Searching the internet, I have found lots of conflicting information from various fitness sites concerning when the human body burns fats, proteins and carbohydrates.

My basic understanding was that the body will first utilize glucose and glycogen stores, followed by fats and finally proteins once the fat and glycogen stores become depleted. Also I believe there is some overlap in this process.

However many fitness websites claim that during high intensity physical activity, the body will prefer to burn protein over fat.

Scientifically speaking, when does the body ordinarily burn the following food groups (proteins, fats and carbohydrates) and how does exercise influence this process?

If you are able to provide graphs or data to back up your answer this will be extra helpful because I am bored of all the speculation I see on other sites.

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    $\begingroup$ I do not know about exercise but I can tell you that in case of starvation, body first uses glycogen/other carbs as energy source and then fat and lastly proteins(as proteins have other important functions,too). I also want to point out that proteins might be used for the first week of starvation but then fat takes over and only after all the fat has been consumed is protein utilisation started substantially. $\endgroup$ – biogirl Dec 9 '13 at 11:26
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    $\begingroup$ Even though I am not sure, it seems ridiculous to me that "protein is used during heavy exercise" as proteins are generally the last resort and they have other more important functions. $\endgroup$ – biogirl Dec 9 '13 at 11:28


The system that regulates body energy store consumption is hugely complicated, but is mainly a cooperation between hormones released by the pancreas (insulin - lowers blood sugar and glucagon - raises blood sugar) and the liver (as the bodies main glycogen store and factory for various energy related tasks).

Fed State

In the fed state (high blood glucose e.g. after a meal), the body produces a large amount of insulin. When the ratio of insulin:glucagon is high (≈0.5):

  • Dietary glucose is absorbed by the liver when glucose concentrations are over 8mM. This glucose is then either
    • used in respiration by the liver
    • converted to glycogen to be stored in the liver
    • converted into fatty acids which are then transported out to the periphery in very-low-density lipoproteins (VLDLs)
  • Adipose tissue (fat tissue) takes up glucose in the blood. This glucose is then:
    • used in respiration by the adipose tissue
    • converted into fatty acids then tri-acyl-glycerols (TAGs) - fat. VLDLs from the liver can also be absorbed by adipose cells to be used for this.
  • Skeletal Muscle again directly absorbs glucose from the blood
    • During contraction glucose is used directly in respiration to fuel the activity
    • In preparation for future contraction it is converted into glycogen and stored in the cell
    • Amino acids are incorporated into cellular proteins though if they are superfluous then they can be used in respiration
  • The brain directly absorbs glucose and uses it in respiration

Early Fasting State

Where the insulin:glucagon ratio has dropped to around 0.15

  • In the liver
    • Glycogenolysis is activated to release glucose from stored glycogen to compensate for falling plasma glucose levels (signalled by elevated glucagon)
    • Triglycerides in the liver are preferentially broken down for hepatic respiration - fatty acids are transported in from the blood to meet the livers own energy demands
    • The liver begins the process of gluconeogenesis - making glucose from non-carbohydrate precursors
  • Adipose tissue
    • Rapidly starts lipolysis - stored fats are broken down into free fatty acids and glycerol.
    • Some of the free fatty acids are used for the adipose tissues own respiratory demands
    • The majority is released to the periphery to be utilised by other tissues
    • The glycerol released can't be used by most tissues, but is taken up by the liver where it can be used to make glucose by gluconeogenesis
  • Skeletal Muscle
    • Uptake of glucose from the periphery is reduced to preserve glucose for cells that can't easily use other fuels - e.g. the brain or red blood cells
    • Free fatty acids become the main fuel for skeletal muscles to conserve the glucose for elsewhere
    • Unlike in the liver, glycogenolysis is not activated as there are no glucagon receptors in muscle tissue. This means glucose is only produced from glycogen when muscles are actively contracting, as before.
    • At this point proteins can start to be broken down, their skeletons used as an immediate source of energy and amino acids released to be taken up by the liver and used in gluconeogenesis
  • The brain directly absorbs glucose and uses it in respiration. Fatty acids can not be used as these do not cross the blood brain barrier

Late Fasting State

If there has still been no glucose intake, the insluin:glucagon ratio falls even further, to around 0.05.

  • Liver
    • No further glucose has entered the liver and within 24h all hepatic glycogen stores have been depleted
    • Gluconeogenesis becomes the principle source of all plasma glucose - the liver creates new glucose from:
      • Amino acids - muscle & liver protein breakdown
      • Glycerol - from adipose tissue
      • Lactate - from red blood cells and muscles
    • High amounts of fatty acids are converted into Acetyl CoA to be used in respiration by the liver, but too much Acetyl-CoA is produced. The remainder is converted into ketone bodies that are useless to the liver, so jettisoned to be used by other tissues (esp. the brain)
  • Adipose tissue
    • As the fast continues into a few days, adipose tissue adapts to producing large amounts of free fatty acids. This becomes sufficient for most body tissues
    • These fatty acids are used preferentially by all tissues that can use them, conserving glucose for the brain
  • Skeletal Muscle
    • Fuelled almost entirely by fatty acids and ketone bodies from the liver and adipose tissue
    • Protein breakdown continues to release carbon skeletons and amino acids for gluconeogenesis - but this is inhibited in the presence of high levels of ketone bodies to prevent unnecessary muscle wastage (still allowing the human to hunt)
    • Amino acids are incorporated into cellular proteins though if they are superfluous then they can be used in respiration
  • The brain
    • Once ketone levels reach a critical level, they can cross the blood brain barrier to be used as a supplementary energy source
    • However, it is not sufficient alone, so a net source of glucose is needed from somewhere in the body

Progressing to starvation

This process can continue for a number of weeks, with the body eventually exhausting fatty acids in adipose tissue and being unable to produce ketone bodies in the liver, losing the inhibition of proteolysis causing a last ditch response of muscle breakdown for energy. Death is usually from heart failure resulting from cardiac muscle breakdown.


This chart might be useful in summary, it shows where the glucose levels in the blood are coming from at various stages, the gluconeogenesis line indicating protein and fatty acid breakdown:

Glucose sources in fed fasting starved states


Exercise has to be very intense to have any effect on this system in normal situations.

  • Resting Muscle metabolises stored fatty acids for its own energy. Glycogen stores are replenished from glucose ready for more vigorous contraction.
  • Brisk Walking fatty acid metabolism again provides almost all the energy
  • Sprinting Glycogen stores are used, respiration is almost entirely anaerobic as blood vessels are constricted by the muscle activity and ventilation has not had time to increase. Lactic acid is produced, which can be used by the liver for gluconeogenesis
  • Middle distance running: aerobic metabolism takes over as the body adjusts to the higher oxygen demand. Lactate is still the major end product

One example of where this may be more noticeable is during a marathon.

  • 0 Mins - Muscle is resting at the start line, as above.
  • 10 mins - Muscle and liver glycogen released to power muscle contraction
  • 2 Hours - A marathon requires roughly 700g of glycogen to complete, however the liver can only store around 500g. This is largely depleted after roughly 20 miles. Blood glucose levels fall rapidly and the body switches to fatty acid metabolism. This only provides around 60% of the power output and pace falls off (known as hitting the wall). At this point, virtually every reserve power source is being used simultaneously to keep the body going.
  • Finish - Muscle and liver glycogen are entirely depleted. If glucose levels haven't been maintained by other means, hypoglycaemia results causing confusion, hallucinations and even coma and death.

Energy sources in a marathon

  • $\begingroup$ Thanks for your informative answer. Let us say someone is on a calorie deficit diet, and undergoes exercise that night. They would supposedly have low glycogen stores, and would rely on gluconeogenesis. What proportion of gluconeogenesis will be fatty acids, and what proportion will be from protein? Is this ratio always the same or is it dependent on intensity? $\endgroup$ – Kenshin Dec 9 '13 at 13:23
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    $\begingroup$ @Chris I would anticipate greatly more from fatty acids than protein breakdown until one runs out of fat. You'll be able to release enough energy through fatty acid usage for any intensity of exercise assuming that you still have fat stores available. $\endgroup$ – Rory M Dec 9 '13 at 21:57
  • $\begingroup$ @Chris The only gluconeogenesis that is possible from fat breakdown is that derived from the glycerol of the triacylglycerol. Fatty acids are metabolised to acetyl CoA, and it is not possible to synthesise glucose from them because for every acetyl group that enters the TCA cycle, two CO2 molecules are lost before the 'start' of gluconeogenesis (oxaloacetate). A starving subject would rely on fatty acids (directly and as ketones) and glucose derived from glycerol, lactate and amino acids. Despite the switchover to using fat, a net loss of protein is unavoidable. $\endgroup$ – Alan Boyd Dec 10 '13 at 8:41
  • $\begingroup$ Dear @RoryM i really liked your answer and just want to kindly ask you further readings (even technical) about this field of human nutriton. $\endgroup$ – MauroM Sep 21 '15 at 10:03
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    $\begingroup$ The answer should cite its sources, crucial for fact checking and good science. Where's the graph and the table from, for example? That said, this answer is top notch and the missing references are small fry. Well answered. $\endgroup$ – SeanJ Oct 18 '16 at 10:27

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