Sugar is the undisputed energy source of the body. The brain only process sugar as an energy source and so elaborate mechanisms are in place to make sure that blood sugar does not fall below safe limits. Normal limits are about 0.84 grams per litre or 5 mmol/litre of glucose. Despite the fact that it is our primary source of energy there is a limited amount of sugar present in the blood which could be exhausted in about 40 minutes when resting.

Hypoglycemia, low blood sugar levels, rarely happens in healthy humans. This is because we have mechanisms that maintain blood sugar at normal levels and methods of producing glucose when our diet is extremely low in glucose and starch. The liver and the kidney can produce glucose from amino acids, in a process called gluconeogenesis, during times of starvation or low carbohydrate. As a result of these mechanisms blood glucose does not fall greatly as a result of fasting, when sleeping, or mild exercise.

• Overnight fasting and light work do not normally reduce blood sugar levels below normal levels.

• Consumption of a large sugar meal causes an increase in blood glucose, but these is kept in check and not allowed to rise to abnormal levels.

• Normal levels are again reached within 2 hours after ingestion of a glucose load.

Glucose is stored in the liver and muscles in the form of glycogen. Below is the average time that stores of glucose will last under conditions of starvation and mild exercise such as walking. However, keep in mind that a person will fall into a coma way before the limits are reached as the brain's requirements for glucose are not satisfied.

The liver and kidneys are the only tissues that can release glucose into the blood stream.  After eight hours sleep glycogen stores are depleted even though blood sugar remains constant. After a good breakfast that contains carbohydrates glycogen in the liver is replenished. Glycogen is then used to keep the blood glucose at constant levels during the mornings activities and is once again topped up by a carbohydrate intake at lunch.

If breakfast is not eaten the body responds by secreting stress hormones adrenalin, glucagon and growth hormones all of which cause the synthesis of glucose through amino acids derived from the breakdown of muscle and liver protein. High levels of stress hormones is not an ideal situation for learning or any activity that requires calmness and a need to contemplate. Gluconeognesis is very active at about 10-12 hours after the last meal providing enough glucose for moderate activity for up to several days of starvation.

The diagram above shows hepatic glucose release during starvation.

Over the course of a night's sleep, which substance in the body is depleted? blood glucose liver glycogen muscle protein liver protein

Which of the following stays constant as we fast or engage in physical exercise? blood glucose liver glycogen muscle protein liver protein muscle glycogen

Which of the following best describes why we lose muscle mass when we fast? gluconeogenesis testosterone secretion muscle protein synthesis muscle glycogen breakdown

Skipping breakfast after a long nights sleep results in low levels of liver glycogen but normal levels of blood sugar normal levels of liver glycogen but low blood sugar levels muscle protein increase muscle glycogen depletion and blood sugar depletion

The stress hormones adrenaline, growth hormone and glucagon are most likely to be released as a result of a build up of glycogen and blood glucose after skipping breakfast after eating a high carbohydrate breakfast after a rise in liver protein

Gluconeogenesis can be performed by the liver only muscles only liver and kidneys muscle and kidney brain only

The brain has glycogen as its only food source fat and glycogen as its two major food sources two major sources of food glucose as its only food source

After 1 day of starvation what is the major source of blood glucose? pyruvate amino acids liver glycogen glycerol

After 5 days of starvation what is the major source of blood glucose? pyruvate amino acids liver glycogen glycerol