Fasting has become one of the most talked-about trends in the fitness world, with surveys showing that nearly 24% of adults in the UK have tried some form of intermittent fasting in the past year. From intermittent fasting (IF) to prolonged multi-day fasts, more people are experimenting with periods of food abstinence in the hope of improving body composition, boosting metabolism, and even enhancing longevity. Social media is packed with influencers swearing by fasting as a way to improve mental clarity and shred fat while still building muscle. But how does fasting actually affect strength training and fitness performance? And are there any concerns when it comes to balancing fasting with the body’s nutritional requirements during training?
The Rise of Fasting in Fitness
Fasting isn’t new. Humans have practised it for thousands of years, whether due to food scarcity, religious practices, or cultural traditions. However, in recent years, fasting has gained traction in the fitness industry due to its potential benefits for fat loss, insulin sensitivity, and even autophagy – a cellular process that helps clean out damaged cells.
Many people adopt intermittent fasting because it’s simple, although some struggle with initial hunger, energy dips, and maintaining workout intensity in a fasted state. Instead of worrying about multiple meals a day, they eat within a specific window—often an 8-hour period—and fast for the remaining 16 hours. Others experiment with 24-hour fasts or longer, believing that extended fasting promotes deeper metabolic shifts. The promise of eating less while still maintaining muscle and losing fat is undeniably appealing. But does it really work that way when you’re lifting weights or pushing through high-intensity workouts?
Fasting and Strength Training: The Nutritional Balancing Act
Strength training places a significant demand on the body, requiring adequate protein intake for muscle repair and carbohydrates for energy. When you train, you create microscopic tears in your muscles, which need nutrients—especially amino acids—to recover and grow stronger. Fasting naturally limits nutrient availability, so the big question is: can you maintain performance and muscle mass without regular fuel?
Some argue that training in a fasted state increases fat oxidation, making the body more efficient at using fat for energy. This can be useful for endurance athletes or those looking to shed excess fat. However, others worry that prolonged fasting might lead to muscle loss, reduced strength, and compromised recovery. A recent study delved into the impact of extended fasting on physical performance and metabolism, giving us valuable insight into how the body adapts.
The Study: Seven Days of Fasting and Its Impact on Strength and Endurance
Researchers wanted to investigate how a full week of fasting would affect physical performance and metabolic adaptations in healthy individuals. Thirteen participants (seven men and six women) undertook a seven-day water-only fast. The study measured changes in body composition, muscle strength, endurance capacity, and metabolism.
By the end of the fasting period, participants lost an average of 5.8 kg, with 4.6 kg coming from lean mass and 1.4 kg from fat. This confirms what many already suspect—fasting leads to weight loss, but it doesn’t discriminate between fat and muscle. Surprisingly, maximal isometric and isokinetic leg strength remained unchanged, suggesting that fasting did not impair raw strength. However, peak oxygen uptake (VO2max) dropped by 13%, and peak power output decreased by 16%, meaning endurance took a noticeable hit.
The study also examined metabolic adaptations. Muscle glycogen was reduced by 50%, and carbohydrate oxidation during exercise dropped significantly. Researchers noted a 13-fold increase in pyruvate dehydrogenase kinase 4 (PDK4), an enzyme that inhibits carbohydrate oxidation, further explaining the reduced endurance performance. Interestingly, resting metabolic rate remained stable, but fat oxidation increased dramatically, accounting for 73% of energy expenditure compared to 37% before fasting.
What This Means for Strength and Conditioning Coaches
So, what does this mean for fitness professionals working with clients who fast? First, it’s clear that fasting does not directly impair maximal strength, which is great news for those who prefer to train fasted. However, losing lean mass over extended periods should be a concern, particularly for individuals aiming to build muscle. Coaches should encourage clients to prioritise protein intake during their eating window and avoid excessive fasting durations if muscle preservation is a goal.
Another key takeaway is the significant drop in endurance performance. If clients engage in high-intensity training, metabolic conditioning, or endurance-based activities, fasting may not be ideal—especially prolonged fasting. Coaches should assess their clients’ goals and structure workouts accordingly. For those aiming for fat loss while maintaining strength, intermittent fasting may be beneficial, but for performance-focused individuals, pre- and post-workout nutrition should not be overlooked.
Coaches should also be mindful of recovery. Since fasting limits nutrient availability, ensuring clients consume enough protein and carbohydrates during their eating window is crucial for muscle repair. Strategies like consuming branched-chain amino acids (BCAAs) before training or breaking a fast with a protein-rich meal can help mitigate muscle loss.
Final Thoughts
Fasting can be a useful tool for fat loss and metabolic health, but it may not be suitable for everyone. Individuals with medical conditions such as diabetes, low blood pressure, or a history of eating disorders should approach fasting with caution and seek professional guidance. While strength may be maintained, endurance and high-intensity performance can suffer, and lean mass loss is a real risk. Fitness professionals should take a personalised approach, considering each client’s training goals, fasting duration, and nutritional intake. With the right guidance, fasting can be incorporated strategically without compromising performance or muscle maintenance.
Reference
Kolnes, K. J., Nilsen, E. T. F., Brufladt, S., Meadows, A. M., Jeppesen, P. B., Skattebo, Ø., Johansen, E. I., Birk, J. B., Højlund, K., Hingst, J., Skålhegg, B. S., Kjøbsted, R., Griffin, J. L., Kolnes, A. J., O’Rahilly, S., Wojtaszewski, J. F. P., & Jensen, J. (2025). Effects of seven days’ fasting on physical performance and metabolic adaptation during exercise in humans. Nature Communications, 16(122). Click here to review the full research article.
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