As we have seen that in normal persons, carbohydrate ingested in form of polysaccharides are broken down into monosaccharides (glucose or sugar) to be used as fuel for energy.A part of glucose is also re assembled into poly saccharides glycogen to be stored in muscle tissues,heart tissues and fat cells,to be used as and when required,after breaking down into glucose.
Now , after several hours of fast,say during night time, when all the available glucose is utilised,then the stored glycogen is broken down into glucose and put into circulation.Suppose if the stored glycogen is depleted then body to meet its requirement of glucose,starts making glucose from sources which were originally not glucose, eg.amino acids,pyruvates etc.The process is called gluco neogenesis.
Once the glucose level in blood goes down the first effect is felt by brain (, glucose is not stored by brain ,) and nervous system.We feel irritated,agitated,hands starts shaking,not able to concentrate etc.If blood sugar level is not corrected quickly ,some damage to brain might occur.
There is an inbuilt mechanism in the body to raise blood sugar levels to desirable level.These are all hormones and the most important is called GLUCAGONE.This hormone is also produced by pancreas by its ALPHA CELLS.It acts by breaking glycogen into glucose and putting it into circulation thus raising glucose level.Other hormones are- CORTISOL,EPINEPHRINE .These are stress hormones,which raises glucose levels and prepare us for fight or flight response.GROWTH hormone is also associated with raising blood glucose level.If blood glucose level falls below a certain level it is called hypoglycaemia.
Likewise when glucose levels rise above certain limit it is called hyperglycaemia .When circulating blood sugar remain raised,what can be the causes---(1) intake is high, or(2) insulin production is low or insufficient. Or(3) insulin receptors are not working properly,insulin resistance, or (4) post insulin receptor some problem inside the cell.Last two factors will lead to failure of glucose transporter to reach cell membrane,so glucose can not enter the cell .So all these will lead to high circulating glucose as it is not removed from circulation into cells
One thing more,as we know that glucose transporter activity is insulin dependent.So no insulin and no glucose inside the cell and no glucose in cell no energy.BUT in skeletal muscles these glucose transporters can be activated without insulin ie. this activity is INSULIN INDEPENDENT. skeletal muscle activity or contraction can sent these transporters to the surface of cell to bring glucose in. So walking, excercise and physical activity is a very important aspect of diabetes management.