As a first year MD1 student at WUHS, I found my first two weeks in Biochemistry with Dr. Mesumbe to be a fascinating and enlightening experience. As such, the fed/fasting states of the body were very interesting to me personally. And specifically the fasting state, which will be the topic of this discussion.
The fasting state begins around 3-4 hours after the absorptive state but certainly after 12 hours (post-absorptive states) and is characterized by the pancreas excreting glucagon after detecting a decrease in blood glucose and insulin levels, which is guided by two core objectives: 1) maintaining requisite glucose levels in the plasma to maintain energy levels for glucose-dependent tissues such as the brain and red blood cells, and 2) appropriating fatty acids from adipose tissue and the amalgamation and distribution of ketone bodies from the liver to supply energy to all other tissues like muscle and the kidneys.
As such, gluconeogenesis is the livers natural ability to subsidize the bodies need for glucose. The liver essentially breaks down proteins to amino acids that will be converted to keto acids and to supply acetyl coenzyme A, which is a requirement for ketone body synthesis. Clinically speaking, high levels of ketone bodies found in the blood or urine indicate a starved state.
Since ketone bodies are readily available, the brain will consume nearly 70% of of the available ketone bodies as a source of fuel to preserve glucose levels. Adipose tissue provides the necessary triacylglycerol’s to supply fatty acids and glycerol to the liver to maintain ketone body levels and muscle protein is degraded last to supply amino acids for gluconeogenesis to continue.
As fasting continues into the starvation state or beyond, the kidneys start playing a more active role in gluconeogenesis by providing precursor enzymes like glucose-6-phosphatase. The kidney also plays an important role after extended periods of fasting past the starvation state by excreting protons (NH¬4+) in the urine to compensate for the acidic state of the body due to the increased levels of ketone bodies.