Optimum nutrition is considered by bodybuilders to be important – both for muscle gain, and for fat loss.
In the context of fat loss, here’s what I found in the book
‘Textbook of Medical Physiology’, Eleventh Edition.
The authors are
Guyton & Hall
Since the book is now into its 13th edition, it must be quite a bestseller. In fact, Amazon notes that it’s
the world’s foremost medical physiology textbook.
http://www.amazon.com/Guyton-Hall-Te.../dp/1455770051
The following extracts suggest that decreasing the consumption of carbohydrates is essential – especially for fat loss.
Page 961-962
Insulin Is a Hormone Associated with Energy Abundance
As we discuss insulin in the next few pages, it will become apparent that insulin secretion is associated with energy abundance. That is, when there is great abundance of energy-giving foods in the diet, especially excess amounts of carbohydrates, insulin is secreted in great quantity.
In turn, the insulin plays an important role in storing the excess energy.
In the case of excess carbohydrates, it causes them to be stored as glycogen mainly in the liver and muscles.
Also, all the excess carbohydrates that cannot be stored as glycogen are converted under the stimulus of insulin into fats and stored in the adipose tissue.
In the case of proteins, insulin has a direct effect in promoting amino acid uptake by cells and conversion of these amino acids into protein.
In addition, it inhibits the breakdown of the proteins that are already in the cells.
Page 965
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.
…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.
Page 966
Insulin Deficiency Increases Use of Fat for Energy
All aspects of fat breakdown and use for providing energy are greatly enhanced in the absence of insulin…
This occurs even normally between meals when secretion of insulin is minimal, but it becomes extreme in diabetes mellitus when secretion of insulin is almost zero…
Insulin Deficiency Causes Lipolysis of Storage Fat and Release of Free Fatty Acids. In the absence of insulin, all the effects of insulin noted earlier that cause storage of fat are reversed. The most important effect is that the enzyme
hormone-sensitive lipase in the fat cells becomes strongly activated. This causes hydrolysis of the stored triglycerides, releasing large quantities of fatty acids and glycerol into the circulating blood.
Consequently, the plasma concentration of free fatty acids begins to rise within minutes. This free fatty acid then becomes the main energy substrate used by essentially all tissues of the body besides the brain.
Page 969-970
Role of Insulin (and Other Hormones) in “Switching” Between Carbohydrate and Lipid Metabolism
From the preceding discussions, it should be clear that insulin promotes the utilization of carbohydrates for energy, whereas it depresses the utilization of fats.
Conversely, lack of insulin causes fat utilization mainly to the exclusion of glucose utilization, except by brain tissue.
Furthermore, the signal that controls this switching mechanism is principally the blood glucose concentration.
When the glucose concentration is low, insulin secretion is suppressed and fat is used almost exclusively for energy everywhere except in the brain.
When the glucose concentration is high, insulin secretion is stimulated and carbohydrate is used instead of fat, and the excess blood glucose is stored in the form of liver glycogen, liver fat, and muscle glycogen.
Therefore, one of the most important functional roles of insulin in the body is to control which of these two foods from moment to moment will be used by the cells for energy.
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