When your body is in a fasted state, it shifts from burning glucose/carbohydrates for energy to burning fat as its main source of energy, which is one of the reasons why intermittent fasting has become so popular. Yes, intermittent fasting can help you burn fat, but there's a caveat: it can also cause muscle catabolism (this occurs when your body begins to burn your muscle).
"From a baseline perspective, the way to maximise muscle protein synthesis (a natural process that allows your body to produce protein to repair skeletal muscle damage from exercise and promote skeletal muscle growth) is to technically have some protein every three to five hours. So, if you're not eating for 12 hours, you know you're entering into some state of muscle catabolism," Jason Machowsky, RD, CSSD, CSCS, a board-certified sports dietitian and exercise physiologist at the Hospital For Special Surgery's Tisch Sports Performance Centre, told POPSUGAR.
There's a possibility that you can lose muscle if you fast, but according to Jason, "There are so many other factors to consider as far as whether that brief period of catabolism is really going to be detrimental to your long-term lean body mass."
In Jason's opinion, the potential for muscle catabolism "depends on how many calories and how much protein you're getting in during the period that you're eating. If you're getting in adequate calories, if you're getting in adequate protein during that period of time, I don't think it's going to be a tremendous loss," he said. To find out your daily caloric needs, you can use this equation or you can consult a registered dietitian for a more exact number.
Along with consuming enough calories, specifically protein (here's how much you need in a day), you've got to strength train in order to preserve your muscle mass, Jason explained. Here's a four-week muscle-building program we like.
"The primary thing I tell people is if you're intermittent fasting, you're really restricting your ability to eat an adequate amount of calories and protein to a narrow window of the day, so you need to be more mindful and make sure that you are getting those things in during that period," he said. Bottom line: to prevent burning muscle when fasting, you've got to eat enough calories, eat enough protein, and strength train. As a reminder, everyone's caloric intake will vary based on activity level, age, goals, and other factors. To find out how much you should eat in a day to function at your best and meet your goals, please speak to a registered dietitian.
According to Jason, intermittent fasting is a good option for the average gymgoer who wants to get lean as long as you're able to consume enough calories and protein during your eating window. Most importantly, Jason said if you do choose to fast to burn fat and get lean, you should make sure it's something you can and want to sustain for the long-term.
If you're interested in intermittent fasting, be sure to consult your doctor before making any changes to your current way of eating.