Dietary fat (the lipids)Composition
In the lipids, more than 85-95% are triglycerides (Triacylglycerols). The other includes Cholesterol, Cholesteryl esters, phospholipids, essential unsaturated fatty acids, and fat-soluble vitamins, etc.
Facts about fat:
The lipids, present in the various cells of the human body, constantly undergo changes. They are continually being oxidized for energy, converted to other essential tissue constituents, or stored as reserve fat in adipose tissue. An adult takes about 60-150gm of lipids per day. Fat is energy-rich compounds and provides 9 kcal/ gm. Normally essentially all (98%) of the fat consumed is absorbed, and most are transported to the adipose for storage.
Dietary sources of Lipids:
Animal Sources: Dairy products- Milk, butter, ghee. Meat and Fish, Pork, eggs.
Vegetable Sources: Cooking oils- Sunflower oil, Mustard oil, Groundnut oil, etc. Fats from other vegetable sources.
Digestion in the mouth and stomach:
Digestion in the mouth:
The lingual lipase is secreted by the Ebner’s glands on the dorsal surface of the tongue. Its action continues in the stomach but action is negligible at the mouth.
Digestion in the stomach:
Lingual lipase from the mouth enters the stomach along with the food. It acts on short-chain triglycerides (present in milk, ghee, and butter; more significant in newborn infants).
Gastric lipase (acid-stable) secreted by chief cells, is present in gastric secretion stimulated by Gastrin. It hydrolyzes short and medium-chain (few to 12) fatty acids from dietary triacylglycerols. Up to 30 % digestion of triglycerides occurs in the stomach.
Significance of Lingual and Gastric Lipases:
- Play a significant role in lipid digestion in neonates since milk is the main source of energy.
- Significants of digestive enzymes in pancreatic insufficiency and pancreatic disorders.
- Lingual lipase & gastric lipase enzymes can degrade triglycerides with short and medium-chain fatty acids in patients with pancreatic disorders despite a partial or complete absence of pancreatic lipase.
Digestion of the lipids in the small intestine:
Emulsification of dietary lipid:
Emulsification is a prerequisite for the digestion of lipids which is achieved with the help of bile (bile salts). The lipids are dispersed into smaller droplets which increases the surface area.
Degradation of dietary lipids:
It is due to pancreatic enzymes. The pancreatic enzymes are primarily responsible for the degradation of dietary triacylglycerols, phospholipids, and cholesteryl esters. Secretion of pancreatic juice is stimulated by the acidic chyme which stimulates enteric hormones called secretin and cholecystokinin which causes a release of bile and the enzyme is pancreatic lipase.
Pancreatic colipase enzyme is released to help facilitate lipase enzyme activity. There are mainly three digestive enzymes (lipolytic enzymes) are present in pancreatic juice, they are Pancreatic lipase, cholesterol esterase, and Phospholipase A2 (Lecithinase).
Some other important lipases are:
- Lingual lipase.
- Gastric lipase.
- Pancreatic lipase.
- Intestinal lipase.
- Phospholipase A2.
- Lipoprotein lipase
- Hormone-sensitive lipase.
Gastrointestinal hormones:
- Secretin- Is a hormone that increases the secretion of electrolytes and fluid components of pancreatic juice in the body.
- Pancreozymin of CCK -PZ stimulates the secretion of the enzymes of the pancreas in the body.
- Cholecystokinin of CCK-PZ- that causes the contraction of the gall bladder and discharges the bile into the duodenum.
Bile Salts:
Bile salts are required for the proper functioning of the pancreatic lipase enzyme and help in a combination of lipase with two molecules of a small protein called Colipase. This combination enhances lipase activity. Bile salts are another most important molecule in the body that also helps in the emulsification of fats.
Enzymatic Digestion:
| Enzyme | Substrate | Action | Final Product |
| Cholesterol esterase | Cholesteryl ester | Hydrolysis to cholesterol compounds and fatty acids | Free cholesterol + Free fatty acids |
| Phospholipase A2 | Dietary Phospholipids | Hydrolysis of lysophospholipids | Free fatty acids + Lysolecithin |
| Lipase | Emulsified triacylglycerols | Hydrolysis to monoglycerides | Free fatty acids + 2-monoacylglycerol |
Absorption in the small intestine:
Absorption is the process of moving molecules across a cell membrane and into various cells. Folds, villi, microvilli expand the absorptive surface and most nutrients absorbed here.
The lipid Absorption:
Small lipid fragments such as glycerol & short-chain fatty acids do not need re-esterification. Small lipid fragments do not form micelles and it is absorbed directly into the bloodstream. From blood to the portal vein and finally to the liver where they are immediately utilized for energy. The absorption process is rapid.
Large lipid fragments or particles such as monoglycerides & long-chain fatty acids need re-esterification. Bile salts coat these and other lipids and form droplets called micelles. The lipid digestive products are phospholipids, cholesterol, monoacylglycerols, and fatty acids. These with bile salts form mixed micelles.
Micelles release their lipids, which diffuse openly across the plasma membrane. Resynthesis of triglycerides. Coating with protein and forming packages called chylomicrons.
The chylomicrons are too large to enter blood capillaries in the body and must be first transported in the lymphatic lacteal.
Fat absorption:
Mixed Micelles diffuse across the brush border of intestinal epithelial cells. The micelles merge there (diffusion). The monoglycerides built into triglycerides & covered with protein coats. Packaged into chylomicron. The chylomicrons are emptied into lymphatic capillaries, the lacteal à lymph circulation à thoracic duct ->blood à cells and liver.
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