Question

1. After glycogen reserves are depleted what are the major gluconeogenic precursors of glucose under the conditions of

A. starvation

B. intense exercise

Answer 1

(A). During Starvation: the body responds to reduced energy intake by burning fat reserves and consuming muscle and other tissues. Glycogen stores in the liver and skeletal muscle are replenished and, together with glucose absorbed from the gut, provide the major source of glucose for peripheral tissues for the next few hours. After prolonged periods of starvation, the body will utilize the proteins within muscle tissue as a fuel source. Glucose moieties in muscle glycogen can be used to provide energy for muscle cells, but cannot be liberated as free glucose in the blood for utilization by other tissues. People who usually practice fasting on a regular basis, or those who adhering to energy restricted diets, can prime their bodies to abstain from food while reducing the amount of muscle exhaustion.

(B) Glycogen plays a major role in supporting the energy demands of skeletal muscles during high intensity exercise. Despite its importance, the amount of glycogen stored in skeletal muscles is so small that a large fraction of it can be depleted in response to a single bout of high intensity exercise. Hence, it is generally recommended to have food after exercise to replenish rapidly muscle glycogen stores; otherwise one’s ability to engage in high intensity activity will be effected.

But what in case of glycogen reserves which are depleted after intense exercise? In such case skeletal muscles have the capacity to replenish some of their glycogen at the expense of endogenous carbon sources such as lactate. This is done by the transient dephosphorylation-mediated activation of glycogen synthase and inhibition of glycogen phosphorylase. The muscle glycogen synthesis occurs by an increased oxidation of lactate post-exercise, such as during active recovery from high intensity exercise. During active recovery glycogen resynthesis is impaired in skeletal muscle as a whole because of increased lactate oxidation, muscle glycogen stores are replenished in Type IIa and IIb fibers while being broken down in Type I fibers of active muscles.