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What happens to the fat we take in meals?

When we eat fat, it is digested and absorbed in our gastrointestinal tract. Some fat is not absorbed and excreted.

Fat is one of the three macronutrients of human food, along with carbohydrates and proteins.

Fat is a very good source of energy and essential for cell formation and vitamin absorption. It is vital for good health. However, an excess of fat is hazardous for our health.

Let us delve into the fate of fat in our body.

Digestion of fat: 

Lipids are large molecules, and unlike carbohydrates and proteins, they are not soluble in water and do not mix with the watery blood. So, they like to cluster together in large droplets in a liquid surrounding the gastrointestinal tract. The digestive process breaks those large droplets of fat into smaller droplets, and then enzymes digest lipid molecules using enzymes called lipase.

In the mouth: 

Chewing breaks food into smaller particles and mixes them with saliva. Lingual lipase begins some digestion of triglycerides, cleaving individual fatty acids and glycerol. Lingual lipase remains active in the stomach. 

In the stomach, 

In the stomach, mixing and churning movements help disperse the digestion of food and fat molecules. Gastric lipase also contributes to triglyceride digestion. Lingual lipase and gastric lipase contribute minimally to fat digestion. So, when food enters the small intestine, lipids are undigested and in large droplets. 

In the duodenum and small intestine

 When food enters the duodenum, bile salts, and pancreatic juice come into the duodenum through the relaxation of the sphincter of Oddi. Bile salts are amphipathic molecules with hydrophobic and hydrophilic regions, so they are attached to fats and water. This feature makes them effective emulsifiers, breaking large fat droplets into smaller droplets. So, it increases the surface area for digestive enzymes to act. The conjugated bile salts occupy the lipid/water interface and form micelles above the right concentration. The conjugated bile salts prevent passive reabsorption in the small intestine; thus, bile acids/ salts in the small intestine are high enough to form micelles and solubilize lipids. Micelles are the aggregation of bile salts, fatty acids, and monoglycerides. They are water-soluble. They can dissolve hydrophobic compounds in their interior and make them water-soluble.

Critical micelle concentration indicates both an intrinsic property of the bile acid and the amount of bile acid present necessary to function in micelles' spontaneous and dynamic formation. The pancreatic lipases present in the small intestine digest triglycerides. Triglycerides are broken down into fatty acids, some free glycerol, and monoglycerides (glycerol with one attached fatty acid). Cholesterol and fat-soluble vitamins are not digested.

Fatty acids,  glycerol, monoglycerides, and cholesterol are end products of fat digestion.

 

Absorption of fat

More than 95% of ingested fat is digested and absorbed. Bile salts aggregate around the products of fat digestion to form micelles. Bile acid-containing micelles help lipases digest lipids and bring them near the intestinal brush border membrane, which results in fat absorption.

The end products of fat digestion diffuse across the intestinal cell membrane, and bile salts are actively reabsorbed in the terminal ileum. The bile salts are recycled back into the 'entero-hepatic' circulation.

 

Once inside the enterocytes, monoglycerides and fatty acids are transported into the endoplasmic reticulum, where they are used to synthesize triglycerides. In the Golgi, triglyceride is packed with cholesterol, lipoprotein, and other lipids to form chylomicrons. Chylomicrons are a large structure with a core of triglycerides and cholesterol and an outer membrane of phospholipids interspersed with apolipoproteins and cholesterol. 

Chylomicrons contain protein, therefore, called esterified fatty acids. Chylomicrons are extruded from the Golgi into exocytotic vesicles, which are transported to the basolateral aspect of the enterocyte. The exocytic vesicles fuse with the plasma membrane and undergo exocytosis. The chylomicrons come into space outside the cells, enter the lymphatics, and enter the blood via the thoracic duct. 

Cholesterol is absorbed directly into the lymphatics. 

However, short-chain fatty acids pass directly from the enterocytes into the villus blood capillaries and are transported as free fatty acids (FFA), also called non-esterified fatty acids(NEFA). They are bound to albumin in the blood. 

Site of absorption: Mainly in the upper part of the small intestine, but also absorbed from the whole length of the small intestine.

In infants, the mechanism of fat absorption is not mature, so 10-15 % of ingested fat is excreted in feces. Lipid is significant in the infant diet. Breast milk has about 4% fat, almost to whole cow's milk. Fat provides approximately half of an infant's calories, essential for brain development.

Infants have some particular adaptations that allow them to digest fat. They have plenty of lingual and gastric lipases right from birth, which play a much more important role in infants than adults.

2. Brecontains lipase enzymes that are activated in the baby's small intestine.   

Steatorrhoea: In this condition, the stool is bulky, foul-smelling, pale, and greasy, which is due to an increase in fecal fat up to 50gm/day. Steatorrhoea is caused by improper digestion and /or absorption of fat.