Lipolysis Pathway

Lipolysis pathway is the chemical degradation of fat, either the fat which is contained in our adipose cells in the form of triglycerides or the saturated fat contained in the food we eat. In the first case, it takes place when we burn fat, and we burn fat when we are fasting or when we work out hard or run for more than half and hour; in the second case, it begins in the duodenum, which is the first portion of the small intestine, when we digest the fat-containing food. In both cases, this chemical breakdown of fat put us in the metabolic state called ketosis, as long as we are not consuming carbohydrates, through ketogenesis, which is the synthesis of ketone bodies.

Lipolysis Pathway (Summary)

Lipase, which is an enzyme, is the initiator of decomposition of fat. When the glycogen store in our body has been depleted during fasting or work out, the pancreas α-cells secretes glucagon. Then this hormone travels in the bloodstream and induces the adipose cells to release the triglyceride molecules contained in their cytoplasm. As soon as they are released, lipase breaks down the triglyceride into one molecule of glycerol and three molecules of fatty acids, which are carried in the bloodstream to the liver.

Fatty acid makes their way into the liver hepatocyte cytoplasm, where it is further broken down into acyl CoA. In the mitochondria liver cells, acyl CoA is further converted into acetyl CoA through β-oxidation. Then Acetyl CoA gets into the Krebs cycle (cytric acid cycle) which takes place in the mitochondrion matrix. With the intervention of the enzyme hydroxy-metyl-glutaryl-CoA synthase, acetyl CoA is metabolically transformed into β-hydroxybutyrate, acetoacetate, and acetona, which are ketone bodies. In fasting or when we are on a fat-based diet, ketone bodies are used as fuel by most of our body cells as the levels of glucose in our bloodstream drop.

In fasting, the lower level of glucose is maintained through a process called gluconeogenesis, which is the production of glucose from glycerol and amino acids (proteins). In this case, we do not have glucose spikes, as when we eat carbohydrates, because this metabolic process is very slow.