In reality, aerobic training & cardio should be programmed as a tool. Not only to maintain a caloric deficit, but also promote better oxygen efficiency, cardiovascular health, lower RHR (resting heart rate) & BP (blood pressure), improve capillary density & muscular endurance, increase metabolic health, reduce stress & improve the recovery process, manage weight & fat metabolism, & overall increase your quality of life. But, many of these benefits fly out of the window when cardio is overdone without proper rest & refuel. 

Too much cardio can cause our body to go into a catabolic state, which is a topic I have discussed heavily in the past. Here is a link to my post about what exactly the catabolic state entails, as well as a deeper explanation on the topic: https://www.cptlij.com/blog?id=67be7381d48040a8be7d5099

To keep this as simple as possible, the over utilization of aerobic training can cause the breakdown of muscle tissue instead of building or preserving the lean mass. We generally want to try our best to maintain skeletal mass due to the fact that it is our longevity organ. Here is a blog of mine that dives deeper into the importance of resistance training for longevity: https://www.cptlij.com/blog?id=67da151115cd4e7a11afb4c2

So what generally is going on when we overuse cardio too often?

• Catabolism, which is the breakdown of molecules for necessary energy. 
• Longer bouts of cardio in calorie-restricted states without refuel depletes glycogen storages. 
• Once glycogen runs out, your body looks for alternative fuel—this includes amino acids from muscle tissue.

Your question might be, “doesn’t the body rely on fat for energy when doing cardio? not just glycogen? so why does it shift to a catabolic state or start breaking down muscle tissue instead of just using fat?”

Here’s the thing: 

In Normal Conditions (Low to Moderate Intensity)

• Primary fuel: Fat (especially free fatty acids from adipose tissue)
• Secondary fuel: Glycogen (stored carbs)
• Muscle breakdown? Minimal—your body’s in balance.

The shift begins once glycogen is low, and if fat oxidation can’t keep up with energy demand (like during long cardio or fasted cardio), the body looks for amino acids to maintain blood glucose levels via gluconeogenesis. Where do those amino acids begin to come from? Muscle tissue. Intense cardio = more reliance on glycogen, not fat. When glycogen runs out in high-intensity settings, your body struggles to generate ATP fast enough from fat. Result: Muscle protein is broken down for quicker fuel.

If you're in a calorie deficit and not eating enough protein, the body starts breaking down lean tissue to keep up with metabolic demands—even if fat stores are available. The body spares fat in extreme deficits as a survival mechanism. Chronically high from long sessions or overtraining. Cortisol increases protein breakdown and decreases muscle protein synthesis. It also promotes gluconeogenesis using muscle-derived amino acids.

Keep in mind, Your body prefers fat during cardio, but if:

• You go too long
• Too hard
• Too fasted
• Or under-fueled
• 
  

it uses muscle protein as an emergency energy source. That’s when you enter a catabolic state.

Peer reviewed articles: 

https://link.springer.com/article/10.1186/1743-7075-6-28?utm_

Aerobic fitness does not modulate protein metabolism in response to increased exercise: a controlled trial

https://pmc.ncbi.nlm.nih.gov/articles/PMC6876877/?utm_

Aerobic exercise increases post-exercise exogenous protein oxidation in healthy young males

https://pubmed.ncbi.nlm.nih.gov/16424115/

Aerobic exercise training increases skeletal muscle protein turnover in healthy adults at rest

https://pubmed.ncbi.nlm.nih.gov/9578384/

Exercise-induced changes in protein metabolism

https://pubmed.ncbi.nlm.nih.gov/26083652/

Mechanisms of Cachexia in Chronic Disease States

https://www.mdpi.com/2218-273X/5/2/356?utm_

Impact of Oxidative Stress on Exercising Skeletal Muscle