Carbohydrate Metabolism Overview
By: Dr. PRION

Some of you have been asking quite a few questions regarding metabolism and regulation of the body specifically in the area of carbohydrate, lipid, and protein metabolism. Here is a little overview of what happens with all that sugar your client shoves in their mouth. 

Carbohydrates are the major source of energy for the human body, supplying nearly half of the total caloric intake. The most important dietary carbs are polysaccharides and disaccharides. Polysaccharide digestion begins in the mouth and continues into the small intestine, while disaccharides are digested primarily in the small intestine. Now the fate of glucose depends on the body’s needs, so I will simplify. If you are active the glucose will enter into glycolysis via hexokinase to be metabolized for the ultimate pay-off, ATP. If you are sedentary it will be ultimately shuttled to the liver to be either stored as glycogen or converted to Acetyl CoA which will raise the over all energy charge of the cell. Now, this is where it gets interesting; carbohydrates are able to convert to both the glycerol and fatty acid portion of fat, however, it is apparent that it actually will inhibit lypolysis (break down of fat) more than convert to fat when there is plenty of glycogen available. That’s why it’s a problem. Ultimately, this is all depended on metabolism/activity levels and diet.

In a state of carb depletion, the body will ultimately switch to fatty acids which provide a high pay-off of ATP. The fatty acid will break down to ketones (ketone bodies) in order to supply the CNS (remember that the only source of fuel for the CNS is glucose or ketones). Amino acid turn over increase un-proportionally as well. This is due to the amino acid converting to glucose in order to supply the CNS as well as the red blood cells. There are only two amino acids that are ketogenic (meaning they can become ketones) these are: leucine and lysine. After only 3-4 days of carbohydrate depletion the CNS utilizes 70% ketones for proper function. The early state of fasting (in general) is accompanied by large daily losses of urinary nitrogen, in keeping with the high rate of breakdown of muscle protein and the synthesis of glucose through hepatic (liver) gluconeogenesis (formation of glucose through other intermediates).

Hope this gives you a better idea of the big picture.