Organ Fuel Selection

1. Fuel requirements of the organs of which the body is composed vary with the demands on the organ whether this be metabolic, exercise, excretion or thinking.

2. The intestine is very active metabolically and activity increases after a meal. The resulting increased blood flow, uses some 20-25% of the body’s oxygen uptake. This high oxygen usage is accompanied by high protein synthesis rates and turnover of epithelial cells. Glutamine , glucose and ketones are taken up by intestinal tissues as important oxidative substrates. In the colon, short chain fatty acids especially butyric acid are important sources of fuel.

3. The liver is important in the metabolism and storage of nutrients, and is the first organ after the intestine to which absorbed nutrients are exposed.

4. The acinus is the anatomical unit of the liver and depends upon blood from the hepatic arterial and portal venous branches, drains into the hepatic vein and secretes into the lymph and bile duct. There is an oxygen gradient from zone 1 next to the portal triad to the periphery of the acinus adjacent to terminal hepatic veins (zone 3); this has a reduced oxygen supply and consequently is more vulnerable to injury, whether viral, toxic or anoxic.

5. The liver contains a multitude of enzymes, sited within the liver structure in a manner that reflects metabolic needs. The periportal zone mitochondrial enzymes predominantly catalyse glucose production, oxidative energy metabolism, amino acid utilization, urea formation, bile acid and haem metabolism. The pericentral zone is important in glucose uptake and glutamine metabolism. The excretory system of the liver for chemicals, e.g. bile acids, cholesterol, bilirubin glucuronide, hormones and drugs, is the bile canaliculi. Bile is also an important contributor to the digestion of fat in the duodenum. The liver is the main site of synthesis of plasma proteins including albumin, fibrinogen, prothrombin, other clotting factors and caeruloplasmin.

6. The brain uses glucose almost exclusively for its metabolism, over half of the body’s glucose utilisation is by the brain. The principle metabolism ( 90%) of glucose by the brain is to CO2 and water, the remainder passing through the pentose phosphate pathways or by glycolysis to lactate and pyruvate

7. Muscle consists of a variety of fibre types, type I and type II A and B. ATP utilisation and resynthesis is crucial to muscle activity. The principal substrates for this action are phosphocreatine, muscle glycogen and blood glucose, ketones and unesterified fatty acids.

8. At rest the principle fuel is non esterified fatty acids which continue to be important during exercise. With moderate exercise plasma glucose utilisation increases to 10-30%. During sustained exercise the dependence upon carbohydrates increases according to the amount of carbohydrate in the diet before the exercise and also the fitness of the person. The time interval after the last meal and the amount of glycogen stored in the muscles will dictate the relative uptake from exogenous and endogenous sources of fuel

9. The kidney weighs less than 0.5% of body weight and receives 20-25% of the cardiac output and uses 10% of O2 consumption. This high activity is to replenish ATP used during renal excretory activity. The commonest substrates are glucose and lactate, though other substrates, ketone bodies, free fatty acids, citrate, glutamine and glycerol are used in significant amounts.

10. The highly active skin tissue has no critical energy source.

11. During pregnancy the ovum grows into the foetus and the size of the supportive placenta is very important. The energy requirements alter with the stage of development of the foetus.

12. During lactation the mammary gland is a very active gland converting glucose triacylglycerols and amino acids into milk constituents for export from the mother,


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