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Can you tell me what are all the cases when muscle would be cannibalized? I've heard the two following cases will cannibalize muscle, but I still don't quite understand.

Aerobic Exercise

Arnold Schwarzenegger says in The New Ecyclopedia of Modern Bodybuilding:

Too much aerobics also encourages the body to metabolize lean body mass -- muscle -- to create additional energy. The body can actually cannibalize muscle tissue, especially white fiber "power" muscle in order to fuel aerobic exercise.

Why would the human body evolve to cannibalize muscle during cardio? Why not just take energy from the food in your stomach (granted that you do have leftover food)?

Skinny People

Scooby from Youtube says that the body's energy stores are three tiered - Food > Fat > Muscle. If energy can not be obtained from one tier, the body seeks the next tier. Since muscle is the last tier, and virtually everyone has some amount of fat, how is the muscle tier ever reached? I suppose the muscle tier will be reached for anorexic people, how about for everyone else?

High Intensity Training

Jamez said this in an article on optimal rep range:

Remember the muscle-building process described in Grow Baby, Grow? Microtrauma stimulates increased protein synthesis, and muscle growth is positively affected by a number of hormones that are released after weight training. High volume, multiple set programs cause more microtrauma and greater hormone secretion-so the end result is more muscle!

This probably explains why I was unimpressed with HIT. Although HIT uses high reps, you only perform one "hardcore" set per bodypart. I actually lost muscle and began to feel like I didn't even train!

His train of logic isn't so clear. What about HIT causes muscle loss?

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I don't think the right term is cannibalized @JoJo ;-) –  Ivo Flipse Apr 2 '11 at 8:58
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It's not the traditional sense of cannibalization, but this term was borrowed to describe this effect. I've heard it used in many places. Heck, Arnold himself used it. –  JoJo Apr 2 '11 at 16:09
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A more nuanced description than "cannibalize" is that muscle rebuilding rate decreases. Every process in the body is dynamic equilibrium, including tissue damage and cell death. Muscles break down all the time on the micro level, and they get repaired depending on the NEED (ie, are they used a lot) and the RESOURCES (ie, do you have the amino acids available, after repairing vital organs etc.). –  J. Winchester Apr 2 '11 at 16:11
    
Your questions are so great, JoJo. I don't have time to answer this one now, but if no one else answers it in the next day or so, I can shed some light on this topic for you :) p.s. Is your avatar a photo of you? Do you compete? –  Barbie Apr 3 '11 at 4:34
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"Catobolism" is a term that loosely describes the mechanism that breaks down muscle tissue. Based on this, I think it's not inconceivable that someone might mistakenly use the term "Cannibalize" instead of "Catabolize". That being said, "cannibalism" does seem to be an adequate description of what's happening inside. –  Joel Cornett Aug 19 '12 at 0:20

3 Answers 3

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To make a long story short: you will start burning muscle when you run out of carbs and dietary protein. Also, your body will only keep as much protein in the muscles as it has to: if you're not training in a way that requires a muscle to maintain it's size, the body will leech out some of its protein and use it for something else. If you want more details, here's the long story.

Background. To understand why protein is drawn from the muscles to be used as energy, you have to understand a little bit about how energy is made in the body (metabolism). In order for fat or carbs (or protein, as a last resort) to be fully converted into energy (i.e. burned), they have to go through a process called the citric acid cycle, which releases energy through a number of reactions. This is the predominant energy system at rest and during prolonged aerobic exercise, and the primary source of energy in these states is usually fat. But here's the thing: one of the intermediary products of the citric acid cycle (oxaloacetate) requires a derivative from either carbs or protein. Because of this, to metabolize fat, you have to break down at least some carbs or protein. A common summary of this is "at burns in a flame of carbohydrates." Your body prefers to burn carbs over protein, but it has limited carb stores. If it runs out of those, it will start breaking down protein (into ketone bodies) to be used as fuel. This is also important for fueling your brain once you run out of carbs, since it can only use carbs or ketone bodies as fuel (not fat). It kind of makes sense that your body would evolve the ability to use protein for energy as a back-up energy source considering there's so much of it in your body.

Aerobic Exercise. Prolonged cardio can deplete your carb stores (colloquially known as "hitting the wall"). For example, if you do a marathon without carb-loading in the days before the event and without consuming carbs during the race, you will run out of carbs in your body and start burning protein. According to the "hitting the wall" article on Wikipedia, carb stores can come depleted in under two hours of moderate exercise. The harder you work out, the faster you'll deplete your stores, because as you work out harder, you begin to burn a larger proportion of your energy as carbs instead of fat.

It makes sense that you would burn protein that you recently ate first, but even if that's all you burn, now you don't have that protein around to help you build muscle (not to mention all the other stuff in your body). Also, if you don't have an immediate use for protein, your body converts it irreversibly into fat (i.e. your body can't convert fat back into protein), so depending on when you ate, you may not have as much protein available as you think.

Food > Fat > Muscle. This is hard to discuss in detail, because it's an oversimplification, but I can at least comment on the spirit of it. During starvation (or dieting) muscle is preserved as much as possible, but based on the info above you can see how this hierarchy wouldn't always apply; there are some things protein can be used for that fat can't be.

High Intensity Training. I'm not very familiar with weight training HIT, but I get the author's logic. As I understand it, HIT is when you do one high-rep set to exhaustion (per muscle group). However, according to the author, doing multiple sets is more beneficial for muscle growth. I assume this is because HIT doesn't overload the muscles in the right way (there are a few different ways to overload the muscles that all correspond to different goals. Check out this ACSM position stand if you want to know more). Your body doesn't waste resources making a muscle any bigger than it has to be. If you don't constantly require your muscle to be big/strong (by doing the right type of training), your body will take unneeded protein from the muscle and use it for something else.

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+1 Amazing answer. –  annelie Apr 4 '11 at 12:35
    
Some people say that muscles are used before fat....quora.com/… so during cardio if you use up all the carbs, you start using your muscles for energy. –  mabounassif Nov 30 '12 at 20:32
    
I did.not.knew.protein.became.fat that easily. This mean that to get strong I must do exercise, and then eat protein during repair phase? –  speeder Dec 27 '13 at 0:48
    
Getting strong does require exercise, though after you're exercise or repair phase you should just have small sugary drink to replenish your glucose level, with a balanced diet. –  Chad Jan 2 at 0:47
    
Could you explain why carbs or protein are required to metabolize fat? I'm no expert in the citric acid cycle but my understanding is that it's fed with Acetyl CoA which can be produced from fatty acids? –  Luis May 14 at 19:47

I don't necessarily buy any of the above explanations. A migrating bird needs to fatten up in order to make the migration. If it was to consume some of the muscle proteins along with the fat, it would never make it from i.e. Canada to Central America.

I just don't believe that human evolution has allowed the body to be less capable of survival after a short term fast/starvation (as long as there is fat deposits to burn off).

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I would answer it in simpler terms.It is easier for your body to store energy in the form of fat rather than muscles as it takes less amount of energy for your body to form fat rather than muscles. So , muscles are bigger source of energy for your body than fat. Said that , when the body senses that it requires more energy , in situations of long period of cardio or weight lifting which cannot be fulfilled by fat as source, it starts using your muscles for its energy requirement rather than fat.

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