When exercising, our bodies take energy from our blood sugar, then uses glycogen as a source for energy, and finally burns fat or muscle when the glycogen is used up.

A nutritionist told me that whether your body will start burning muscles or fat depends on the timing in which the two sources before (blood sugar and glycogen) were burnt. If this process take the proper time, then it will burn fat as desired.

I would like to know how to make sure we will burn fat when working out instead of muscles.

  • 1
    I liked the post and it is base on science except for "evolutionarily we are developed" as evolution studied all of these stuff and designed us to be able to store and function and breath. .. From my personal experience, eating less carbs burned more muscles especially the meal post the workout.
    – user6810
    Sep 24, 2013 at 2:42

12 Answers 12


Energy metabolism is not a very well understood system in the sense that while the biochemical reactions are well known, their dynamics is highly variable depending on the individual. I find it disturbing that so many people have their own understanding of how their body work, without any sound reasoning behind it. Below I'll try to give some background information to the chemistry of it.

The biochemistry behind it is essentially very complicated and is often over-simplified. The truth is, different parts of the body use different sources for energy. A common example is the brain, which can ONLY use glucose as the energy source.

To argue against @camara90100's post, ATP is NOT an energy source but instead an energy carrier. ATP molecule is carries three phosphate groups as its name suggests. By breaking the these bonds (i.e. ATP -> ADP + P) energy is released which is used in some other reaction in the body. When the body "burns" sugars, or anything else, it uses the energy to synthesize more ATP molecules, or to reverse the original reaction.

Whether or not lactic acid is produced from breaking sugars is dependent on the oxygen supply to the surrounding tissue, if you cannot supply the tissue with enough oxygen a less than optimal reaction will take place where one of the byproducts is lactic acid. Lactic acid build-up in the tissue will ultimately lead to "cramps" as your body is telling you stop what you are doing since your metabolism cannot keep up with the physical activity you put yourself through.

Further more, there is an interplay between simple sugars and complex ones (carbs), as well as between carbs and fat. Excess blood sugar is processed in the liver to produce glycogen which is a long-term-storage of sugars. However glycogen is not the only way to store fuel, evolutionarily we are developed to "store energy" in case food becomes scarce. In that sense it's important to understand that fat is not a undesired trash molecule, but a perfectly healthy part of the metabolism. I recall reading some article on a critical limit on body fat index and normal brain function, where the authors have discussed individuals with extremely low body fat percentage were performing less than average on intellectual tasks.

Long story short, I don't believe that you can "guarantee" that you're burning only fat and no proteins during some training, especially considering that all these reactions I've described (and many more) have different rates on different individuals. Individuals with higher metabolic rates will end up breaking down muscle tissue through physical training instead of building muscle mass if they cannot keep up with the food intake. So I suggest you look over your diet so that you do not consume excess amounts of fat or carbs, and plan your training so that it matches with your own metabolic rate.

PS: Sorry for the long post but I hope it helps people get a better grasp of things.


I will do my best to address this question in a practical manner. Namely, I think the best way to burn fat and spare muscle while training is to construct a hypocaloric diet and workout plan with muscle preservation in mind.

Steps I would take:

  1. Ensure diet is hypocaloric so that you actually lose weight over time.

  2. Continue weight training while dieting. Based on the use it or lose it principle, frequent weight training will help spare existing muscle tissue since it is needed to do work.

  3. Consume adequate protein. Many bodybuilding references will suggest >= 1 gram protein/lb lean body mass. Others will suggest more or less but you get the point. Nice to make sure that if proteins are needed, they are available via diet.

  4. A moderate caloric deficit will give you a better chance at sparing muscle tissue than a drastic deficit.

  5. As a general note, if you are well above your bodyfat setpoint, expect to have better success burning more fat than muscle while training. As you move to lower and lower bodyfat levels, expect to experience more trouble maintaining muscle mass on a diet. A takehome message from this point might be to bulk and cut around your bodyfat setpoint to help avoid muscle loss.

With a good workout routine, adequate protein intake and hypocaloric diet, I would simply measure results over time. Adjust if you find you are losing muscle tissue instead of fat.

It may seem as though I didn't address question directly. However, based on my experience, for a given training day, the factors above largely determine whether I will experience muscle loss or fat loss. I am not sure I see a good reason to complicate a simple process any further.

  • 3
    Comment from V/R Arnold: One quick note - I think the author meant one GRAM protein/lb lean body mass. 1 KG of protein would mean consuming 2.25x your body weight in protein.
    – Ivo Flipse
    Jan 2, 2013 at 8:27

Endurance athletes are aware of the increase in protein metablolism. Therefore there is a trend to prefer energy sources to come from foods that contain fat and protein. For example eggs and peanut butter. When I refer to endurance I am talking about no or little sugars consumed through the exercise. For instance if you cannot exercise for more than an hour and a half without requiring sugar and caffeine it would be wise to start training your body not to rely on sugar unless during race day or at an event. A good barometer is if you can't walk around in your feet for say 2 hours without requiring sugar or good then your endurance fitness is way lower than it should be. The key is to incorporate low intensity exercise for longer periods over an hour. Sorry if this is a bit off topic.


Protein, Carbs and Fat:

Protein - energy source - building blocks for muscle

Carbs - digests fast - fast energy source - building blocks for cells, including muscle

Fat - digests slow - high energy source - acts as cushion for organs


Short and intense exercise - Increases testerone level (men have higher testerone level than women usually) - Builds muscle, when there is enough protein and carbs available - Burns carbs and fat

Long exercise - Burns carbs and fat

Things to avoid:

  • Food in plastics, especially when heated, so don't heat up your meals in plastic holders (it will polute your food with an estrogen-like substance)
  • Not enough activity, kills muscles that are not used and with less muscle it takes longer to burn fat
  • High estrogen, which tells you body to store more fat (women have higher estrogen levels, which is normal)
  • 3
    Explaining down votes helps new users to improve their answers and other users which part of the answer is bad or wrong.
    – Baarn
    Sep 6, 2013 at 15:39

I recently started a nutrition plan based on very low carbs and low calorie with daily vigorous exercise, so it has made me curious about these body processes (fat and protein metablolism/catabolism). One thing is definite, it is a very complicated process and apparently not completely or widely understood, even by "experts"--so opinions abound. In addition, everyone has slightly varying metabolic responses, a part of genetics, so there may not be one correct model.

One simple fact is sure. It takes more energy for the body to break down muscle (not glycogen) than to breakdown stored fats for energy. Your body nets less energy breaking down muscle. It does not make sense for the body to "use" muscle protein energy when stored fats are available. This assumes you maintain a full-body workout to maintain overall muscle tone.

For my part, I have definitely lost belly fat and increased strength and tone in the six weeks since I have started this nutrition plan. It might be harder to maintain as stored fat is used, but I can't see how this would affect the metabolic cycle.


I second camara's answer. This part of the reason why eating sweets is generally not good if your are dieting, because sugars are first in the "burning order" of things. Carbs are generally second, fat is third, and protein is last since protein really isn't an energy source but used for repair.

Carbs and sugar are very close to the same but sugar is considered a simple-carb, it burns/is broken down faster and eaiser thus sought after first bu the body.

  • Sugar
  • Carb
  • Fat
  • Protein

Read this it's excellent: http://idealfitnessofct.com/idealfitnessofct.com/Protein,Carbs%26_Fat.html

This is good too: http://ezinearticles.com/?How-Long-Does-it-Take-to-Burn-Off-Carbs-Before-Burning-Fat?&id=2142117

* Sugars are simple molecules of carbohydrates, so it's just carbs (galactose, fructose, glucose), Protein (Amino acid, oligosaccaride, monosaccaride), Fats (free fatty acid, monoglyceride), not "Sugar and carbs" **

The body burns carbohydrates and fat together, and efficiently. Oncethe body's glycogen runs out, the body will start burning fat and protein to keep up with its requirement.

  • 1
    sugars are carbs. you probably meant simple and complex carbs which also impact the GI differently Sep 6, 2014 at 9:40

I'm directly answering the question "I would like to know how to make sure we will burn fat when working out instead of muscles."

One way no one mentioned is the use of BCAAs

One theory as to how BCAAs exert their substantial fat burning and muscle building effects is this: When present in high amounts during exercise, the body senses high levels of BCAA in the bloodstream which is typically a sign of excessive muscle breakdown. So the body stops muscle breakdown and uses more fat for fuel. At the same time the extra BCAAs in the blood stimulate insulin so the BCAAs are driven directly to the muscle. So the result is people lose body fat and gain muscle at the same time. If my hunch is correct, in order to benefit the most from the fat loss aspect of BCAAs you should make sure you limit carbohydrate consumption during the 2 hour window before your workout.

BCAAs probably exert most of their anabolic effects through anti-catabolic activity. In short, they suppress the use of muscle proteins for fuel, thereby sparing the breakdown of muscular protein. In part this is because they can sacrifice themselves as fuel. With less muscular protein being broken down by the body during training, the net result is increased protein synthesis and more muscle for you! In a study done on obese people put on a starvation type diet, BCAA supplementation was found to induce anabolism and nitrogen sparing so the subjects burned body fat instead of lean muscle mass, thus sparing muscle protein.

Unknown study:

Supplementation of BCAAs has been shown to trigger significant and preferential losses of visceral body fat. Located in the deeper layers of the body under the subcutaneous fat, this visceral fat tends to be resistant to dieting and is hard to lose. In one study, 25 competitive wrestlers were divided into 1 of 3 diet groups: a diet high in BCAAs, a diet low in BCAAs, and a control diet. The wrestlers stayed on the diets for 19 days. The results showed that the high BCAA group lost the most body fat, 17.3% on average. Much of the fat lost was in the abdominal region. This may give credence to BCAAs effectiveness at "spot reducing" the abs. In another study 2 groups of climbers were divided into a BCAA supplemented group and a control group. Both groups lost weight but the BCAA group actually gained muscle mass while losing fat and the other group lost muscle mass.

source: http://www.bodybuilding.com/fun/inmag13.htm

Though I do agree with "you can't guarantee that you're only burning fat and not muscle".


So much bro science in here :(

The guy who thinks sugars are different to carbohydrates lol?!

You just need to have eaten before your workout so you have readily usable blood sugar and protein (*BCAAs broken down protein) then your body will use these instead of going catabolic and using its internal stores.

Also maintaining high BCAA* levels always will ensure your body doesn't break down its own stores to move out for healing damaged areas or to use for energy.


The most simple answer: Never make your stomach ask you to feed it(don't make it growl). Feed it with small meals (6-7 meals every 2-3 hours). You'll stay lean and pack muscles with a decent workout plan.

  • 4
    Have any sources to back up your claim?
    – Ivo Flipse
    May 27, 2012 at 17:51
  • 2
    That article is one of the worst mixes of barely common sense and outright ignorance I've seen in a while.
    – JohnP
    Aug 15, 2012 at 17:49
  • The 'growl' feeling is simply your stomach shrinking/contracting as it empties. It has no relationship to hunger. Sep 27, 2013 at 18:48
  • This myth has been put to rest long time ago. Or so i thought...
    – ruslaniv
    Apr 14, 2016 at 10:28

Stay at a -mild- caloric deficit.

Maintain your lifts.

Enjoy watching yourself lose fat and not muscle.


I was wondering about the same thing a while ago and I read this article that explained this issue in a very detailed way, http://www.healthguidance.org/entry/13020/1/How-Bodybuilding-Works--The-Science-of-Bodybuilding.html

Your body will start burning muscles only under extreme conditions when you're pushing your self in training so much burning fats is not fast enough to keep you going. According to the article, there are different forms of energy the body use during a workout.

The first form is ATP short for Andenosine Triphosphate, a power source that only lasts for 3 seconds of expanding and contracting then the body look for a source of energy elsewhere. However using creatine can help recombining the already used ATP and using it again which basically make it 13 seconds of ATP based energy instead of only 3.

Then the body moves on to a new source of energy which carbs which is a little slower form energy than ATP, the body first have break it down to ATP than use but this process creates lactic acid which causes burning sensation you feel in your muscles when we push yourself in running or in the gym.

In order to lose fat you'll need to push yourself beyond the first 2 energy forms and cause your body to start looking for fat as a source of energy however and according to the article there is not specific timing or order on which when your body will switch from carbs to fat, its genetically determined by your body’s physiology.

And eventually after you train for too long you run out of carbs and your body fat is not producing enough power to keep you going, your body starts to burn muscles for energy.


The body never burns muscle for energy, except in maybe in the most extreme cases of starvation and I mean holocaust starvation where you're dying. This is a myth.

  • 1
    Can you add some evidence to substantiate your answer?
    – rrirower
    Feb 13, 2017 at 18:56

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