Diet and Liver Glycogen Levels

A student experiment


Acrobat PDF file can be downloaded here.

Carbohydrate is stored as glycogen in both muscles and the liver.  It is liver glycogen that is the first line of defense when blood glucose concentration falls.  That large glycogen reserve in our muscles cannot be released to the circulation.  Hepatic gluconeogenesis takes over after some hours, using lactate and amino acids to form glucose.  It should be emphasized that the level of blood glucose will be maintained within narrow limits as long as a source of amino acids is available.  The only real question is " will these come from me or my food"?  The body's proteins are its largest "glucose reserve".  Without an adequate blood sugar level, (> 2.5-3 mmol/l)  brain activity is impaired.  Interestingly. it is the transport of glucose into the brain that becomes rate-limiting at low blood glucose levels, not the initial phosphorylation og glucose by hexokinase.

The question addressed in this student exercise is "does the choice of diet after a fast alter the liver's ability to replace glycogen used to replace blood glucose?".  All of the rats used in this exercise were fasted overnight.  Three of these were then offered either bread (a carbohydrate-rich diet), dried fish (a high-protein diet) or margarine (a fat-rich diet) and allowed to eat for one day.  Their livers were then removed and glycogen was extracted.  This stored carbohydrate represented the glucose these animals obtained from the diets that was in excess of their immediate energy needs.  

Our students analyze the glycogen content of the liver extracts and usually get the following results:

Diet

Liver glycogen

mg/gram liver

24 hour fast

7.8

fast + 24 h. bread

66.8

fast + 24 h. dry fish

21.3

fast + 24 h. margarine

7.1

 

How do we explain these results?