Effect of Insulin on Fat Metabolism

Effect of Insulin on Fat Metabolism

Although not quite as visible as the acute effects of insulin on carbohydrate metabolism, insulin’s effects on fat metabolism are, in the long run, equally important. Especially dramatic is the long-term effect of insulin lack in causing extreme atherosclerosis, often leading to heart attacks, cerebral strokes, and other vascular accidents. But first, let us discuss the acute effects of insulin on fat metabolism.

Insulin Promotes Fat Synthesis and Storage

Insulin has several effects that lead to fat storage in adipose tissue. First, insulin increases the utilization of glucose by most of the body’s tissues, which automatically decreases the utilization of fat, thus functioning as a fat sparer.
However, insulin also promotes fatty acid synthesis. This is especially true when more carbohydrates are ingested than can be used for immediate energy, thus providing the substrate for fat synthesis.Almost all this synthesis occurs in the liver cells, and the fatty acids are then transported from the liver by way of the blood lipoproteins to the adipose cells to be stored.

 The different factors that lead to increased fatty acid synthesis in the liver include the following:

1. Insulin increases the transport of glucose into the liver cells.After the liver glycogen concentration reaches 5 to 6 per cent, this in itself inhibits further glycogen synthesis. Then all the additional glucose entering the liver cells becomes available to form fat. The glucose is first split to pyruvate in the glycolytic pathway, and the pyruvate subsequently is converted to acetyl coenzyme A (acetyl-CoA), the substrate from which fatty acids are synthesized.



2. An excess of citrate and isocitrate ions is formed by the citric acid cycle when excess amounts of glucose are being used for energy. These ions then have a direct effect in activating acetyl-CoA carboxylase, the enzyme required to carboxylate acetyl-CoA to form malonyl-CoA, the first stage of fatty acid synthesis.


3. Most of the fatty acids are then synthesized within the liver itself and used to form triglycerides,the usual form of storage fat. They are released from the liver cells to the blood in the lipoproteins.
   Insulin activates lipoprotein lipase in the capillary walls of the adipose tissue, which splits the triglycerides again into fatty acids, a requirement for them to be absorbed into the adipose cells, where they are again converted to triglycerides and stored.

Role of Insulin in Storage of Fat in the Adipose Cells.

 Insulin has two other essential effects that are required for fat storage in adipose cells:

1. Insulin inhibits the action of hormone-sensitive lipase. This is the enzyme that causes hydrolysis of
the triglycerides already stored in the fat cells.
Therefore, the release of fatty acids from the adipose tissue into the circulating blood is inhibited.

2. Insulin promotes glucose transport through the cell membrane into the fat cells in exactly the same
ways that it promotes glucose transport into muscle cells.
 Some of this glucose is then used to
synthesize minute amounts of fatty acids, but more important, it also forms large quantities of a-glycerol phosphate.
This substance supplies the glycerol that combines with fatty acids to form
the triglycerides that are the storage form of fat in adipose cells.
Therefore, when insulin is not available, even storage of the large amounts of fatty acids transported from the liver in the lipoproteins is almost blocked.
 
 References:
© Copyright Guyton and Hall Textbook of Medical Physiology

References:
© Copyright Guyton and Hall Textbook of Medical Physiology - See more at: http://how-to-get-rid-of-diabetes.blogspot.com/2014/07/insulin-promotes-glucose-metabolism.html#sthash.tKI5KMwU.dpuf