Metabolism Distribution and Regulation

Once absorbed, carbohydrates are carried to the liver (A). There, fructose and galactose are converted into glucose. Some of the absorbed glucose reaches the peripheral bloodstream, where it is recognized by pancreatic receptors. This triggers increased insulin secretion by the p-cells and reduces glucagon secretion.

These hormonal changes provide a signal which affects the entire metabolism: absorption of glucose into the liver, muscle cells, and fatty tissues is increased, and its conversion into storage forms enhanced. Once inside the liver, glucose is phosphorylated into glucose-6-phosphate (G6P) and trapped since G6P is unable to pass through the membrane. G6P is then metabolized either into energy, into free fatty acids (FFAs) and triglycerides (TG), or into glycogen—as long as sufficient storage capacity remains. Due to its hydrated state, glycogen requires lots of space; therefore, the liver can store only about 100 g glucose in this form. Up to 0.5 kg glycogen can be stored in muscle. Hydrolyzed glycogen cannot leave the muscle cells to add to blood glucose and is therefore no longer available to the rest of the body.

Due to this limited storage capacity, all carbohydrates consumed beyond energy need are converted to and stored as fat. Fat storage is practically unlimited. Yet, the idea that mostly carbohydrates make people fat is wrong. Basically, all energy nutrients, consumed in excess of energy need, are converted to fat. Since carbohydrates first have to be metabolized for this purpose, their conversion is actually most ineffective. Some of the carbohydrate energy is lost in postprandial thermogenesis. This does not apply to fatty acids, which can be directly integrated into fat cell triglycerides. If any nutrient can be said to be fattening, it tends to be the lipids.

Towards the end of the absorptive phase or while fasting, when plasma glucose levels drop due to lack of new supplies and/or rapid use, hormonal regulation is reversed (B). Plasma insulin levels drop while glucagon secretion increases. Glucagon stimulates liver glycogen breakdown and increases the enzymatic activity required for the reversal of glycolysis (gluconeogenesis from amino acids). The lower plasma insulin causes breakdown of muscle glycogen, making glucose available to the muscles. Catecholamines released during stress or physical activity can have the same effect. Furthermore, increased glucagon levels make fatty acids available through triglyceride hydrolysis. Alternatively, this can be induced by the sympathetic nervous system.

- A. Distribution and Regulation (Post-absorptive) -

Glycogen

Muscle

- A. Distribution and Regulation (Post-absorptive) -

Glycogen

Adrenaline Triglycerides Hydrolysis

Muscle

Fatty tissues

Triglycerides

Fatty tissues

Triglycerides

B. Distribution and Regulation (Fasting or Starvation)

Blood cells

B. Distribution and Regulation (Fasting or Starvation)

Blood cells

Triglyceride Metabolism

Adrenaline (stress) Muscle

Triglycerides

Noradrenaline Fattytissues

(sympathetic nervous system)

Adrenaline (stress) Muscle

Triglycerides

Noradrenaline Fattytissues

(sympathetic nervous system)

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Responses

  • Arabella
    Which cell in kidney make triglyceride?
    5 years ago

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