Tissue Specific Energy Metabolism

The brain depends almost exclusively on glucose for energy. Since it cannot store compounds for oxidation, it has to receive a constant supply of glucose. To make this possible, a minimum blood glucose level has to be maintained at all times (A). The brain uses about 120 g of glucose per day; during phases of prolonged fasting or starvation it can use ketone bodies instead, but only to a limited extent.

Muscle tissue, on the contrary, possesses large glycogen stores. When broken down, it is converted into glucose-6-phosphate, which cannot be further hydrolyzed. It is metabolized exclusively via the glycolytic pathway and cannot pass into the blood as blood glucose. Particularly in cases of sudden spurts of activity, glucose is the predominant energy supply molecule for muscle cells. Under anaerobic conditions, lactate forms, which is then released into the bloodstream. Triglycerides stored in fatty tissue are the most important form of stored energy for humans. To esterify fatty acids, the fatty tissues need activated glycerol. The enzyme glycerokinase, however, which ist required to make activated glycerol, is not available in fatty tissue. The glycerol released during ongoing triglyceride hydrolysis cannot be used to make new fat. Instead, to make new fat, activated glycerol must be provided by glycolysis. Fat synthesis in cells is, therefore, possible only if there is sufficient glucose available—a fact used by many so-called "diets." The liver is the body's metabolic control center. It can take up large amounts of glucose, store it as glycogen, and make it available to stabilize blood glucose levels. As long as there is a sufficient supply of energy nutrients, the liver also synthesizes fatty acids, esterifies them into lipids, and sends them off to the peripheral tissues as lipoproteins. During starvation or prolonged fasting, however, the liver increasingly converts fatty acids into ketone bodies (B). The liver starts to synthesize them as soon as the supply of acetyl-CoA falls below levels that can be processed in the citrate cycle. These ketone bodies are used as a source of energy in all tissues except the liver itself. At the same time, amino acids resulting from protein breakdown are used for gluconeogene-sis in order to maintain the necessary minimum supply of glucose. Theoretically, human fat stores would supply enough energy for two months. However, only about 3 kg of protein can be mobilized, covering the nervous system's glucose needs for about 15 days. Only due to the flexible nature of nerve cells is fasting beyond that duration possible (C). Nerve cells can drastically reduce their glucose use during prolonged fasts, making up for the resulting energy deficit by using ketone bodies. Thus, fewer amino acids are needed for gluconeogenesis, protecting the protein in lean body tissue. This mechanism enables humans to survive several weeks of fasting or starvation .

Tissue-Specific Energy Metabolism

I- A. Fuel Reserves of a 70 kg (154 lb) man (kcal)

Blood

Liver

Brain

Muscle

Fatty tissues

Glucose or glycogen

60kcal

390kcal

8 kcal

1 1200 kcal

80 kcal

Triglycerides

45 kcal

450kcal

0 kcal

450kcal

135 000kcal

Proteins available as fuel

0kcal

390kcal

0 kcal

1 24 000kcal

37kcal

- B. Fuel Metabolism in the Liver During Prolonged Fasting or Starvation -

Urea transported to kidneys and

- B. Fuel Metabolism in the Liver During Prolonged Fasting or Starvation -

Urea transported to kidneys and

Energy Metabolism During Fasting

C. Fuel Metabolism During Fasting or Starvation

Fuel used by brain

Glucose produced by gluconeogenesls

C. Fuel Metabolism During Fasting or Starvation

Fuel used by brain

Glucose produced by gluconeogenesls

Glucose Ketone bodies Breakdown of Glucose promuscle protein duced in liver

Amount formed or used in 44 hours (g)

Fasting Glucose Ketones
Days of fasting
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Responses

  • hannu mieto
    How cell produce energy during fasting?
    6 years ago
  • Christian
    Which tissue cannot store glucose?
    5 years ago

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