Digestion of Lipids Overview

Triacylglycerols are digested by lipases:
- Lingual lipase secreted by Ebner’s glands on the tongue, has minimal effect due to short food retention in the mouth.
- Gastric lipase optimum pH is 5.5, active mainly in infant stomachs (pH: 5) acting on milk fat.
- Pancreatic lipase is the most important lipase, removing fatty acids at carbon 1 and 3, yielding a mixture of 2-monoacylglycerol and free fatty acids.
2-monoacylglycerols can be:
- Absorbed directly (72%).
- Converted to 1-monoacylglycerols by isomerase enzyme.
  - 6% absorbed as 1-monoacylglycerol.
  - 22% hydrolyzed by pancreatic lipase into glycerol and fatty acids, which are then absorbed.
Steatorrhea is a condition of increased fat content in stool.
- Causes: Deficiency of any factor essential for lipid digestion or absorption, including:
  - Pancreatic lipase deficiency:
    - Undigested fat in stool.
    - No loss of fat-soluble vitamins.
  - Bile salts deficiency:
    - Digested fat present in stool.
    - Loss of fat-soluble vitamins.
  - Defective intestinal mucosa cells.

Tissue Lipids: Present in cell membranes and the brain, never oxidized for energy during starvation.
Depot Fat (Adipose Tissues):
- Sites: Under skin, breast, around kidney, in omentum & mesentry.
- Types:
  - White adipose tissue: Composed mainly of triacylglycerols (TAG).
  - Brown adipose tissue:
    - Brown due to high blood supply and mitochondria content.
    - Contains thermogenin (uncoupler protein), common in animals exposed to cold.
Functions of Depot Fat (Adipose Tissues):
- Energy source during starvation.
- Vitamin D3 synthesis (7-dehydrocholesterol under skin).
- Protection against cold.
- Protection of bony prominences.
- Kidney support.

Carried out by three enzymes in adipose tissue:
- Hormone-sensitive triacylglycerol lipase (HS lipase).
- Diacylglycerol lipase.
- Monoacylglycerol lipase.
Causes of Lipolysis:
- Starvation.
- Diabetes Mellitus.
- Low carbohydrate diet.
- Certain infectious diseases (tuberculosis due to high catabolic state).

Key enzyme: Hormone-sensitive triacylglycerol lipase (HS lipase).
HS lipase exists in two forms:
- Active form: Contains phosphate.
- Inactive form: Lacks phosphate.
Lipolytic factors:
- Lipolytic hormones (anti-insulin):
  - Epinephrine and norepinephrine.
  - Glucagon.
  - ACTH (adrenocorticotrophic hormone).
  - TSH (thyroid stimulating hormone).
  - Growth hormone.
- Methyl xanthine (caffeine).
- Sympathetic nervous system.
Anti-lipolytic factors:
- Insulin.
- Nicotinic acid and prostaglandin E.

Β-oxidation: Occurs in mitochondria of liver, not brain.
Fatty acid transport into mitochondria:
- Fatty acyl CoA synthetase (thiokinase): Converts fatty acid to acyl CoA in the cytosol.
- Carnitine shuttle: Carnitine carries fatty acids from cytoplasm to mitochondria.
  - Carnitine acyl transferase I: Transfers fatty acid from acyl CoA to carnitine, forming acyl carnitine.
  - Carnitine Acylcarnitine translocase: Exchanges fatty acyl-carnitine for carnitine across the mitochondrial membrane.
  - Carnitine acyl transferase II: Transfers fatty acid from carnitine to coenzyme A.
Energy production from β-oxidation:
- Palmitic acid (16 C): Net energy gain = 129 ATP.
- Calculation formula: (N/2 – 1) X 5 ATP) + (N/2 X 12 ATP) – 2 ATP (where N is the number of carbon atoms in the fatty acid).
Importance of β-oxidation:
- Energy production.
- Production of acetyl CoA.
- Ketone body formation.

Sources:
- Lipids: Oxidation of fatty acids and ketone bodies.
- Carbohydrates: Glucose oxidation → Pyruvate → Acetyl CoA.
- Proteins:
  - Ketogenic amino acids: Directly give acetyl CoA or indirectly give acetoacetate → Acetyl CoA.
  - Glucogenic amino acids: → Pyruvate → Acetyl CoA.
Fate:
- Oxidation: Through Krebs' (citric acid cycle).
- Lipogenesis: Formation of fatty acids and triacylglycerols.
- Ketogenesis: Synthesis of ketone bodies.
- Acetylcholine synthesis.
- Cholesterol synthesis: Precursor for:
  - Bile acids.
  - Vitamin D3.
  - Steroid hormones (glucocorticoids, mineralocorticoids, male sex hormones, female sex hormones).