ABSTRACT
The brain cannot tolerate low blood glucose for even short periods and so the first priority of metabolism in starvation is to provide sufficient glucose to both the brain and red blood cells, both of which are absolutely dependent on this fuel source. As blood glucose levels begin to fall, insulin secretion is reduced and its counter-regulatory hormone, glucagon, is secreted, leading to mobilisation of triacylglycerols in fat and gluconeogenesis by the liver. Concentrations of acetyl-CoA and citrate rise, which reduces glycolysis. Muscle uptake of glucose reduces secondary to the lack of insulin and, therefore, muscle also shifts to using fatty acid as fuel. Ketones become the major fuel source for the brain. The effective conversion of fatty acids into ketones by the liver and their subsequent use by the brain markedly diminishes the need for glucose and, therefore, reduces muscle breakdown.
