Under what conditions does the body burn fats, proteins and carbohydrates?
Introduction

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 &amp; 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.

Summary

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:



Exercise

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.