Specialty: Pediatrics
Discuss  the  physiological  principles by which fats are absorbed from alimentary tract
Rationale: Introduction:
Fats, proteins and complex carbohydrates (CH) can not be taken up from the gut in the form that these nutrients appear in the diet.  A process of breakdown of food starts in the mouth by mastication followed by the digesting processes.  Different nutrients are broken down into small absorbable units, which can be absorbed from the intestine.  Digestion starts in the mouth, is carried further in the stomach and is completed in the small intestine.  Absorption of nutrients mainly take place from the upper ½ of the small intestines.

Digestion and absorption of fat:

By far the most common fats in the diet are the neutral fats (triglycerides), each molecule contains a glycerol nucleus and three fatty acids.  The rest of the fats are made up of phospholipids, cholesterol and cholesterol esters, (cholesterol is a sterol, thus not a fat, but has the characteristics of a fat, is derived from fats, and metabolised like fat).

Pancreatic lipase is the most important enzyme involved in the digestion of fats.  Lipase is also secreted in the mouth and the stomach, the lipase from the Ebner glands on the dorsal surface of the tongue digests a substantial amount of the triglycerides, while the gastric lipase is normally of little value in the digestion of fat.

Emulsification of fat, very important in the digestion of fat, begins with the mechanical action within the stomach, and is fully accomplished in the duodenum by the detergent action of the bile salts.  In the emulsification process the fat globules are broken down into smaller units, allowing water-soluble enzymes to work on the fat globule surface.  The bile salts’ carboxyl end which is highly water-soluble protrudes from the surface of the fat globule, while the sterol end, which is fat soluble, is directed down into the fat globule.

Three enzymes from the pancreas take part in the digestion of fats, these being; lipase, carboxyl-ester-hydrolase and phospholipase A2.  Lipase hydrolyses triglycerides and long chain fatty acids to di- and monoglycerides plus free fatty acids and a small amount of glycerol, all these products are insoluble in water.  Phospholipase A2 is secreted as an inactive proenzyme and has to activated by trypsin to the active enzyme that will hydrolyse glycerol-phosphates.

Fatty acids and monoglycerides are not soluble in water, and as such not absorbed into the enterocytes.  By binding of bile salts to fatty acids and monoglycerides, aggregates of micelles are formed.  The micelles are absorbed into the enterocytes together with glycerol.  It was always thought that the uptake of micelles were by passive diffusion, but there is now some evidence that carrier substances are involved in their transport.  The products of fat digestion get absorbed from the first part of the jejunum, while bile salts only gets absorbed in the distal part of the ileum.  After the absorption of monoglycerides and fatty acids into the enterocytes, these substances are then used to form triglycerides in the membrane of the smooth endoplasmic reticulim.  The newly formed triglycerides combine with cholesterol, phospholipids and protein to form chylomicrons, which gets released from the enterocytes into the lymphatic system.  Fatty acids with less than 10 – 12 carbon atoms pass directly from the enterocytes into the portal system, while those with more than 12 carbon atoms have to be changed back into triglycerides and pass ianto the lymphatic system as part of chylomicrons.

Short and medium triglycerides (MCT) form only a small part of the fats in a normal diet, and their absorption is different from that of the triglycerides:

i)	MCT are taken up directly into the enterocytes, so bile salts are not needed for absorption.
ii)	MCT is in part absorbed as triglycerides, so digestion by lipase is not needed.
iii)	MCT are mainly transported in the portal vein and not the lymphatic system.

On a moderate fat intake, 95% or more of the fat ingested fets absorbed from the intestinal tract.  Fat-soluble vitamins are handled in a manner similar to that of other fats