The common fear that cardiovascular exercise, or cardio, immediately “eats” muscle is a misunderstanding of how the body manages its energy stores. While muscle loss can occur, it is not the default outcome of standard aerobic activity. Cardio only begins to compromise muscle when specific, demanding conditions are met. Maintaining muscle mass while engaging in cardio is entirely possible and requires an understanding of the body’s internal fuel hierarchy.
How the Body Prioritizes Fuel During Exercise
The human body is highly efficient and has a well-defined order for selecting energy sources during physical activity. The immediate fuel for muscle contraction is adenosine triphosphate (ATP), generated primarily through the breakdown of carbohydrates and fats.
During aerobic exercise, the body relies heavily on stored glycogen (carbohydrate) and fat. At rest and during very low-intensity activities, fat is the dominant energy source. As exercise intensity increases toward a moderate level, the body shifts to a roughly equal contribution of fat and carbohydrate for ATP production.
When intensity rises further, carbohydrate becomes the preferred fuel because its metabolic pathway is faster. This reliance on fat and carbohydrate is a protective mechanism. The body reserves protein—the building block of muscle tissue—for its structural and regulatory functions, only turning to it as an emergency fuel source when other stores are severely depleted.
Conditions That Trigger Muscle Breakdown
Muscle breakdown is triggered when the body’s primary fuel tanks—glycogen and fat—are insufficient to meet energy demands. This process is known as gluconeogenesis, where the liver converts non-carbohydrate precursors, including amino acids derived from muscle protein, into glucose. This catabolic state is a response to specific metabolic stress, not the result of a short run or cycle.
One major trigger is prolonged, high-volume cardio, such as marathon training, where muscle and liver glycogen stores are eventually exhausted. When a session extends beyond 90 minutes, especially at a moderate-to-high intensity, the body’s reliance on protein for energy production increases significantly. This is a direct attempt to maintain blood glucose levels.
A severe or prolonged caloric deficit, combined with exercise, also forces the body into a catabolic state. If an individual is consistently under-eating, particularly without adequate protein intake, the body must break down muscle tissue to acquire the necessary amino acids for gluconeogenesis and energy. Doing long-duration cardio in a completely fasted state can similarly accelerate this process, as immediate fuel from food is unavailable.
Protecting Muscle Mass While Doing Cardio
Protecting muscle mass requires a strategic approach that combines smart nutrition with thoughtful workout structure. Consuming enough calories overall is the foundation, as a negative energy balance is the primary driver of muscle catabolism. It is important to consume adequate protein to provide the necessary amino acids for muscle repair and synthesis, minimizing the need to break down existing muscle.
The timing of carbohydrate and protein intake also plays a significant role. Consuming carbohydrates before and after a cardio session helps replenish glycogen stores, which directly spares muscle protein from being used for energy.
Incorporating regular resistance training is a powerful signal to the body to preserve muscle. Strength training promotes muscle protein synthesis, effectively counteracting the catabolic effects of endurance exercise. Moderating the duration and intensity of cardio, particularly keeping sessions under the 60- to 90-minute mark, prevents the severe depletion of glycogen.