The idea that running automatically “eats” away at muscle mass is a common concern, but muscle loss is not inevitable. With strategic management of nutrition and training, runners can effectively maintain or even build muscle while pursuing endurance goals. Understanding the biological triggers for muscle breakdown and growth is the first step in preventing unwanted losses.
The Conditions That Lead to Muscle Loss
Muscle loss in runners is typically not caused by the act of running itself, but rather by placing the body in a state of prolonged energy imbalance. The primary trigger is a substantial caloric deficit, where the energy expended exceeds the energy consumed. When the body is forced to find fuel, it first turns to stored carbohydrates (glycogen) and fat, but eventually, muscle protein becomes a fuel source.
This process is exacerbated by high-volume endurance running, such as training for a marathon or ultramarathon, especially when under-fueled. Long, sustained efforts deplete glycogen stores, forcing the body to break down protein into glucose for fuel (gluconeogenesis). Even a modest daily calorie deficit, combined with the high energy demands of training, can lead to a catabolic environment where muscle tissue is sacrificed.
The Cellular Mechanism of Muscle Preservation
At the cellular level, muscle mass is regulated by a constant push-pull between two major signaling pathways: AMPK and mTOR. Prolonged or intense endurance exercise activates Adenosine Monophosphate-activated Protein Kinase (AMPK). AMPK acts as the cell’s energy sensor; when it detects low energy stores, such as depleted glycogen, it initiates catabolic processes and simultaneously inhibits muscle growth.
Conversely, muscle growth and repair are driven by the Mammalian Target of Rapamycin (mTOR) pathway, activated by resistance training and nutrients, especially amino acids. Since AMPK and mTOR are antagonistic, muscle loss occurs when the energy-depleted state of endurance running causes AMPK signaling to dominate. The goal is to limit AMPK dominance while maximizing mTOR activation through specific nutritional and training interventions.
Essential Nutritional Strategies
Preventing muscle breakdown requires maintaining an adequate total caloric intake to avoid a significant energy deficit. For endurance athletes, this means consuming enough calories to match the high energy expenditure of training, often requiring a high intake on heavy training days. A moderate daily deficit, such as 300 to 500 calories, should be the maximum if weight loss is a secondary goal.
Adequate protein consumption is paramount, as it provides the necessary building blocks for muscle repair and growth, which activates the mTOR pathway. Endurance athletes should consume between 1.2 and 2.0 grams of protein per kilogram of body weight daily. Spreading this intake across multiple meals, including a post-exercise dose of 20 to 30 grams, helps ensure a steady supply of amino acids for continuous muscle protein synthesis.
Carbohydrate availability is equally important, as consuming enough carbohydrates spares protein from being used as fuel. Pre- and peri-workout carbohydrate intake ensures the body has a readily accessible energy source, limiting the need to activate the AMPK pathway. Consuming 30 to 60 grams of carbohydrates per hour during runs longer than 90 minutes is a standard strategy to protect muscle tissue.
Structuring Your Training for Muscle Maintenance
To effectively counteract the catabolic signals of endurance running, runners must strategically integrate resistance training into their routine. Weightlifting provides the mechanical stimulus necessary to activate the mTOR pathway, overriding the muscle-degrading effects of AMPK. Focusing on compound movements like squats, deadlifts, and presses two to three times per week helps maintain or build muscle mass.
The timing of training sessions also influences the cellular signaling balance. Since endurance exercise activates AMPK, performing resistance training too closely after a long run may blunt the anabolic signal of the lifting session. Ideally, separating challenging runs and resistance workouts by at least a few hours, or placing them on separate days, allows the body’s growth pathways to operate without inhibition. Incorporating high-intensity interval training (HIIT) can also be beneficial, as it provides a strong cardiovascular stimulus without the prolonged catabolic effect of long distance runs.