Lipid Digestion and Absorption: Nutritional Biochemistry Study Notes

Lipid Digestion

Fatty Acids

Fatty acids are essential biomolecules that serve as key energy sources and structural components in living organisms. Their origin in the human body can be traced to three main sources:

• Diet: Fatty acids are ingested through foods such as fats, oils, and nuts.

• Lipolysis: This is the breakdown of triglycerides stored in adipose tissue, releasing fatty acids for metabolic use.

• De Novo Synthesis: The body can synthesize fatty acids from acetyl-CoA via fatty acid synthesis pathways.

Additional info: Fatty acids vary in chain length and degree of saturation, which influences their metabolic fate and physical properties.

Challenges in Lipid Digestion

Lipids are hydrophobic molecules, making their digestion in the aqueous environment of the gastrointestinal tract challenging. The body overcomes this by emulsifying lipids with bile salts, which increases their accessibility to digestive enzymes.

• Emulsification: Bile salts break large lipid droplets into smaller micelles, increasing surface area for enzyme action.

• Factors Affecting Digestion: Chain length, saturation, and placement of fatty acids influence how efficiently they are digested.

Sites and Enzymes of Lipid Digestion

• Mouth: Lingual Lipase is secreted, especially important in neonates, and acts on pre-emulsified milk fats. Reaction: $\text{Triacylglycerol} + H_2O \rightarrow \text{Diacylglycerol} + \text{Fatty Acid}$

• Stomach: Minimal lipid digestion occurs. Mechanical churning creates a coarse emulsion, and fat slows gastric emptying.

• Small Intestine: The primary site for lipid digestion (about 90%). Hydrolysis and de-esterification of lipids occur here, facilitated by bile salts forming micelles.

Key Digestive Enzymes

• Pancreatic Lipase: Hydrolyzes triglycerides (TAGs) at the sn-1 and sn-3 positions, producing 2-monoacylglycerol (2-MAG) and free fatty acids (FFA). Reactions:

  • $\text{TAG} + H_2O \rightarrow \text{DAG} + \text{FFA}$

  • $\text{DAG} + H_2O \rightarrow \text{2-MAG} + \text{FFA}$

  Regulation: Inhibited by bile salts, enhanced by Ca2+ and colipase.

• Colipase: A protein secreted by the pancreas that binds lipase in a 1:1 ratio, anchoring it to bile acids on emulsified lipid droplets and facilitating lipase action.

• Phospholipase A2 (Lecithinase): Acts at the C1 and C2 positions of phospholipids (PL), releasing fatty acids. Reaction: $\text{Phospholipid} + H_2O \rightarrow \text{Lysophospholipid} + \text{FFA}$

• Cholesterol Esterase: Removes fatty acids from cholesterol esters at low pH (lumen) and adds fatty acids at higher pH (mucosa). Reaction: $\text{Cholesterol Ester} + H^+ \rightleftharpoons \text{Cholesterol} + \text{FFA}$

Hormonal Regulation

• Cholecystokinin (CCK): A peptide hormone (33 amino acids) produced in the duodenum and jejunum. It is stimulated by digestion products of fat and protein, causing contraction of the gallbladder (releasing bile) and stimulating pancreatic secretions, including bicarbonate.

Lipid Absorption

Micelle Formation and Absorption

After digestion, lipids must be absorbed into mucosal cells (enterocytes). Coarse emulsion particles are too large to be absorbed directly, so bile salts break them into tiny micelles (about 5 nm), which can diffuse between villi (50 nm apart).

• Bile Salts: Synthesized from cholesterol in the liver, stored in the gallbladder, and released into the small intestine upon stimulation by CCK.

• Derivatization: Most bile acids are conjugated with glycine or taurine, lowering their pKa and increasing solubility at the duodenal pH (5.8–6.5).

• Micelle Structure: Bile salts have hydrophobic and hydrophilic regions, allowing them to encapsulate lipids and facilitate their interaction with the aqueous environment and digestive enzymes.

Additional info: The critical micellar concentration (CMC) is the minimum concentration of bile salts required for micelle formation.

Role of Intestinal Bacteria

• In the colon, bacteria further modify bile acids by removing hydroxyl groups at the C-7 position, forming secondary bile acids. These are associated with increased risk for colonic cancer (mechanism unclear).

Transport Across Enterocytes

• Lipid breakdown products (FFA, MAG, cholesterol) diffuse into enterocytes.

• Inside enterocytes, lipids are re-esterified and packaged into chylomicrons for transport via the lymphatic system.

• Chylomicrons: Lipoprotein particles composed mainly of TAGs, cholesterol esters, and apoproteins, responsible for transporting dietary lipids from the intestine to other tissues.

Summary Table: Key Enzymes in Lipid Digestion

Enzyme	Substrate	Site of Action	Products	Regulation
Lingual Lipase	Milk fats (TAGs)	Mouth	DAG, FFA	Active in neonates
Pancreatic Lipase	TAGs	Small Intestine	2-MAG, FFA	Inhibited by bile salts, enhanced by Ca2+, colipase
Phospholipase A2	Phospholipids	Small Intestine	Lysophospholipid, FFA	Acts at C1, C2 positions
Cholesterol Esterase	Cholesterol esters	Small Intestine	Cholesterol, FFA	pH-dependent activity

Clinical Relevance

• Defects in bile salt production or secretion can impair lipid digestion and absorption, leading to malabsorption syndromes.

• Alterations in intestinal bacteria can affect bile acid metabolism and may influence disease risk.