Intermittent fasting, also known as IF [1], is one of the latest diet crazes that has everyone's attention. With talk of incredible weight loss and tons of before-and-after photos on Instagram [2], it's hard to not pay attention.
But maybe you're not trying to lose weight, but you do want to change your body with some new muscle gains. Trying to lean out and build some definition? This could be your ticket. "IF is an excellent tool to use while trying to gain muscle and get stronger," said clinical/sports nutritionist and personal trainer Autumn Bates, CCN, CPT [3]. "Multiple studies show that IF helps to boost growth hormone, which assists in forming muscle," she told POPSUGAR. "What's also unique about IF is that it boosts fat burning while still maintaining (and forming new) muscle. I use IF with many of my athlete clients to boost performance levels while increasing muscle mass and losing fat mass."
Precision Nutrition coach and personal trainer Austin Lopez, CSCS [4], confirmed that fasting stimulates human growth hormone, known as HGH. "I personally love intermittent fasting (IF) for all sorts of reasons," he said, noting that the hormone stimulation from fasting "in addition to an increase in insulin sensitivity from not constantly ingesting carbohydrates produces the fat-loss effect we recognise from IF."
That fat loss can lead to muscle gains [5], according to Bates. "The longer you use IF, the more efficiently your body begins to burn the fat calories that are stored in your body," she said. "As you become more metabolically fit, your body will turn to stored fat as a source of energy rather than glycogen. Your body can store many more fat calories than it can glycogen calories, so IF helps to tap into that for sustained energy levels."
But there is a bit of a catch, Lopez says. "What I have found with regards to building muscle [through IF] is a little complicated," he said, explaining that the primary use of IF is to "cut weight," which is typically counterintuitive if your goal is gaining muscle. "Basically, if you end up eating one less meal, you will end up losing weight — thermodynamically sensical," and from this, he typically sees muscle maintenance vs. muscle gain.
"Where I have seen the most lean mass gain is from people who continue eating the same or more calories, but fit all of their eating into a relatively small eating block (seven to nine hours a day where you can eat [6])," he said. "This gives you the benefit of the fasting, plus the benefit of a high-calorie diet," which you need to build new lean muscle tissue. So with this approach, you're burning fat and gaining muscle simultaneously. "This is no small feat, as you realise you are constantly eating in your eating block." Easier said than done, apparently!
Here's another key to success: "If you choose to use IF to help gain muscle, I recommend working out during your fasting period (preferably in the morning)," said Bates. She noted that this tactic "will help your body take advantage of the boosted growth hormone levels that you achieve during the fasted state." And the biggest consideration? Be mindful of what you're eating. "Just make sure that once you are ready to break your fast, you eat a nutrient-dense meal with healthy fats, proteins, and carbs [8]."