Researchers looked at 33 studies comparing the ketogenic diet (keto) to other diets to see if it affects muscle. The keto diet is popular for weight loss, but people worry it might make muscles weaker or smaller. The good news: the study found that keto doesn’t significantly reduce muscle mass or strength in adults. However, it did show a small decrease in lean body mass. The keto diet appears to change how your body uses fat for energy while keeping your muscles relatively intact, though more research is still needed to be completely sure.

The Quick Take

  • What they studied: Whether the ketogenic diet (a low-carb, high-fat eating plan) affects muscle size, strength, power, and endurance in regular adults
  • Who participated: The analysis combined results from 33 different research studies that compared people eating keto to people eating other types of diets
  • Key finding: The keto diet did not significantly reduce muscle mass or strength compared to other diets. However, there was a small decrease in lean body mass (the weight that isn’t fat). Muscle power, endurance, and aerobic fitness were not meaningfully affected.
  • What it means for you: If you’re considering keto for weight loss and worried about losing muscle, this research suggests your muscles should stay relatively strong and large. However, you may lose some lean body mass, so combining keto with strength training is still a good idea. Talk to your doctor before starting any new diet.

The Research Details

This was a meta-analysis, which means researchers searched through multiple databases (PubMed, Embase, Cochrane Library, and Web of Science) to find all studies comparing the ketogenic diet to other diets. They included only studies that had a control group—meaning they compared keto eaters to people eating differently. They excluded studies that didn’t have a comparison group. The researchers looked at three main things: how much muscle people had, how strong they were, and how well their bodies could use oxygen during exercise. They used statistical methods to combine results from all 33 studies together to see the overall pattern.

By combining many studies together, researchers can see the bigger picture instead of relying on just one study. This approach is more reliable because it reduces the chance that one small or flawed study will mislead us. The researchers also checked the quality of the evidence using a system called GRADE, which helps readers understand how confident we should be in the results.

This meta-analysis included 33 controlled studies, which is a solid number. The researchers used proper statistical methods and checked their results with sensitivity analysis (a way to make sure the findings hold up even when you change the analysis slightly). However, the studies included had different types of people, different diet plans, and different ways of measuring muscle, which can make combining them tricky. The researchers noted that more high-quality studies are still needed.

What the Results Show

The main finding was that the ketogenic diet did not significantly reduce muscle mass compared to other diets. When researchers measured muscle mass, they found almost no difference between people eating keto and people eating other diets. Similarly, muscle strength (measured by exercises like squats and bench press) and muscle power (measured by jumping ability) were not significantly affected by the keto diet. This was good news for people worried about losing muscle on keto.

However, there was one notable finding: people on the keto diet did show a small but statistically significant decrease in fat-free mass (the weight of your body that isn’t fat, including muscle, bone, and water). This decrease was small—less than half a kilogram on average—but it was measurable. This suggests that while overall muscle mass stayed similar, the composition of body weight changed slightly.

The keto diet did appear to change how the body uses fuel. It increased fat oxidation, meaning the body burned more fat for energy. This is actually one of the reasons keto is effective for weight loss. Despite this change in fuel use, the muscles themselves remained relatively strong and functional.

Other measurements related to aerobic fitness and endurance were not significantly affected by the keto diet. This includes VO2max (how much oxygen your body can use during intense exercise), treadmill time to exhaustion (how long you can run before getting too tired), and perceived exertion (how hard the exercise feels). These findings suggest that the keto diet doesn’t meaningfully harm your cardiovascular fitness or endurance capacity.

This research aligns with growing evidence that the ketogenic diet may not be as harmful to muscle as previously thought. Earlier concerns about keto causing significant muscle loss appear to be overstated based on this comprehensive review. However, the small decrease in fat-free mass suggests that while muscle tissue is preserved, some loss of overall lean body weight can occur, possibly due to water loss or other factors. This fits with what we know about how keto affects the body’s water balance.

The studies included in this analysis had important differences. Some involved athletes, others involved regular people. Some studies lasted weeks, others lasted months. The ways researchers measured muscle varied between studies. Additionally, the analysis couldn’t determine whether the results would be the same for people with certain health conditions or for very long-term use of the keto diet. The researchers also noted that the quality of evidence was moderate, not high, meaning we should be somewhat cautious in our conclusions. More large, well-designed studies are needed to give us even stronger answers.

The Bottom Line

Based on this research, the ketogenic diet appears safe for muscle preservation during weight loss in adults (moderate confidence). If you’re considering keto: (1) Combine it with strength training to maintain and build muscle, (2) Ensure adequate protein intake, (3) Monitor how you feel and your performance in activities you care about, (4) Consult your doctor before starting, especially if you have health conditions. The diet may help you lose fat while keeping muscle relatively intact, but it’s not a substitute for exercise.

This research is relevant for adults considering the keto diet for weight loss who are concerned about muscle loss. It’s particularly useful for people who aren’t professional athletes but want to stay strong and healthy. However, this analysis focused mainly on short to medium-term studies, so people planning very long-term keto use should discuss this with their healthcare provider. People with certain medical conditions, pregnant women, and those with a history of eating disorders should consult their doctor before trying keto.

Changes in muscle composition typically take weeks to months to become noticeable. If you start keto combined with strength training, you might notice changes in how your clothes fit within 2-4 weeks, though actual muscle changes take longer. Most of the studies in this analysis lasted 4-12 weeks, so that’s a realistic timeframe to assess whether keto is working for your goals.

Want to Apply This Research?

  • Track weekly body weight, waist circumference, and strength metrics (like how many push-ups or squats you can do). This helps you see if you’re losing fat while maintaining muscle, rather than just watching the scale. Record these measurements every 7-10 days.
  • If using a nutrition app, log your daily protein intake to ensure you’re getting enough (aim for 0.7-1.0 grams per pound of body weight). Combine this with logging 2-3 strength training sessions per week. This combination—adequate protein plus resistance exercise—helps preserve muscle while on keto.
  • Set up monthly check-ins to assess: (1) How strong you feel in daily activities, (2) Your performance in chosen exercises, (3) How your clothes fit, (4) Your energy levels. Use the app to create a simple dashboard showing weight loss, muscle strength progress, and protein intake trends over 8-12 weeks to see the full picture of how keto is affecting your body composition.

This research summary is for educational purposes only and should not replace professional medical advice. The ketogenic diet is not appropriate for everyone, including pregnant women, people with certain medical conditions (such as type 1 diabetes, liver disease, or kidney disease), and those with a history of eating disorders. Before starting the ketogenic diet or any new diet plan, consult with your healthcare provider or a registered dietitian. Individual results vary based on genetics, exercise habits, diet adherence, and overall health. This analysis represents current research but is not a guarantee of personal outcomes.