Lipids - Fat Metabolism

Question 1

Synthesis and Storage of Triglycerides

Answer 1

• The body synthesises triglycerides whenever caloric intake exceeds energy requirements.
• Excess dietary energy is converted to triglycerides via lipogenesis, and excess ingested fat is taken up by adipose tissue.
• Lipogenesis takes place in adipose tissue and the liver.
• Adipocytes are supplied by an extensive network of blood vessels.
• They acquire TGs from circulating lipoproteins, chylomicrons and very low density lipoproteins (VLDLs).
  lipo = fat, genesis= formation

Question 2

Lipogenesis

Answer 2

• Lipogenesis is the process through which acetyl-CoA is converted to triglycerides for storage in fat.
• Fatty acids are synthesised when there is an excess of carbohydrates.
• Acetyl-CoA, created from glucose during glycolysis, as well as from fats and amino acids, is built up with the addition of 2-carbon units to form palmitic acid (C16).
• Three fatty acids are bound to glycerol and stored as triglycerides.
• The sites of fatty acid synthesis are the liver, adipocytes, kidneys and lactating mammary glands.
  acetyl-CoA = a component of cell respiration that adds acetyl groups to reactions

Question 3

Lipolysis

Answer 3

• When dietary energy is limited, the fatty acids from triglycerides are mobilised from adipocytes into circulation.
• Triglycerides are hydrolysed by lipase into fatty acids and glycerol for use in the body.
• Lipolysis is stimulated by:
  o Adrenaline, noradrenaline.
  o Adrenocorticotropic hormone (ACTH).
  o Glucagon and growth hormone.
  o Thyroid-stimulating hormone (TSH) and thyroxine.
• Insulin antagonizes the lipolytic effects of these hormones. As a result, insulin resistance (e.g. Type 2 diabetes) = central adiposity
  lipo = fat, lysis= breakdown

Question 4

Fatty Acid Catabolism

Answer 4

• Fatty acids can be broken down to produce energy.
• Fatty acids cross the cell membrane, traverse the cytosol and reach the mitochondria.
• Carnitine facilitates the transport of fatty acids across the mitochondrial membrane.
• The fatty acids undergo beta-oxidation and are broken down into 2-carbon blocks as acetyl-CoA, which is oxidised via the Krebs cycle to CO2 and H2O.
• Energy is then generated using the electron transport chain.
  catabolism = breakdown

Question 5

Ketone synthesis

Answer 5

When carbohydrate levels are low, fat becomes the primary fuel for energy production. Ketone synthesis becomes necessary because the brain cannot metabolise fatty acids.
* Ketones are made when glucose is in short supply. This occurs overnight, and during dieting or fasting.
* By a process known as ketogenesis, acetyl-CoA is converted to the ketones acetoacetate orβ-hydroxybutyrate(β-OHB).
* Acetoacetate can undergo decarboxylation to another ketone acetone.
* Acetone build-up gives a characteristic sweet smell to the breath.

Question 6

Ketosis

Answer 6

Low carbohydrate diets initiate a fundamental shift in the body’s primary fuel source from glucose to fat.
* This allows energy needs to be met by utilising fat (fatty acids or by ketone bodies).
* For most adults, this happens when carbohydrates are restricted to less than around 40 g a day.
* Ketosis is linked with health benefits including weight loss, and the management of epilepsy, Parkinson’s and Alzheimer’s disease.
* Nutritional ketosis is different from ketoacidosis —an unstable and dangerous condition that occurs when there is insufficient pancreatic insulin response to regulate serum β-OHB.
decarboxylation = removal of a carboxyl group