Lipids constitute about 15-20% of the body weight. Lipids are a major source of energy for our body. Lipid metabolism is concerned mainly with the fatty acid and cholesterol. The main source of fatty acids is dietary lipids.
In the first phase of fatty acid metabolism, fatty acids are oxidized to acetyl CoA by the process of β-oxidation or esterified with glycerol, forming triglyceride, stored in the adipose tissue, serve as the body’s principal fuel reserve.
In the second phase of fatty acid metabolism, acetyl CoA synthesized by the process of β-oxidation may undergo the following fates;
(1) Acetyl CoA is oxidized to carbon dioxide via the citric acid cycle and produces large amount of energy.
(2) It is the precursor for the synthesis of cholesterol and other steroids.
(3) Acetyl CoA is used to synthesis the ketone bodies in the liver that are important fuels in prolonged fasting state.
As a whole, lipid metabolism can be divided into two categories:
A) Catabolic pathway of lipid metabolism:
- Beta-oxidation of fatty acid – Fatty acids are oxidized to acetyl CoA by the process of β-oxidation.
- Hydrolysis of triglyceride – Triglyceride is hydrolysis into glycerol and free fatty acid.
A) Anabolic pathway of lipid metabolism:
- Lipogenesis (Synthesis of fatty acid) – fatty acids may be synthesized from acetyl CoA derived from glucose or amino acids metabolism by the process of lipogenesis.
- Ketogenesis (Formation of ketone bodies) – Acetyl CoA arise from β-oxidation of fatty acid is used to synthesis of ketone bodies in the liver by the process of ketogenesis.
- Cholesterol synthesis – Cholesterol is synthesized from acetyl CoA arises from β-oxidation of fatty acid.
- Triglyceride synthesis – Triglyceride is synthesized by the esterificaton of free fatty acids with glycerol, which is stored in the adipose tissue.
- Steroid synthesis – Steroid is synthesized from acetyl CoA arises from β-oxidation of fatty acid.