There have been many articles and answers (even on this site) that exercising while on a calorie-deficit diet (or weightlifting/exercising for more than an hour) will not result in bigger muscles as the body will begin to burn the muscles for energy after a certain point.


  • Is this valid for all bodies? Meaning that will this happen regardless of whether your body fat percentage is 20% or 10%?
  • Why (Only applicable if the assumption is valid for all bodies)? I understand the rationale if the body fat percentage is less than a certain level; however, for a body with lots of body fat, the body already has enough fat storage for the body to use. Why would it use the muscles for energy?


  • I think it has to do with cortisol. Extended exercise raises cortisol and this causes your body to be conservative with fat and less apt to build muscle. (It's hypothesised/known? to be a factor in why being stressed correlates with being overweight.) I'll just leave this here as a hint for anyone wanting to do some deeper research to write a full answer. I don't have the time.
    – Tyler
    Commented Jan 14, 2015 at 7:05
  • This seems very useful. bodybuilding.com/fun/…
    – Tyler
    Commented Jan 14, 2015 at 7:20
  • Your "Why" bullet seems to make the assumption that energy requirements are resolved in a specific order. I don't believe that is correct. I agree with Tyler that cortisol is a trigger for protein catabolism.
    – rrirower
    Commented Jan 14, 2015 at 14:06
  • @Tyler Thanks for the link; that has given a starting point for further research. Commented Jan 15, 2015 at 15:43

2 Answers 2


Ok, firstly you're dealing with the human body. You have several different paths to turn to when it comes to producing energy, but they boil down to three:

  • ATP-PC Pathway
  • Glycolytic Pathway
  • Oxidative Pathway

These energy systems all use and produce ATP (that's the main source of energy) in some way. To get ATP, they either break down:

  • Carbohydrates
    • These are typically the primary source of energy, yields 4 kcals per gram
  • Fats
    • Typically the secondary source, found everywhere but also large stores in the body. It's the most energy dense at 7 kcals per gram
  • Proteins
    • Typically tertiary, 4kcals per gram

The energy systems work like three dimmer switches. When you're doing something like an endurance sport, i.e. marathon running, you'll switch to oxidative, which will use fat as an energy source. When you're just sprinting or Olympic lifting, you'll use the ATP-PC pathway, which uses carbs. However, you never just turn off one. You'll always use all three-just to differing degrees. Fats, carbs, and proteins can also fuel each pathway.

Now, the critical thing to note here, is that we are not machines. Your body does not burn just fat, or just protein. These energy systems all work at the same time. It's more of a question of what you are doing and for how long that dictates what source of energy your body needs to burn first.

The reason your body will cease to build and repair muscle in a caloric deficit environment is because it needs to save those energy sources over a longer period of time. It's primary job is survival first-and if it's not receiving enough calories, it will hang onto energy stores for later metabolism, vs just repair. Building and repairing muscles costs calories, and the processes for repair are semi-unrelated to metabolism (for the sake of this answer anyway), or your day-to-day activities.

So for your questions, TL,DR:

  • Is this valid for all bodies?

    • Yes, but how much it turns to protein really depends person to person-activity levels, sport, body composition, diet-they all play a role. However, what you're concerned about, catabolism, is a real thing and will happen to anyone who starves themselves of energy needed. You can't get bigger without a caloric surplus. If you're trying to change your body, it's recommended to eat right and train right. Once again, you're not a machine. It's not a black and white, if this, then that environment.
  • Why?

    • You want to know why we just don't use fat 24/7. Think about our ancestors-to survive they needed to eat whatever they could get their hands on. Their bodies learned to use whatever they could get their hands on. If all we needed to eat was fat, what would happen to us if we ran out of nuts or animals?

Now I'm really simplifying this, there's so much more detail about how to answer your question, but it sounds like you want to change your body to run on fat. You can do that, but before you do, take a look at any Olympic athlete-the ultimate expression of prime physical condition. They eat balanced diets, and they train. That's the kind of philosophy you should be looking at, not fad diets.

EDIT: This is a difficult question to answer succinctly.

  • You say this is valid for all bodies, does that mean you believe there are no circumstances where the body will burn fat while building muscle? In the case of an obese beginner, for example, can they not add muscle and burn fat simultaneously?
    – Moses
    Commented Jan 14, 2015 at 19:29
  • No, there are certainly circumstances where the body will burn fat and build muscle. Sorry if my answer doesn't make it clear enough-human metabolism isn't the simplest subject to talk about. An obese beginner will DEFINETLY burn fat and build muscle (BUT NOT FAT ALONE)-it takes an obese beginner more calories to move their body (because you have more weight dedicated to non-functional (in relation to movement) priorities). Once you build muscle, your body will actually require more calories to maintain that muscle. Strength training is for everyone. There's too many benefits to ignore. Commented Jan 14, 2015 at 19:34
  • And remember, there are more calories per gram in fat, so that will be a good source of energy for some time. Because it costs that obese person more to move, I think it's safe to say that their body will tend to burn fat a little more than let's say me, because I have fewer stores to draw from. Commented Jan 14, 2015 at 19:35
  • Finally, you won't be able to sustain training hard and starving yourself. Your body won't have the surplus to repair itself, and you'll end up hurt. Our bodies don't work like that. Commented Jan 14, 2015 at 19:43
  • How would one explain the fact(?) that resistance training during calorie deficit leads to a higher proportion of fat loss to muscle loss, using this model?
    – Mårten
    Commented Jan 15, 2015 at 13:12

If your carbohydrate intake is too low, your glycogen levels within your skeletal muscle could very well be depleted to the point where your protein stores (muscle) is being used for fuel. Even in a calorie deficit, you need some carbohydrates to fuel your activities. It is a real balancing act.

  • 1
    It's not that simple.
    – rrirower
    Commented Jan 14, 2015 at 16:32

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