ItsTheWoo said Right. I was aware of that. I was trying to point out that the math was wrong. And I was worried that someone might read the post, dramatically increase their calorie consumption, and run into problems. I didn't make that clear. But it got me thinking and I've done some research.

Following are some excerpts (in red) from Bettelheim, Brown, & March, Introduction to General, Organic, and Biochemistry, 6th ed, 2001. I'm going to summarize metabolism of carbs, fats, and proteins below, hopefully in a manner everyone can understand, and maybe clarify the metabolic advantage concept of low carb diets. Also keep in mind that "energy" used by the cell is in the form of ATP molecules, which are utilized in the mitochondria. All ATP is the same, whether it comes form carbs or fats.

Here is a link to the mentioned textbook.
http://www.brookscole.com/cgi-brook...pline_number=12

"To convert these compounds to energy [carbs, lipids, proteins], the body uses a different pathway for each type of compound. However, all these diverse pathways converge to one common catabolic pathway."

Lipids, proteins, and carbohydrates alike are broken down into small "chunks" containing 2 and 4 carbons before they enter the citric acid cycle (aka the Krebs cycle) and the electron transport chain to make ATP. So we are interested in how much energy is yielded from each type of molecule.

Carbohydrates (whether they be disaccharides - simple sugars - or polysaccharides like starch) are broken down into monosaccharides (glucose, etc.). Glycogen (also a carbohydrate) is also broken down in the same way. This pathway is glycolysis.

Lipids are broken down into glycerol (aka glycerine) and fatty acids, and sometimes into monoglycerides. These small molecules are then oxidized to produce energy. "The specific pathway by which energy is extracted from glycerol involves the same glycolysis pathway as that used for carbohydrates." Cells use beta-oxidation to break down fatty acids and produce energy.

"[Amino acids] serve as building blocks for proteins as needed and, to a smaller extent (expecially during starvation), as a fuel for energy. In the latter case, the nitrogen of the amino acids is catabolized through oxidative deamination and the urea cycle and is expelled from the body as urea in the urine. The carbon skeletons of the amino acids enter the common catabolic pathway either as alpha-keto acids or as acetyl coenzyme A."

Now I'm going to attempt to compare the energy yield from each type of molecule. Energy yeild is through the number of molecules of ATP produced from each building block (refer to chapter 27 in the mentioned textbook).

Glucose metabolism yields 36 ATP molecules from one molecule of glucose, or 6 per carbon.

Catabolism of glycerol yields 20 ATP from each glycerol molecule, or 6.7 per carbon.

Fatty acids are long-chain carboxylic acids (similar to acetic acid, citric acid, etc.), but with more carbons, usually between 10 and 20 per molecule. These are broken down into 2-carbon "chunks". Stearic acid, a typical fatty acid produces 146 ATP molecules, or 8.1 per carbon.

The combined ATP yield of both glycerol and fatty acids result in the increased caloric value of fats. A typical fat molecule has one glycerol and three fatty acid chains.

There are 20 different amino acids which can be modified to enter the citric acid cycle. I did not find a "per carbon" or "per molecule" yield for ATP for these in my resource book.

So, more energy is yielded per carbon from molecules of lipids (fats).

However, utilization of glucose is preferred by the body. When it is not available, fatty acids are used. Glucose is broken down into pyruvate, and then into acetyl CoA which enters the citric acid cycle. Through a series of steps oxaloacetate is made, which then reacts with the "next" acetyl CoA molecule entering the cycle. Oxaloacetate is required for the citric acid cycle to continue cycling. "What has happened in the entire process [the citric acid cycle] is that the original two acetyl carbons of acetyl CoA were added to the C4 oxaloacetate to produce a C6 unit, which then lost two carbons in the form of CO2, to produce, at the end of the process, the C4 unit oxaloacetate. The net effect is the conversion of the two acetyl carbons of acetyl CoA to two molecules of carbon dioxide."

Here is a graphical link to metabolism for extra info. http://www.pantethine.info/whatis/chemistry.php

The 2-carbon fragments produced from fatty acid metabolism are also acetyl CoA. These enter the citric acid cycle. "Unfortunately, low glucose supply also slows down the citric acid cycle. This happens because oxaloacetate is produced from the carboxylation of pyruvate. This oxaloacetate normally enters the citric acid cycle where it is essential for the continuous operation of the cycle. But if there is no glucose, there will be no glycolysis, no pyruvate formation, and therefore no oxaloacete production." BUT... not all of the acetyl CoA is used in the citric acid cycle. I believe this is where the metabolic advantage comes in, not through any more "calories" being needed to metabolize lipids over carbs.

The excess acetyl CoA are made into "ketone bodies" (ketone bodies include acetone, acetoacetate, and beta-hydroxybutyrate) and sent into the bloodstream by the liver. Cells use them in the common catabolic pathway to produce energy by muscle cells and neurons.

SUMMARY: Lipids produce more energy in the body than carbs or proteins, even considering the different metabolic pathways they must use to be broken down into the molecules used in the common catabolic pathway. But not all the lipids consumed in a low carb diet are able to enter the common metabolic pathway, thus the metabolic advantage.


I really hope this helps!

Joan