A 45-yrs-old man undergoes a gastrectomy for recurrent peptic ulcer disease. He has a history of excessive alcohol ingestion. Which nutritional substrate is depleted earliest?
|A||Branched – chain amino acids|
|B||Non – branched-chain amino acids|
a. In the healthy adult, principal sources of fuel during short-term fasting (<5 days) are derived from muscle protein and body fat, with fat being the most abundant source of energy.
b. The normal adult body contains 300 to 400 g of carbohydrates in the form of glycogen, of which 75 to 100 g are stored in the liver. Approximately 200 to 250 g of glycogen are stored within skeletal, cardiac, and smooth muscle cells.
c. The greater glycogen stores within the muscle are not readily available for systemic use due to a deficiency in glucose-6-phosphatase, but are available for the energy needs of muscle cells.
d. Therefore, in the fasting state, hepatic glycogen stores are rapidly and preferentially depleted, resulting in a fall of serum glucose concentration within hours (<16 hours).
f. While glucagon, epinephrine, and cortisol directly promote gluconeogenesis, epinephrine and cortisol also promote pyruvate shuttling to the liver for gluconeogenesis.
g. Precursors for hepatic gluconeogenesis include lactate, glycerol, and amino acids such as alanine and glutamine.
h. Lactate is released by glycolysis within skeletal muscles, as well as by erythrocytes and leukocytes.
i. The recycling of lactate and pyruvate for gluconeogenesis is commonly referred to as the Cori cycle, which can provide up to 40% of plasma glucose during starvation
j. Lactate production from skeletal muscle is insufficient to maintain systemic glucose needs during short-term fasting (simple starvation). Therefore, significant amounts of protein must be degraded daily (75 g/d for a 70-kg adult) to provide the amino acid substrate for hepatic gluconeogenesis.
k. Proteolysis during starvation, which results primarily from decreased insulin and increased cortisol release, is associated with elevated urinary nitrogen excretion from the normal 7 to 10 g per day up to 30 g or more per day.