Each of many different human hormones can by itself raise the concentration of glucose in the blood  The reason for this is probably a metabolic quirk of the brain  To see this, consider that although most human cells can produce energy from fats and proteins, brain cells can use only glucose
Although most human cells can produce energy from fats and proteins, brain cells can produce energy only from glucose.
 Thus, if blood glucose levels fall too low, brain cells will rapidly starve, leading to unconsciousness and death