Lipids are absorbed from the intestine and undergo digestion and metabolism before they can be utilized by the body. Most of the dietary lipids are fats and complex molecules that the body needs to break down in order to utilize and derive energy from.
Digestion of Lipids
Digestion of fats comprises of these major stages:-
- Emulsification of Fats
- Digestion of Fats
- Fat Metabolism
Absorption of lipids
Short-chain fatty acids (up to 12 carbons) are absorbed directly.
Triglycerides and dietary fats are insoluble in water and thus their absorption is difficult. To achieve this, the dietary fat is broken down into small particles that increases the exposed area for rapid attack by digestive enzymes.
Emulsification of Fats
Dietary fats undergo emulsification that leads to liberation of fatty acids. This is brought about by simple hydrolysis of the ester bonds in triglycerides.
Fats are broken down into small particles by detergent action and mechanical mixing. The detergent action is performed by digestive juices, but especially by partially digested fats (fatty acid soaps and monacylglycerols) and by bile salts.
The bile salts such as cholic acid contain a side that is hydrophobic (repellent to water) and another water loving or hydrophhillic side. This allows them to dissolve at an oil-water interface, with the hydrophobic surface in contact with the lipid to be absorbed and the hydrophilic surface in the watery medium. This is called the detergent action and this emulsifies fats and yields mixed micelles.
Mixed Micelles serve as transport vehicles for less water soluble lipids from food and also for cholesterol, fat-soluble vitamins A, D, E, and K.
Digestion of Fats
After emulsification the fats are hydrolyzed or broken down by enzymes secreted by the pancreas. The most important enzyme involved is pancreatic lipase. Pancreatic lipase breaks down primary ester linkages, the 1 or the 3 ester bonds. This converts triglycerides to 2-monoglycerides (2-monoacylglycerols). Less than 10% of triglycerides remain unhydrolyzed in the intestine.
Short chain fatty acids enter the circulation directly but most of the fatty acids are reesterified with glycerol in the intestines to form triglycerides that enter into the blood as lipoprotein particles called chylomicrons.
Lipoprotein lipase acts on these chylomicrons to form fatty acids. These may be stored as fat in adipose tissue, used for energy in any tissue with mitochondria using oxygen and reesterified to triglycerides in the liver and exported as lipoproteins called VLDL (very low density lipoproteins).
VLDL has a similar outcome as chylomicrons and eventually is converted to LDL (low density lipoproteins). Insulin simulates lipoprotein lipase.
During starvation for long periods of time the fatty acids can also be converted to ketone bodies in the liver. These ketone bodies can be used as an energy source by most cells that have mitochondria.
Fatty acids are broken down by Beta oxidation. This occurs in the mitochondria and/or in peroxisomes to generate acetyl-CoA. The process is the reverse of fatty acid synthesis: two-carbon fragments are removed from the carboxyl end of the acid. This occurs after dehydrogenation, hydration, and oxidation to form a beta-keto acid.
The acetyl-CoA then converts to ATP, CO2, and H2O using the citric acid cycle and releases energy of 106 ATP. Unsaturated fatty acids require additional enzymatic steps for degradation.