How intermittent fasting connects to muscle loss, muscle building, and muscle maintenance
Intermittent fasting—an eating routine that focuses on when you eat rather than what you eat—is known for its associations with a wide range of potential physical and mental health benefits for your body. In particular, this eating routine has gained a lot of attention for its potential impact on metabolic health and body composition: It’s consistently linked to positive weight loss and fat loss results.
When it comes to overall body composition and health, though, there’s another factor to consider: muscle mass and muscle health. Some studies suggest that intermittent fasting can lead to muscle loss while others show it can support muscle preservation.
So, which is it?
Although intermittent fasting can lead to muscle loss, it doesn’t have to—and with some careful planning, it can in some cases even support muscle growth. In this article, we explore why fasting can contribute to muscle loss, why muscle mass is important, and how to maintain and even build muscle mass and muscle strength while fasting. We also spotlight some warning signs of potential muscle loss to watch out for in case you need to adjust your approach to keep meeting your goals.
The Relationship Between Fasting and Muscle Mass
Let’s start with an overview about how fasting and muscle mass relate to each other—and how things like fasting periods, calorie restriction, protein intake, and weight loss come into play.
Why Intermittent Fasting Can Lead to Muscle Loss
Whether or not you’re specifically fasting for weight loss or fat loss, any eating routine that involves restricting energy intake (for instance, eating for fewer hours and likely consuming fewer calories than you would when you’re not intermittent fasting) can lead to changes in weight and body composition. While elements like water weight and visceral fat tissue are commonly considered within that process, other components of the body contribute to these measurements, too—including muscle mass. In other words, any eating routine that involves potential changes to weight can also mean changes to muscle mass, as muscle is one of many internal elements that make up your overall body composition.
Even more simply: body composition = fat mass + muscle mass + bone mass + total body water. So, if you’re doing anything that may change your body composition, muscle mass can be impacted—and that includes intermittent fasting.
Does Fasting Cause Muscle Loss or Reduced Muscle Function?
Though intermittent fasting can impact muscle mass and muscle building, it doesn’t necessarily lead to muscle loss or compromise muscle strength.
That also means fasting doesn’t inherently cause muscle breakdown or impaired muscle function; it’s just often the scapegoat. In reality, several factors—like a calorie deficit, rapid weight loss, inadequate protein intake, and a lack of resistance training—are the culprits behind any muscle loss that may happen.
Fasting doesn’t cause muscle loss — these do
Intermittent fasting is often blamed, but the real culprits are four specific factors you can control.
When you consume fewer calories than your daily needs and in turn get less energy from food, your body is forced to use alternative fuel sources—including breaking down muscle tissue (a process known as muscle catabolism). Typically this process only kicks in if you’re hitting a calorie deficit of 20 percent or more, which can definitely happen during intermittent fasting, but it’s not a guarantee. Some research also suggests traditional calorie-restricted diets can actually cause more muscle loss than intermittent fasting.
Rapid weight loss can trigger muscle catabolism, too. If you’re losing more than one percent of your initial body weight per week, you’re more likely to experience muscle loss than fat loss since your body can’t rely solely on fat tissue for rapid refueling needs.
Similarly, if you’re not consuming enough protein (roughly between 0.45–0.68 gram per pound of body weight) to support muscle synthesis, your body will break down existing skeletal muscle to access the amino acids and glycogen stores it needs for vital functions.
How Much Protein Do You Need While Fasting?
Enter your body weight to get your personal daily protein target for muscle preservation while fasting.
That’s roughly equal to
Based on 0.45–0.68g per lb of body weight. Spread across meals for best muscle synthesis.
The same goes for a lack of strength training. Without resistance exercises (like lifting weights), the body has no signal to maintain muscle mass, which means it’s more likely to burn that muscle for energy. As you age, you also naturally experience sarcopenia (the progressive loss of skeletal muscle mass, muscle strength, and muscle function), and resistance training is the most effective and evidence-based intervention for it.
Why Is Muscle Mass So Important?
While muscle may be obviously crucial for athletes or people who are highly physically active, the importance of muscle mass—and maintaining it through any eating routine—extends far beyond general performance capabilities.
Let’s dig into why muscle mass is so mission-critical for our bodies.
Metabolic Health and Weight Management
Muscle is a metabolically active tissue that burns more calories than body fat, even at rest, and over 80 percent of glucose from food is processed by muscles. Preserving muscle mass not only maintains a higher resting metabolism—which prevents a yo-yo effect where the metabolic rate slows down after weight loss—but also improves both blood pressure and insulin sensitivity and reduces the risk of type 2 diabetes and other metabolic conditions.
Overall Functional Health
Beyond metabolic regulation, skeletal muscle is critical for immune system support, reducing frailty and falls / fractures, and promoting a longer lifespan. Muscle also produces myokines, which support brain health and combat chronic, low-grade inflammation associated with many diseases.
Physical Strength and Function
Higher lean muscle mass correlates directly to increased strength and physical performance, especially in older adults. Muscles also support joints and bone density by acting as dynamic stabilizers and shock absorbers, which in turn reduces strain and the risk of injuries while also improving balance and coordination during movement.
How to Build Muscle and Maintain Muscle Mass While Intermittent Fasting
Since intermittent fasting doesn’t intrinsically compromise skeletal muscle or lean muscle mass, approaching the process is less about asking “How much muscle do you lose when fasting?” and more about considering how to prevent muscle loss—and potentially even support muscle growth—when fasting.
So, whether you’re aiming for muscle preservation or muscle building, here are five ways to mind your muscles while fasting.
1. Build enough protein intake into meals.
Dietary protein is necessary for building new muscle and maintaining existing muscle, and there’s quite a bit of research that suggests protein intake reduces lean body muscle loss during weight loss and supports gains in lean body mass. When you structure your protein intake, it may also affect its impact: some studies suggest that incorporating some protein into every meal is more effective for continual muscle synthesis than saving the bulk of your protein for an evening meal ahead of your fasting period.
2. Don’t forget about carbs in food intake, too.
While protein is critical for muscle growth and repair, carbohydrates are also essential nutrients and act as the primary fuel source for intense training—which means keeping your glycogen stores well-stocked is crucial for preventing your body from breaking down muscle tissue for energy intake.
3. Focus on resistance training.
There’s a decent amount of evidence that shows resistance-based exercise programs are most effective for both increasing lean mass and preserving lean mass (as well as muscle strength) during caloric restriction. One systematic review and meta-analysis even suggests that resistance training can prevent nearly 100 percent of calorie-restriction-induced muscle loss in older adults.
4. Don’t completely ignore other forms of physical activity.
Although cardio can sometimes get a bad rep for impacting muscle strength and muscle breakdown, general exercise can act as a protector against muscle mass loss (as well as a general physical and mental health boost). Some studies also suggest a combination of fasting and endurance exercises can produce favorable outcomes in terms of muscle biochemistry and metabolism linked to insulin sensitivity.
5. Aim for slow and steady weight loss.
Working toward gradual weight loss and avoiding overly restrictive eating routines will keep your body from thinking it’s entering a starvation era and should start snacking on your muscles for energy. Aiming for no more than 2.2 lbs of weight loss (or one percent of your body weight—whichever is lower) per week promotes higher-quality body fat loss while preserving muscle mass.
How fast can you safely lose weight?
Lose too fast and your body starts burning muscle. Enter your weight to find your personal safe ceiling.
Signs of Muscle Loss to Watch Out For
Methods for assessing human skeletal muscle mass have been a discussion for centuries. As muscle mass is an internal component of body composition—and one that is continually masked by fluctuations in other elements of body composition, like water, fat, and glycogen—it’s not a straightforward concept to measure like total body weight.
Even if you may not be able to tell for sure that you’re experiencing muscle loss, keep an eye out for the following signs. They may be the warning bell that your current eating routine isn’t working out for you.
1. Losing more weight than expected each week.
Early rapid weight loss may stem from losing water weight linked to glycogen depletion, but if you continue to see more significant drops, it’s likely because you’ve cut your calories too severely and your body has turned to muscle protein to replace the depleted glucose stores as a source of fuel.
2. Feeling tired and like things are more difficult.
Over time, inadequate nutrition can prevent the body from repairing and maintaining muscle tissue, which can lead to subsequent structural muscle loss and increased fatigue in performing daily tasks. In turn, muscles operate with a “use it or lose it” functionality: If things feel more difficult and you start leading a less active life, muscles can atrophy.
3. Emotional shifts and low motivation.
Decreased skeletal muscle mass, strength, and function—particularly as you age—can contribute to various mood disorders like depression and anxiety. Likewise, chronic stress can contribute to prolonged muscle tension (and, in turn, muscle fatigue) while elevated cortisol from chronic stress, often both a cause and an effect of moodiness inhibits protein synthesis, which directly affects muscle strength and mass.
Even if these changes ultimately aren’t connected to any lean muscle mass loss, it’s still a good idea to speak to your doctor if you notice any of them—or any other significant changes to your health or body mass—cropping up.
TL;DR
There’s still some scientific back and forth on whether intermittent fasting can be associated with muscle mass loss and how fasting might impact muscle health. Largely, the evidence suggests fasting can lead to muscle loss, but it’s neither a cause nor a guarantee—and it can also still support muscle building, too.
Any eating routine that involves restricting calories or simply eating less than you might normally (as with fasting) can lead to changes in body composition, but getting adequate protein and carbs in your diet, building workouts around resistance training and some general physical activity, and approaching any weight loss or fat loss goals with a slow and steady mentality will support muscle preservation.
With its connections to metabolic health, weight management, functional health, and physical strength and performance, skeletal muscle mass plays an important role in both our bodies and our lives. If you notice any rapid weight loss or changes to your energy level or mood, make an appointment with your primary care provider to assess whether muscle loss is something to investigate and discuss your options to help maintain muscle mass.
