From the Guidelines
After 24 hours of fasting, the primary metabolic process to maintain blood glucose is gluconeogenesis. This process occurs mainly in the liver, where non-carbohydrate substrates such as amino acids, lactate, and glycerol are converted into glucose 1. During prolonged fasting, glycogen stores (which typically last only 12-24 hours) become depleted, forcing the body to synthesize new glucose molecules. The liver uses amino acids derived from muscle protein breakdown, glycerol from adipose tissue lipolysis, and lactate from anaerobic metabolism to create glucose that can be released into the bloodstream.
Some key points to consider in this process include:
- The role of the kidney in endogenous glucose production, which becomes more significant during prolonged starvation periods 1
- The regulation of gluconeogenesis by hormones like glucagon and cortisol, which increase during fasting, while insulin levels decrease 1
- The importance of gluconeogenesis in maintaining adequate blood glucose levels for the brain and other glucose-dependent tissues during extended periods without food intake
The process of gluconeogenesis becomes increasingly important as fasting continues beyond 24 hours, working alongside ketogenesis to provide alternative fuel sources for the body 1. Overall, gluconeogenesis is the primary metabolic process that maintains blood glucose levels after 24 hours of fasting, and its regulation is crucial for ensuring the body's energy needs are met during prolonged periods without food intake.
From the Research
Metabolic Processes After 24 Hours Fasting
After 24 hours of fasting, the primary metabolic process to maintain blood glucose levels is:
- Gluconeogenesis: This process involves the generation of glucose from non-carbohydrate sources such as amino acids, lactate, and glycerol 2, 3.
- Glycogenolysis: Although glycogen stores are depleted after 24 hours of fasting, glycogenolysis still contributes to glucose production, albeit to a lesser extent 4, 3.
Key Findings
- Studies have shown that gluconeogenesis contributes more to glucose production in diabetic subjects than in control subjects during fasting 3.
- Fasting increases fat utilization and lowers the rate of muscle glycogen depletion, and blood glucose concentration is maintained at normal levels during exercise after fasting due to increased gluconeogenesis and decreased utilization of glucose in the muscle 2.
- The quantity of glucose contributed by gluconeogenesis declines during fasting, but it remains a crucial process for maintaining blood glucose levels 3.