Digestion of Dietary Lipids

Study Notes

Adults consume approximately 60-150 grams of lipids per day, with over 90% composed of triacylglycerol (TAG).
The remaining lipids include cholesterol, cholesteryl esters, phospholipids, and free fatty acids (FFAs).

Lingual Lipase: Secreted by Ebner's glands on the tongue.
- Plays a minimal role in digestion due to the short time food remains in the mouth.
Gastric Lipase: Optimal pH is 4-6, rendering it inactive in the adult stomach due to high acidity.
- May be significant in infants' stomachs, acting on mother's milk fat containing short or medium-chain fatty acids.
Pancreatic Lipase: Secreted into the intestine, it prefers TAG with long-chain FAs.
- Most important lipase for TAG digestion.
- Digests the primary ester bonds (positions 1 and 3), hydrolyzing them into two fatty acids and 2-monoacylglycerol.
- Requires emulsifying agents like bile salts and phospholipids for optimal action.
- Emulsification breaks down large globules into smaller ones, increasing the surface area exposed to the enzyme.
Intestinal Lipase: Acts on 1-monoacylglycerols converting them into glycerol and FFAs.
- End products of triacylglycerol digestion include:
  - 2-monoacylglycerols (72%)
  - 1-monoacylglycerols (6%)
  - Glycerol and free fatty acids (22%)

Cholesterol itself is not digested and absorbed as is.
Cholesterol esters are digested by cholesterol esterase secreted in pancreatic juice, yielding cholesterol and free fatty acids.

Phospholipids may be absorbed as such or digested by phospholipase enzymes (A1, A2 (B), C and D).
These enzymes hydrolyze phospholipids into FFAs, glycerol, phosphate, and a nitrogenous base (e.g., ethanolamine, choline, serine, inositol).

Short-chain fatty acids and glycerol are absorbed at the intestinal lining and pass through the portal circulation to the liver.
Long-chain fatty acids are re-esterified into triglycerides and combined with protein to form chylomicrons, which travel in the lymph.
Cholesterol is absorbed in its free form; it is then re-esterified in the intestinal mucosa and passes along with triglycerides in chylomicrons to the lymph vessels and systemic circulation. This results in a milky appearance of serum after fatty meals.
Phospholipids are resynthesized in the intestinal mucosa and become part of chylomicrons.

Glycerol and FFAs are taken up by various tissues for the following functions:
- Formation of depot fats.
- Oxidation for energy production.
- Synthesis of biologically active compounds.

Steatorrhoea: Condition characterized by increased fat content in stool (> 5 grams/day).
- Caused by deficiency of factors essential for lipid digestion or absorption, such as pancreatic lipase, bile salts, or a healthy intestinal mucosa.