<Originally Posted by geneman>
My Biochemistry may be rusty, but I'm pretty sure protein metabolism does not provide for conversion of unused amino acids to fat. Hence the success of all-protein diets in reducing fat.
<response from Koffee>
My biochem is just as rusty, but first, I'm not going to hijack a thread to argue for or against the high protein diet. If you feel like you have to debate it, start another thread on the topic.
I do believe I oversimplified- proteins are long chains of amino acids, but at the same time, we have to consider the protein diet itself, as well as what's beinc consumed with the high protein diet. As far as the "success" of the all-protein diets in reducing fat, a protein diet often results in an increase in fat intake. Excess protein is not used efficiently by the body, and can promote an increased metabolic burden on the kidneys and liver. A high protein-high fat diet, deficient in carbohydrate, causes a metabolic shift in favor of ketosis.
Besides that, when we eat protein in our diet, the main source of protein is from animal fats. There are not many people on Atkins who are vegetarians! When proteins are ingested, they are broken down into water, carbon dioxide, and nitrogen. The nitrogen part of protein metabolism is used for amino acids in muscle and tissue repair, as well as to drive the Krebs Cycle (to a small extent). BUT we are not just ingesting long chains of amino acids- often, these animal fats are higher in saturated fats. As we overconsume protein, we increase the amount of saturated fats in the body. I remember hearing once that people on high-protein diets are consuming up to 34% of their total calories in the form of protein and up to 53% of total calories from fat. So, if someone is eating a higher protein diet, they're actually consuming more fat, which will be stored in the body in excess. Since there isn't a lot of energy expenditure promoted in Atkins, then if you think about it, there will be excessive amount of fat storage in the body.
Beyond this, there is more to it with Atkins and other high protein diets, but I'm not going to discuss it in this thread. The point of THIS thread is to discuss recovery, not the alleged success of protein rich diets. I had hoped to skip a longer explanation of proteins, so I did simplify things a bit, but I suppose I should have just broke it down so I wouldn't have to then hijack a thread about the "success" of protein diets.
My intention was not to support high-protein diets. I simple wanted to point out that unused amino acids are not converted to fat during normal metabolism. And it turns out I was wrong. Here's the explanation I dug up in an old Biochemistry text book. You were on the right track in stating that high-protein diets cause a shift in favor of ketosis;
We now turn to the fates of the carbon skeletons of amino acids after the removal of the alpha-amino group. The strategy of amino acid degradation is to transform the carbon skeletons into major metabolic intermediates that can be converted into glucose or oxidized by the citric acid cycle. The conversion pathways range from extremely simple to quite complex. The carbon skeletons of the diverse set of 20 fundamental amino acids are funneled into only seven molecules: pyruvate, acetyl CoA, acetoacetyl CoA, a-ketoglutarate, succinyl CoA, fumarate, and oxaloacetate. We see here a striking example of the remarkable economy of metabolic conversions, as well as an illustration of the importance of certain metabolites.
Amino acids that are degraded to acetyl CoA or acetoacetyl CoA are termed ketogenic amino acids because they can give rise to ketone bodies or fatty acids. Amino acids that are degraded to pyruvate, a-ketoglutarate, succinyl CoA, fumarate, or oxaloacetate are termed glucogenic amino acids. The net synthesis of glucose from these amino acids is feasible because these citric acid cycle intermediates and pyruvate can be converted into phosphoenolpyruvate and then into glucose. Recall that mammals lack a pathway for the net synthesis of glucose from acetyl CoA or acetoacetyl CoA.
Of the basic set of 20 amino acids, only leucine and lysine are solely ketogenic. Isoleucine, phenylalanine, tryptophan, and tyrosine are both ketogenic and glucogenic. Some of their carbon atoms emerge in acetyl CoA or acetoacetyl CoA, whereas others appear in potential precursors of glucose. The other 14 amino acids are classed as solely glucogenic. This classification is not universally accepted, because different quantitative criteria are applied. Whether an amino acid is regarded as being glucogenic, ketogenic, or both depends partly on the eye of the beholder.
Clearly protein metabolism does result in the net creation of fatty acids. The efficiency with which new fat is formed from broken down amino acids is still a bit unclear to me but obviously will depend upon the amino acid composition of the protein in our diets as conversion to acetyl CoA appears to be more effecient for a class of amino acids. Obviously this is not something that people think about when considering diet. However, the bottom line is (and I think you'd agree from your last post) that the contribution of fat from the protein source (i.e. fatty foods) is likely to far outweigh de novo fat synthesis from the metabolic conversion of ketogenic amino acids.