The muscle sparing effects of carbohydrates occur via several different mechanisms. When the body is in a low energy state, it may try to produce energy by converting amino acids to glucose. Carbohydrates prevent this since they can be easily broken down (and converted if need be) to glucose molecules. Carbohydrates then spare dietary protein from oxidation and these proteins can be stored rather than oxidized.
Carbohydrates are also very muscle sparing during exercise. When one lifts heavy weights, the primary pathway that is used to produce ATP (cellular energy currency) is the anaerobic or glycolytic pathway (as the name implies this pathway operates in the absence of oxygen). The only substrate for this pathway is glucose, which can be obtained from dietary carbohydrates or by breaking down glycogen (the cell's stored form of glucose).
If one is on a ketogenic or extreme "low carb" diet however, the body will need to utilize another source to synthesize glucose from. Since glycogen levels are low on a ketogenic diet, the body will actually convert amino acids to glucose and this glucose will be used in the anaerobic pathway to produce ATP. These amino acids will come from dietary protein, amino acids from the cellular amino acid pool, and from muscle tissue. The latter situation is where one would experience muscle loss. Dietary protein would be sacrificed for ATP production and the depleted amino acid pool would not bode well for protein synthesis rates, thus causing a net loss in muscle mass.
Carbohydrates are also muscle sparing because they are a cause of insulin release. Now I know your thinking, "but Layne, you just said in your intro that low insulin levels were great for fat burning!?" Yes, you are correct. I did indeed say that low insulin levels are good for fat burning. Insulin inhibits lipolytic (fat burning) activity and must be kept low if one wishes to burn a maximal amount of fat.


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