Intermittent Fasting? You Might Be Losing Muscle & Here's Why
Intermittent fasting has become one of the most popular eating patterns for weight management and metabolic health. For some people, it works well1. Others? Not as much. If your goal is to build muscle (or maintain the muscle you already have), fasting may make that harder. In fact, a new systematic review and meta-analysis found that both younger and older adults lost some lean muscle while fasting2.
That's because certain aspects of fasting protocols can create conditions that make muscle maintenance more challenging. Here's why.
A calorie deficit without enough protein can lead to loss of muscle
One reason intermittent fasting can be effective for weight loss is that it naturally reduces the amount of time available to eat. For many people, that leads to a calorie deficit, and therefore, lower protein intake.
When calories are reduced, the body doesn't exclusively lose fat. Some lean mass (like muscle) is often lost alongside it. This is one reason experts generally recommend prioritizing protein (and strength training) during any weight-loss effort.
The challenge is that fitting enough protein into a shortened eating window can be difficult. If you're only eating one or two meals per day, you have fewer opportunities to consume the amount of protein needed to support muscle maintenance and recovery.
Muscle health also depends on protein timing
Building and maintaining muscle relies on muscle protein synthesis (MPS), the process of repairing and building muscle tissue. Protein-rich meals (specifically leucine-rich meals) help stimulate MPS. That's why it's often recommended to spread protein intake through the day (such as eating 30+ grams of protein for breakfast, lunch, and dinner) rather than having protein only at dinner.
When someone follows a fasting protocol (even when they fast for 16 hours) that limits them to just one or two large meals, they may stimulate MPS fewer times throughout the day compared to someone eating three or four protein-rich meals.
If you fast, meal & workout timing are especially important
Fasting doesn't automatically lead to muscle loss. Resistance training and sufficient protein intake can go a long way toward protecting lean mass. That said, workout timing becomes especially important. For example, completing a hard strength-training session and then waiting several hours before eating may delay the delivery of amino acids your muscles need for recovery and repair.
If you enjoy fasting and want to continue doing it, consider aligning your eating window so that you can consume a protein-rich meal within a few hours of training.
RELATED READ: What Women Need To Know About Fasting, From An MD
A shorter fasting window may be the sweet spot
While 16:8 fasting gets much of the attention, longer fasting windows aren't necessarily better. A more moderate eating schedule, such as a 10- to 12-hour eating window, may offer some of the structure and metabolic benefits people enjoy from fasting while providing more opportunities to consume protein and support muscle recovery.
This approach can be particularly helpful for adults who strength train regularly, are actively trying to build muscle, or are concerned about age-related muscle loss.
The takeaway
Intermittent fasting may be an effective strategy for some people. But if building or maintaining lean mass is a priority, it's important to look beyond the fasting window itself. Prioritizing protein intake, spreading that protein across multiple meals, timing meals around workouts, and strength training can all help support muscle health3. While it's nice to give your GI system an extended break (think 10 to 12 hours if possible), extended fasts can come with some unintentional downsides without careful planning.