Metabolic States

Absorptive State

‘Fed’ state.

Occurs after eating when digesting food. Lasts ~4 hrs.

Amino acids & glucose go to the bloodstream. Fats go to lymphatic system.

Effects:

• Food digestion results in a rise in blood glucose

  • Absorption of glucose (breakdown product of carbohydrates) into the bloodstream

• Presence of elevated blood glucose & parasympathetic activity triggers insulin release

  • β cells (beta) from pancreas release insulin

• Stress can inhibit parasympathetic activity, and in turn inhibit insulin release. This is how conditions such as diabetes can develop.

Insulin

• Released into the bloodstream

• Stimulates liver hepatocytes, muscle cells & adipose cells to absorb glucose from the bloodstream.

  • Once glucose is absorbed, glycolysis occurs immediately to ensure a concentration gradient is maintained.

  • Higher glucose concentration in bloodstream

  • Lower concentration inside cells

• Hepatocytes & muscle cells convert glucose to glycogen (glycogen synthesis)

• Inhibits liver gluconeogenesis

  • Prevents creation of more glucose

• Stimulates adipose cells to store excess lipids as blood glucose can provide adequate energy to the body

Absorptive State

Post-absorptive State

‘Fasting’ State

After digestion, absorption & storage of nutrients has occured.

Occurs often overnight when sleeping or skipping meals (>8 hrs)

Effects:

• Blood glucose levels drop

  • Breakdown of glycogen polymer to release glucose monomers into the bloodstream

• Presence of decreased blood glucose triggers glucagon release

  • α cells (alpha) from pancreas release glucagon

Glucagon

• Inhibits the synthesis of glycogen

• Stimulates breakdown of glycogen to glucose which is released into the bloodstream to be used by tissues, such as in the brain

  • Inhibits glycolysis to flip the concentration gradient

  • Higher glucose concentration inside cells

  • Lower concentration in bloodstream

  • ensures glucose is always flowing into the blood, to prevent blood glucose levels reaching 0.

    • Ensures fuel is available for cells at all times.

• Stimulates gluconeogenesis to create even more glucose in the bloodstream

• Triggers release of lipids by adipose cells

  • Lipids can be used to create glucose, ketone bodies & ATP

Post-absorptive State

Metabolism states

Switch between absorptive & post-absorptive states

Absorptive summary

After feeding, blood glucose levels rise, the pancreas releases insulin to stimulate the uptake of glucose by liver, muscle cells, and fat cells, to promote its conversion to glycogen.

Post-absorptive summary

As the post-absorptive state begins, falling glucose levels trigger the pancreas to release glucagon to turn off glycogen synthesis in the liver and stimulate its breakdown into glucose. The glucose is released into the bloodstream to serve as a fuel source for cells throughout the body.