Metabolic fuels in the fed and fasting states

Energy expenditure is relatively constant throughout the day, but food intake normally occurs in two or three meals. There is therefore a need for metabolic regulation to ensure that there is a more or less constant supply of metabolic fuel to tissues, regardless of the variation in intake. See section 10.5 for a more detailed discussion of the hormonal control of metabolism in the fed and fasting states. 

The increased concentration of glucose and amino acids in the portal blood stimulates the P-cells of the pancreas to secrete insulin, and suppresses the secretion of glucagon by the a-cells of the pancreas. Insulin has four main actions  

In the liver, glucose uptake is by carrier-mediated diffusion and metabolic trapping as glucose 6-phosphate (section 3.2.2.2), and is independent of insulin. The uptake of glucose into the liver increases very significantly as the concentration of glucose in the portal vein increases, and the liver has a major role in controlling the amount of 

In the fasting state (sometimes known as the post-absorptive state, as it begins about 4-5 hours after a meal, when the products of digestion have been absorbed) metabolic fuels enter the circulation from the reserves of glycogen, triacylglycerol and protein 

---

What is not known is what signals hunger or satiety to these hypothalamic centres. It may be the relative concentrations of glucose, triacylglycerols, non-esterified fatty acids and ketone bodies available as metabolic fuels in the fed and fasting states (section... 

Figure 5.8 Plasma concentrations of metabolic fuels in the fed and fasting states and in starvation.

---