Does Ketosis Affect Muscles? The Science Behind Keto and Muscle Health

January 1, 2026 •

5 min read

According to a 2024 review in the journal Nutrients, while a ketogenic diet can preserve muscle mass during fat loss, it may not be the optimal strategy for muscle hypertrophy during a caloric surplus. This reveals the nuanced ways in which ketosis affects muscles, which depend on an individual's goals and training type.

Quick Summary

Ketosis shifts the body's metabolism to use fat for fuel, impacting muscle function, mass, and exercise performance. Effects vary depending on adaptation time and activity intensity, with endurance and resistance training seeing different outcomes. Electrolyte imbalances are a common side effect affecting muscle health.

Key Points

Initial Performance Drop: During the first few weeks, a drop in high-intensity exercise performance is common due to depleted muscle glycogen.
Improved Endurance: After adaptation, endurance athletes may experience better fat oxidation, potentially improving performance in prolonged, steady-state activities.
Impaired High-Intensity Output: The reliance on fat for fuel makes it less efficient for powerful, short-burst activities that rely on readily available muscle glycogen.
Electrolyte Imbalance Risk: Reduced insulin on keto can cause the body to flush electrolytes like sodium and potassium, leading to muscle cramps and fatigue.
Muscle Mass Preservation: With sufficient protein and resistance training, ketosis can help preserve muscle mass, though it may not be the optimal diet for maximizing muscle growth.
Adaptation is Key: Effects on muscles are highly dependent on the body's adaptation period. Initial side effects subside as the body becomes more efficient at using ketones for fuel.
Supplementation for Muscle Health: Strategic supplementation with electrolytes (sodium, potassium, magnesium) and adequate protein intake are important for managing muscle health on keto.

How the Body's Metabolism Changes in Ketosis

When following a ketogenic diet, carbohydrate intake is drastically reduced, forcing the body to enter a metabolic state called ketosis. In this state, the liver breaks down fat into molecules known as ketone bodies, primarily beta-hydroxybutyrate (βHB), which are then used for energy by the brain, heart, and skeletal muscles. This metabolic shift, known as keto-adaptation, fundamentally alters how muscles are fueled and can take several weeks to complete.

During keto-adaptation, the body's primary energy source shifts from glucose to fat and ketones. For muscles, this means relying less on stored glycogen and more on fat oxidation during rest and lower-intensity exercise. Enzymes related to fat metabolism increase their activity, while those involved in carbohydrate metabolism decrease.

Short-term effects on muscle function

In the initial weeks of ketosis, often called the 'keto flu' phase, many individuals experience a temporary drop in exercise performance and energy levels. This is primarily due to depleted muscle glycogen stores, which are the body's most readily available energy source for high-intensity, short-burst activities like sprinting or heavy weightlifting. Fatigue and a perceived increase in effort during workouts are common complaints during this adaptation period.

Decreased Anaerobic Performance: The body's capacity for high-intensity, anaerobic activities is reduced, as these activities rely heavily on muscle glycogen for fuel.
Risk of Electrolyte Imbalance: The initial reduction in carbohydrate intake and subsequent drop in insulin levels cause the kidneys to excrete more water and electrolytes, such as sodium, potassium, and magnesium. This can lead to muscle cramps, spasms, and general fatigue.
Keto Flu Symptoms: Along with muscle-related issues, other temporary side effects like headaches, brain fog, and nausea can occur as the body adjusts to the new fuel source.

Long-term effects on endurance and strength

For well-adapted athletes, the effects of ketosis on muscles become more nuanced and specific to the type of exercise.

Endurance Exercise: Keto-adapted endurance athletes show significantly higher rates of fat oxidation during prolonged, submaximal exercise. This increased efficiency in burning fat for fuel can conserve limited glycogen stores, theoretically prolonging performance. Some studies have shown maintained or even improved endurance performance in keto-adapted athletes, particularly in ultra-endurance events where consistent energy supply is crucial. However, other research indicates that the keto diet impairs exercise economy, requiring more oxygen to perform the same amount of work, which can negatively affect performance in elite athletes.

Strength and Power Exercise: The impact on high-intensity, power-based sports is generally less favorable. The downregulation of carbohydrate metabolism and lower muscle glycogen stores can compromise the rapid energy production needed for maximal strength efforts and sprints. Studies on resistance-trained individuals show mixed results, with some suggesting that while fat loss can be maximized, muscle hypertrophy may be blunted compared to a high-carbohydrate diet, even with adequate protein intake.

Ketosis and Muscle Mass: Preservation vs. Growth

For most individuals, a well-formulated ketogenic diet does not lead to significant, long-term muscle loss, provided protein intake is adequate and resistance training is maintained. Research suggests that ketone bodies, particularly βHB, may have a protein-sparing effect by reducing the rate of muscle protein breakdown. Some studies in resistance-trained men have shown that while on a keto diet with a caloric surplus, fat mass decreased, but muscle mass gains were less pronounced compared to a non-keto group.

However, a key factor in body composition changes is initial water and glycogen loss. When starting keto, reduced muscle glycogen leads to a significant loss of water, which is often misinterpreted as muscle loss. This initial drop in lean mass stabilizes as the body adapts. For individuals primarily focused on maximizing muscle growth (hypertrophy), a ketogenic diet may not be the most efficient strategy due to its potential to inhibit anabolic pathways, such as mTORC1, which are crucial for muscle building.

Comparison: Keto vs. High-Carb Diets for Muscle Health


Feature	Ketogenic Diet	High-Carbohydrate Diet
Primary Fuel Source	Fat and ketone bodies.	Glucose from carbohydrates.
High-Intensity Performance	Often decreased due to limited anaerobic capacity.	Optimized and fueled by readily available muscle glycogen.
Endurance Performance	Can improve fat oxidation and may benefit ultra-endurance, but effects on exercise economy can be negative.	Provides a quick and efficient fuel source, maximizing performance across a wide range of intensities.
Muscle Mass Preservation (during fat loss)	Can be effective for preserving lean mass, especially with adequate protein.	Often effective, especially when paired with resistance training.
Muscle Hypertrophy (Growth)	Potentially less efficient due to lower glycogen and inhibited anabolic signaling pathways.	Supports optimal muscle growth and repair due to insulin and glycogen availability.
Electrolyte Balance	Requires deliberate focus on replenishing sodium, potassium, and magnesium to prevent cramps.	Generally easier to maintain, as diet includes a wider variety of foods and less fluid excretion occurs.

Managing Muscle Issues on a Ketogenic Diet

For those who experience muscle-related side effects on keto, several strategies can help mitigate the issues:

Prioritize Electrolyte Intake: Focus on foods rich in sodium, potassium, and magnesium to prevent muscle cramps and spasms. Incorporate salty broths, avocados, spinach, and nuts into your diet. In some cases, a high-quality, sugar-free electrolyte supplement may be necessary, especially during the initial adaptation phase or with intense exercise.
Ensure Adequate Protein: Consume enough protein to support muscle maintenance and minimize breakdown. A daily intake of 1.6 to 2.2 grams of protein per kilogram of body weight is often recommended for active individuals on a ketogenic diet.
Adjust Exercise Intensity: During the first few weeks, scale back on high-intensity exercise and focus on lower-intensity, steady-state activities like walking, jogging, or cycling. As your body becomes more keto-adapted, you can gradually reintroduce higher intensity training.
Consider a Cyclical Approach: Some athletes use a cyclical or targeted ketogenic diet, strategically reintroducing carbohydrates around intense workouts to replenish glycogen stores and support peak performance.
Stay Hydrated: Increased fluid excretion on a keto diet can lead to dehydration and affect muscle function. Drink plenty of water throughout the day to support overall hydration.

Conclusion

In summary, the question of whether ketosis affects muscles has a complex answer. The diet fundamentally alters muscle energy metabolism, shifting the primary fuel source from carbohydrates to fat and ketones. This leads to a distinct set of effects on muscle mass and exercise performance that vary based on the individual's keto-adaptation status, training intensity, and fitness goals. While endurance athletes may experience enhanced fat oxidation and potentially maintained performance after adaptation, those in high-intensity sports may face performance limitations due to reduced glycogen availability. Critically, maintaining adequate protein and electrolyte intake is essential to manage potential side effects like muscle cramps and support overall muscle health. For individuals aiming to maximize muscle hypertrophy, traditional high-carbohydrate diets may offer superior results, but a well-managed keto diet can effectively preserve muscle mass during fat loss. Ultimately, understanding these effects allows individuals to make informed decisions about tailoring their diet to meet their specific athletic and health objectives. For further reading, consult the Journal of the International Society of Sports Nutrition for studies on the ketogenic diet's effect on body composition and athletic performance.

Frequently Asked Questions

Muscle cramps, especially during the initial phase of ketosis, are often caused by an electrolyte imbalance. When carbohydrate intake is low, the body excretes more water, which also flushes out essential minerals like sodium, potassium, and magnesium needed for proper muscle function. Replenishing these electrolytes through food or supplements can help prevent cramps.

Not necessarily. While some initial lean mass may be lost due to decreased glycogen and associated water, studies suggest a well-formulated ketogenic diet with adequate protein can help preserve muscle mass, especially when combined with resistance training. However, it may not be the best strategy for maximizing muscle growth (hypertrophy).

The research is mixed, but many studies suggest that while you can build muscle on keto, it may not be as efficient as a traditional, high-carbohydrate diet, especially during a caloric surplus. The mechanisms driving muscle growth, like the mTOR pathway, may be less activated in a ketogenic state.

Ketosis can negatively impact performance in high-intensity, anaerobic activities like heavy strength training or sprinting. These activities rely on muscle glycogen for rapid energy, and with lower glycogen stores on keto, your peak power and strength can decrease.

Yes, many endurance athletes successfully use the keto diet. After a period of keto-adaptation (often several weeks), the body becomes highly efficient at burning fat for fuel, which can be advantageous for long-duration, steady-state exercise. However, some athletes may notice a decrease in exercise economy and find high-intensity efforts more challenging.

To support your muscles, focus on consistent and adequate protein intake, replenish electrolytes (sodium, potassium, magnesium) through nutrient-dense foods or supplements, and ensure proper hydration. Consider adjusting your training intensity during the initial adaptation period to reduce muscle stress.

Full keto-adaptation can take several weeks. While the initial metabolic shift occurs relatively quickly (within days), optimizing the body's ability to efficiently use fat and ketones for fuel, especially during exercise, is a longer process. Performance levels typically return to normal or improve for endurance activities after this period.