The postabsorptive state commences when the last nutrient is absorbed from the previous meal and continues until the next meal or for approximately 12 h during a normal overnight fast. Metabolically, it is the period when there is a transition from exogenous energy consumption to reliance on endogenous energy sources. The release from the liver of approximately 200-250 g of glucose per day or 8-10 g per hour balances the rate of glucose utilization of the brain and other tissues. During an overnight fast, a significant proportion of the glucose requirements are met by the breakdown of liver glycogen. The remaining glucose is formed from noncarbohydrate sources: glycerol (from triacylgyl-cerols), pyruvate, and lactate (from muscle).
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