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Metabolic Patterns of our Organs

Each of our organs has a unique metabolic profile!

 

The metabolic patterns of our brain, muscles, adipose tissue, kidney, and liver are strikingly different.

 

Brain
  1. Glucose is our first fuel of choice for the brain, except during prolonged “sugar” starvation.
  2. The brain does not store fuel and hence requires a continuous supply of glucose.
  3. In the state of prolonged “sugar” starvation, ketone bodies generated by the liver replace glucose as fuel for the brain.
  4. Most fatty acids do not serve as fuel for the brain because they are bound to albumin in plasma and so do not traverse the blood-brain barrier.
  5. Ketones are exceptional fatty acids: When somebody gets Alzheimer’s, their brain has trouble metabolizing glucose. Research shows that ketones cross the blood-brain barrier and are actually a twice efficient fuel than glucose which might help explain the cognitive benefits some Alzheimer’s patients report (since their brains no longer have to rely on a damaged glucose mechanism for fuel). Benefits for other well known neurological disorders such as Parkinson’s disease and Autism are also reported.
  6. Brain consumes about 420 kcal (1760 kJ) of energy daily (120 g of glucose).
  7. In the resting state, brain’s energy need accounts for some 60% of the of total energy utilized by the whole body.
  8. 60-70% of the energy needed by the brain is used to power transport mechanisms that maintain the Na+-K+ membrane potential (Sodium-Potassium balance) required for the transmission of the nerve impulses.
  9. Overall, glucose or fat metabolism in the brain remain unchanged during mental activity. Local increases are detected when a person performs certain mental tasks.
Muscle

The major fuels for muscle are

  • glucose,
  • fatty acids, and
  • ketone bodies.
  1. KETO muscular-tissueUnlike the brain, muscle has a large store of glycogen (1,200 kcal, or 5,000 kJ).
  2. About three quarters of all glycogen in the body is stored in muscle.
  3. Glycogen is converted into glucose 6-phosphate for use within muscle cells for bursts of activity.
  4. The metabolic patterns of actively contracting and resting skeletal muscle are different.
  5. The actively contracting skeletal muscles shift part of the metabolic burden to the liver in a series of interchanges known as the Lactic acid cycle (Cori cycle).
  6. In resting muscle, fatty acids are the major fuel, meeting 85% of the energy needs.
  7. Unlike skeletal muscle, the heart muscle has virtually no glycogen reserves. Heart muscle functions almost exclusively aerobically (requires oxygen). It has a high density of mitochondria, an organelle in which the biochemical processes of respiration and energy production occur.
  8. Fatty acids are the heart’s main source of fuel.
  9. Ketone bodies and lactate also serve as fuel for heart muscle.
  10. In fact, heart muscle consumes acetoacetate in preference to glucose.

 

Adipose tissue
  1. The triacylglycerols stored in adipose tissue (fat tissues) are an enormous reservoir of metabolic fuel.
  2. In a typical 155 lbs-man (70-kg), the 33 lbs (15 kg) of triacylglycerols have an energy content of 135,000 kcal (565,000 kJ).
  3. Adipose tissue is specialized for the esterification of fatty acids and for their release from triacylglycerols.
  4. The liver is the major site of fatty acid synthesis. It forms triacylglycerol from fatty acids which are transported to the adipose tissue in lipoprotein particles.
  5. After the fatty acids enter the fat cell, the adipose tissue activates these fatty acids.
  6. Triacylglycerols in adipose cells are continually being hydrolyzed and resynthesized.
  7. Adipose cells need glucose for the synthesis of triacylglycerols.
  8. Glycerol derived from their hydrolysis is exported to the liver.
  9. Most of the fatty acids formed on hydrolysis are reesterified if glycerol 3-phosphate is abundant.
  10. In contrast, they are released into the plasma if glycerol 3-phosphate is scarce because of a scarcity of glucose.
  11. Thus, the glucose level inside adipose cells is a major factor in determining whether fatty acids are released into the blood.

 

The kidney

KETO KidneyAnatomyThe major purpose of the kidney is to produce urine, which serves as a vehicle for excreting metabolic waste products and for maintaining the osmolarity of the body fluids.

  1. The blood plasma is filtered nearly 60 times each day in the kidney.
  2. Most of the filtered material out of the blood is reabsorbed; only 1 to 2 liters of urine is produced.
  3. Water-soluble materials in the plasma, such as glucose, and water itself are reabsorbed to prevent wasteful loss.
  4. The kidneys require large amounts of energy to accomplish the re-absorption.
  5. Kidneys constitute 0.5% of body mass, however consume 10% of the oxygen used in cellular respiration.
  6. Much of the glucose that is reabsorbed is carried into the kidney cells by the sodium-glucose co-transporter.
  7. During sugar starvation, the kidney becomes an important site of gluco-neo-genesis and contributes as much as half of the blood glucose.

 

Liver

The metabolic activities of the liver are essential for providing fuel to the brain, muscle, and other peripheral organs. Indeed, the liver, which can be from 2% to 4% of body weight, is an organism’s metabolic hub. Most compounds absorbed by the intestine first pass through the liver, which is thus able to regulate the level of many metabolites in the blood.

  1. The liver metabolizes carbohydrates by removing two-thirds of the glucose from the blood and all of the remaining monosaccharides. Some glucose is left in the blood for use by other tissues.
  2. In the liver, the absorbed glucose is converted into glucose 6-phosphate. The liver uses little of it to meet its own energy needs.
  3. The liver can store as much as 400 kcal (1700 kJ) in form of glucose 6-phosphate. Excess glucose 6-phosphate is metabolized to acetyl CoA (coenzyme adenine), which is used to form fatty acids, cholesterol, and bile salts.
  4. The liver can produce glucose for release into the blood by breaking down its store of glycogen (gluconeogenesis).
  5. The main precursors for gluconeogenesis are lactate and alanine from muscle, glycerol from adipose tissue, and glucogenic amino acids from the diet.
  6. The liver also plays a central role in the regulation of lipid metabolism. When fuels are abundant, fatty acids derived from the diet or synthesized by the liver are esterified and secreted into the blood in the form of very low density lipoprotein.
  7. In the fasting state, the liver converts fatty acids into ketone bodies.
  8. When fuels are scarce, fatty acids liberated from adipose tissues enter the mitochondrial matrix for conversion into ketone bodies.
  9. The liver also plays an essential role in dietary amino acid metabolism.
  10. The liver absorbs the majority of amino acids for protein synthesis, leaving some in the blood for peripheral tissues.
  11. The liver secretes between 20-30 grams of urea a day.
  12. The liver deliberately avoids using the fuels that it exports to muscle and the brain.
  13. α-Ketoacids derived from the degradation of amino acids are the liver’s own fuel.

 

 

 

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