I have learned that during aerobic exercising the body converts different sources of energy: glucose, fat and muscle. However, different ratios are used in different situations.

So what are the ratios by intensity? I once had a link to a page where this was explained, the Wikipedia article about exercise intensity lacks muscle tissue as energy source.

Also, the ratio of these sources changes with time of activity. I heard that after about 30-40 min of cardio the main source of energy switches from glucose to fat. How does this affect the amount of muscles burned?

  • What!? Calories come from muscle? I don't understand. Do calories come from muscles!?
    – Freakyuser
    Oct 22, 2013 at 4:12
  • basically i meant when body breaks down muscle to use it as energy. Oct 22, 2013 at 15:18
  • 2
    If your body is to the point of breaking down muscle for energy, you are WAY far over length of exercise, or you have some disease state showing up. Fat stores will be used long before muscle, so unless you are 3% body fat and working out for 5-6 hours, you should have plenty before your body turns to muscle.
    – JohnP
    Oct 22, 2013 at 16:21
  • Whether or not you SHOULD be burning muscle is good info, but irrelevant to this question. If you are a lean athlete and need to cut mass to meet a weight requirement down the road, for example, cutting muscle is a valid option.
    – GHP
    Oct 22, 2013 at 20:23
  • i know i'd have plenty but i am looking for a source that listed how much muscle, even when i have plenty, would be burning. Oct 23, 2013 at 3:33

2 Answers 2


Your muscles and brain run on glycogen. More importantly for this question, Glycogen fuels your muscles. When you exercise with high intensity for 20 minutes or more this is when your body switches from a primarily carbohydrate based metabolism to a fat metabolism, as duration increases so does amount of fat burned for fuel.


I suppose high-intensity would be > 70% V02 max Carbs are used as primary fuel here, if low-intensity or under < 30% V02 max, fats are primary fuel.

So if the total energy expenditure is relatively high, 50% from carbohydrates, 40% from fat, and 10% from muscle, which creates lactic acid.

If your prolong your exercise, you shift from carbohydrate dominant metabolizing toward fat metabolism. This is due to an increase in breakdown of triglycerides (cholesterol) or (glycerol + free-fatty-acids).

So in reference to the muscle that is used, it actually comes from the glycogen, as the primary source of carbs (keep in mind you're also burning fat after 20min) for the first hour or so.

85% V02 max and you're burning 60% muscle glycogen, 15% glucose, 20% FFA, <5% muscle triglycerides

Fat burns in the flame of carbohydrates http://www.depts.ttu.edu/hess/mccomb/documents/ess3305/ppt/chap04.pdf

  • I am telling you its ratios. All sources are used but at different ratios. From what i read it was like at beginning 2% from muscle. Oct 22, 2013 at 15:20
  • It is ratios. It's not like eating the steak, then the peas, then the potatoes. It all gets burned at different ratios according to intensity.
    – JohnP
    Oct 22, 2013 at 15:25
  • Oh I see...I'm sure if it's burning glycogen your % of muscle being "burned" is minimal...I'll look for a resource and update. Thanks.
    – Hituptony
    Oct 22, 2013 at 15:33
  • updated answer, provided reference, and explanation. Apologies for first answer, maybe i was thinking too newb on the topic.
    – Hituptony
    Oct 22, 2013 at 15:58
  • -1 again for a rather glaring mistake. Lactate (the actual term for what is commonly called lactic acid) does NOT come from burning muscle. I'm also not sure where you got your quote, as it is not from the link that you provided. If you do look at around slide 33 onward, there is good information on the ratios, what lactate actually is/does (It's a secondary fuel source), and where muscle energy comes from.
    – JohnP
    Oct 22, 2013 at 17:45

The ratios do not change based on duration, the ratios change based on the intensity of the exercise. At low intensities, you can get nearly all of your energy from fat, but the amount of power that can be generated by fat is limited, so as you increase your intensity, you will need to start burning more carbohydrate, and therefore the ratio of carbohydrate to fat will increase.

It might help to think of fat metabolism as a furnace that is very efficient but can't put out much heat; if you need to get the house warmer you might need to turn on the electric heater (carb metabolism) to produce extra heat.

This is somewhat related to duration because long-duration events are generally at a lower intensity than short-duration events, and therefore will depend more on fat as an energy source.

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