From my own reading, there are three ways used by the body to produce energy:

  • Alactic anaerobic (direct degradation of ATP and creatine phosphate for regeneration of ATP)

  • Lactic anaerobic (breakdown of muscle glycogen with lactic acid production)

  • Aerobic (breakdown of carbohydrates and fats primarily and secondarily of proteins)

My questions are:

1) Knowing that the anaerobic mechanisms (alactic and lactic) are triggered when the effort is intense and / or greater than body oxygenation capacity, it seems logical, at rest, the body uses mainly aerobic way for producing energy. Am I right?

2) If 1) is true, what are the conditions (trigger elements, trigger events) which lead to muscle catabolism (so protein degradation) in order to produce energy? ? Is it well the last resort used by the body to produce energy that is to say after degradation of total carbohydrates and lipids?

  • $\begingroup$ (1) is basically correct. I don't see how (2) is related to (1) ? Also, there is no single answer to whether protein is the "last resort used by the body to produce energy", because different tissues of the body have widely different fuel preferences, and also it depends on the extent of fasting, level of exercise, amount of muscle and fat stores, and a bunch of other things. Can you be more specific? $\endgroup$
    – Roland
    Jul 18, 2015 at 19:24
  • $\begingroup$ Precisely, in a bodybuilding context, I would like to know the conditions which lead to muscle catabolism (so protein degradation) in order to produce energy? $\endgroup$
    – backlash
    Jul 19, 2015 at 5:50
  • $\begingroup$ Okay, can you please edit your question then so that it is specific to muscle catabolism during exercise. $\endgroup$
    – Roland
    Jul 19, 2015 at 7:08
  • $\begingroup$ @backlash that's an entirely different question. $\endgroup$
    – pbond
    Jul 24, 2015 at 13:47

2 Answers 2


1) Yes, most cells in the body derive energy from oxidative metabolism, unless oxygen is not available for some reason. There are some exceptions, for example red blood cells lack mitochondria and so have no respiration, and various types of rapidly proliferating cells such as activated immune cells have high glycolysis and lactate production (for reasons that are still not well understood).

2) The main fuel of exercising muscle is glycogen, which can account for 60-80% of energy requirements (article). Triacyglycerols (fat) stored in muscle tissue may account for most of the remainder (artice). Normally, muscle protein is not a major energy source during exercise. Some muscle protein breakdown has been observed after 2-3 hours of intense exercise (think marathon runners), when glycogen stores are depleted. See this review article.

If glycogen is not synthesized and stored normally while resting, then muscle may be more likely to catabolize protein during intense exercise. This can happen to some extent in diabetes, see for example this paper. There are also severe genetic disorders that disrupt glycogen synthesis, but those are very rare. I would guess fasting also makes it difficult to replenish muscle glycogen stores (and muscle triglycerides as well), so strenuous exercise combined with fasting might result in muscle protein breakdown, but I don't know of a study on this.


Prolonged fasting/starvation promotes muscle protein catabolism.

During starvation blood glucose must be maintained in order to provide fuel for red blood cells and brain in particular. Amino acids from muscle breakdown are used as gluconeogenic precursors by the liver and kidney. However fat is also mobilised and fatty acids are converted to ketones by the liver. Most organs/tissues can use ketones, including the brain (but not red blood cells or the liver). This use of the fat store reduces the body's demand for glucose and slows the process of muscle breakdown, allowing survival for weeks rather than days.


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