Your muscles are built for strength, but as fuel runs low, they become an energy source—and it happens quicker than you may expect. Initially, your body taps into glycogen, but should you push hard or skip meals, things shift fast. Stress, exercise intensity, and even sleep play a role in the moments muscle breakdown starts. Curious about how far you’re tipping the scales? The five stages reveal when your effort turns against you—and how to stop it.
Immediate Energy Sources and Glycogen Depletion
Ever contemplate why your energy crashes during a long workout?
It’s likely because your body burns through muscle glycogen initially—your go-to energy source for physical activity. Whenever you push hard, especially in endurance exercise, glycogen depletion happens fast.
Once those stores dwindle, your energy demands force your body to tap into blood glucose to keep you moving. But here’s the hitch: in case you don’t refuel, your system starts breaking down muscle protein for fuel through gluconeogenesis.
That’s when fatigue really kicks in, leaving you feeling drained. Consider it like running low on gas—your body scrambles for alternatives, but it’s not ideal.
To avoid hitting that wall, pace yourself and fuel smartly before and during workouts. Your muscles (and energy levels) will thank you.
Transition to Fat Metabolism and Ketone Production
Anytime your glycogen runs low after hours of exercise or days without enough carbs, your body doesn’t just give up—it shifts gears.
Instead of relying on glucose, it turns to fat for energy, deconstructing fatty acids into ketones. This switch keeps your brain and muscles fueled while sparing your muscle mass.
As your liver ramps up ketone production, it also starts gluconeogenesis, pulling amino acids from protein to make small amounts of glucose.
But here’s the positive update: ketones become your primary energy source, reducing the need to dismantle muscle tissue. Your metabolic processes adapt, prioritizing fat over protein to keep you going.
It’s a survival tactic—your body’s way of protecting muscle while still powering you through low-energy times.
Onset of Muscle Protein Breakdown for Amino Acid Release
While your body’s shift to fat metabolism helps spare muscle initially, prolonged stress—like hours of intense training or days without food—pushes it to start breaking down muscle proteins for fuel.
At the moment glycogen stores run low after prolonged fasting or intense exercise, your system taps into muscle protein to meet energy needs. Amino acids, the building blocks of muscle, get released into your bloodstream through catabolism, a process breaking down tissue. Some of these amino acids convert into glucose via gluconeogenesis, fueling organs like your brain that rely on it.
Cortisol rises, insulin drops, and muscle breakdown accelerates to keep you going. Though BCAAs usually help maintain muscle, their supply dwindles as your body prioritizes survival over repair.
It’s a trade-off—your muscles shrink to keep you alive.
Factors That Accelerate Muscle Catabolism
Each time your body runs low on energy, it doesn’t just burn fat—it turns to muscle for fuel, breaking down proteins to keep you going.
Several factors speed up muscle catabolism. Prolonged fasting or extreme caloric deficits force your body to use amino acids from muscle breakdown for energy once glycogen stores run out.
Intense exercise, especially without enough carbs, ramps up muscle protein breakdown to meet energy demands. High cortisol levels, whether from stress or overtraining, also trigger muscle loss by promoting protein degradation.
Aging plays a role too—sarcopenia kicks in as your body struggles to maintain muscle without proper nutrition and exercise.
To slow this process, balance your energy metabolism with smart eating and activity choices. Your muscles depend on it.
Adaptive Responses to Prolonged Energy Deficit
As your body faces a prolonged energy deficit, it doesn’t just shut down—it adapts.
Initially, it burns through glycogen stores, then taps into fat oxidation for fuel. But at the point those run low, it turns to muscle protein, breaking it down into amino acids for energy.
Cortisol rises, speeding up this process and risking muscle mass loss. Your body also ramps up gluconeogenesis, converting amino acids into glucose to keep you going.
Over time, your metabolic rate might slow to conserve energy, making it harder to maintain strength. While these adaptations help you survive, they’re not ideal for long-term health.
The key is balancing energy intake to avoid pushing your body into this survival mode too often.