Fat Metabolism

Describe the physiology and biochemistry of fat, carbohydrate and protein metabolism

Digestion

Triglycerides are the main constituent of body fat in animals and vegetables, and therefore in dietary fat. They consist of three fatty acid molecules joined by a glycerol molecule.

As fats are not water soluble, they tend to clump together in chyme and are hard to digest due to the low surface area:volume ratio. Emulsification speeds up the digestive process, and occurs via the action of:

• Bile salts
  Many bile salts have a hydrophobic and a hydrophilic end, which give a detergent action. Bile salts bound to fatty acids form a mixed micelle which can be further digested by enzymes or directly absorbed.
• Partially digested fats
• Mechanical action of the stomach

Once emulsified, triglycerides can be hydrolysed by lipase into fatty acids and monoacylglycerol.

Absorption

Absorption occurs in a number of stages:

• Mixed micelles, free fatty acids, monoacylglycerol, and cholesterols are absorbed via facilitated diffusion into the enterocyte
• From the enterocyte:
  • Short-chain fatty acids (those with < 12 carbon atoms) enter the portal vein and travel directly to the liver
  • Long-chain fatty acids are re-esterified and packaged with a layer of protein and cholesterol to form a chylomicron
    Re-esterification maintains the concentration gradient for diffusion of fatty acids, allowing further uptake to occur.
    • Chylomicrons are ejected from the cell into the lymphatics and travel to the systemic circulation
    • Chylomicrons are removed from circulation by lipoprotein lipase
      Lipoprotein lipase is found on capillary endothelium and bound to albumin.
    • Lipoprotein lipase breaks down triglyceride in chylomicrons and VLDL to free fatty acids and glycerol
      This reaction uses heparin as a cofactor.
    • Free fatty acids and glycerol are then free to enter adipose tissue

Storage

Fat is stored as triglycerides, and forms the bulk of energy storage of the body.

Triglycerides are synthesised by the liver:

• Occurs when insulin levels are high and glycogen stores are full
• From excess carbohydrate and amino acids
  These are converted to fatty acids and glycerol, and then esterified to form triglyceride. This is known as lipogenesis.

Metabolism

Free fatty acids can be absorbed by adipocytes for storage, or be β-oxidised to acetyl CoA in the liver, which can enter the citric acid cycle to produce ATP.

References

1. Chaney S. Overview of Lipid Metabolism. University of North Carolina School of Medicine. 2005.
2. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
3. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.