: Why does the body burn muscles instead of fat in a calorie deficit body? Assumption There have been many articles and answers (even on this site) that exercising while on a calorie-deficit diet (or weightlifting/exercising

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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.

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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.