism and digestion. You can increase your metabolism through a process called reverse dieting."
However, if you've been dieting in extreme ways and "dealing with symptoms of severe malnutrition," Liz warned that "refeeding on your own can be dangerous and should be conducted under medical supervision." You need to be especially careful with the amount of food you add as well as the speed. "Essentially, you are slowly increasing your caloric intake week by week to coach your body out of survival mode and allow your body time to increase its metabolism. It is not a quick fix — it happens over a period of months in order to heal any metabolic damage."