Final answer:
After 24 hours of fasting, gluconeogenesis primarily uses amino acids from muscle proteins, glycerol from adipose tissue, and lactate. Glycogen stores are quickly depleted, and hormones like glucocorticoids and GH regulate this energy-generating process. If fasting continues, the body increasingly relies on ketone bodies for energy to conserve protein stores.
Step-by-step explanation:
After 24 hours of fasting, the primary source of energy for gluconeogenesis in the human body transitions from glycogen stores to other substrates. Initially, the liver's glycogen reserve is used to maintain blood glucose levels, but this is typically depleted within 24 hours of fasting. Following this, gluconeogenesis relies mainly on amino acids from muscle proteins, glycerol from triglycerides in adipose tissue, and lactate from anaerobic glycolysis. These substrates are converted into glucose in the liver and, to a lesser extent, in the kidneys. The process is regulated by hormones such as glucocorticoids, growth hormone (GH), adrenocorticotropic hormone (ACTH), and thyroid hormones.
As fasting continues and glycogen stores are depleted, the body increases fatty acid mobilization, which leads to the production of ketone bodies through ketogenesis. These ketone bodies act as an alternative energy source for many organs, reducing the reliance on gluconeogenesis to conserve protein stores. Ultimately, once the fat reserves are exhausted, protein catabolism increases significantly, with amino acids from muscle proteins becoming the principal substrates for gluconeogenesis.