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How the body produces glucose when we are fasting

Describes the key pathways that the body uses to increase blood glucose concentrations in fasting healthy individuals
Glucose is our main source of energy and it is therefore important that there are physiological pathways in place to prevent blood glucose from dropping dangerously low when we are fasting, for example when sleeping at night.

Gluconeogenesis: producing glucose from non-carbohydrate sources

The gluconeogenesis pathway (see metabolism figure below) synthesises new glucose using non-carbohydrate precursors (glycerol from the breakdown of triglycerides, lactate during anaerobic glycolysis and amino acids from muscle protein degradation). 90% of gluconeogenesis occurs in the liver but some occurs in the kidney too. Insulin regulates gluconeogenesis. The newly made glucose is released back into the blood stream to raise blood glucose levels.

Metabolism summary diagram Figure: Metabolism is a complex and interlinked process. Gluconeogenesis and glycogenesis are important pathways in metabolism, to raise blood concentrations.

Glycogenolysis: release of glucose from stored glycogen

In the glycogenolysis pathway (see metabolism figure above), liver glycogen can be broken down to produce glucose which is released back into the blood stream to increase blood glucose concentrations. The combination of these processes enables us to maintain a low but significant level of glucose, despite fasting or even starvation (see figure below).  Release of glucose from stored glycogen chart over 40 days showing maintenance of a low but significant level of glucose despite fasting or starvation Figure: release of glucose from stored glycogen

So you can see from the diagram above that the blood glucose (black line) is kept constant by a combination of processes: release of glucose from the diet (red line), gluconeogenesis (blue) and glycogenolysis (green).

© University of Southampton
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Understanding Insulin

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