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. 860
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. 860