Digestion, Absorption And Utilization Of Lipids

DIGESTION ABSORPTION AND UTILIZATION

Digestion

In the mouth:

Fat digestion starts in the mouth with hard fats beginning to melt when they reach body temperature. The salivary glands at the base of the tongue release a lipase enzyme which digest fat to a less extent in adults.

In the stomach:

In the stomach fat floats as a layer above the others components of swallowed food. As a result little fat digestion takes place.

In the small Intestine:

When fat enters the small intestine, the hormone

cholescystokinin signals the gall bladder to release bile. Bile emulsifies fat and also provides an alkaline medium for the action of pancreatic lipase and intestinal lipase. The triglycerides are acted upon by these lipases and hydrolyzed to monolycerides and fatty acids.

The cholesterol esters are hydrolyzed to give cholesterol and fatty acids.

Triglycerides -> Monoglyceride + fatty acids

Cholesterol esters -> Cholesterol + fatty acids

Absorption and Utilisation

Small molecules of digested triglycerides (glycerol, short & medium chain fatty acids) can diffuse into intestinal cells and are absorbed directly into the blood stream.

Larger molecules(Monoglycerides, long chain fatty acids) merge into spherical complexes known as miscelles. The lipid contents of the miscelles diffuse into the intestinal cells. Once inside the monoglycerides and long chain fatty acids are reassembled to new triglycerides.

Within the intestinal cells the new triglycerides and larger lipids like cholesterol and phospholipids are placed into transport vehicle called chylomicrons.

The intestinal cells then release chylomicrons into the lymphatic system. The lymph circulation empties into the thoracic duct which inturn enter the subclavian vein and subsequently into the blood stream.

The blood transport lipids to the rest of the body and cells absorb them and utilize for energy. This breakdown of fat to yield energy is called lipolysis.

Majority of lipids enter via the lymph to the liver where the protein and lipid (cholesterol, triglycerides) are bound together to form lipoproteins.

There are four types of lipoproteins, they are:

1.     chylomicrons,

2.     very low density lipo protein (VLDL)

3.     low density lipo protein (LDL) and

4.     high density lipo proteins (HDL).

Chylomicrons, VLDL and LDL serve to transport and deposit lipids from the intestine and liver to the tissues for absorption. Low-density lipoprotein, which has the highest cholesterol fraction favours lipid deposition in tissues including blood vessels and hence termed ' bad' cholesterol. HDL cholesterol removes the lipids from the tissues and transports it back to liver for disposal, hence it is termed as ' good cholesterol' . High levels of LDL cholesterol indicates a high risk of cardiovascular disease.

Apart from lipoproteins, triglycerides, cholesterol and phospholipids are synthesized in the liver. This is called lipogenesis.