sharptailhunter

I recently posted a topic about fat metabolism which saw a good number of quality responses. I'm hoping that we can have the same good conversation about protein.

I have a good understanding of how protein is used as a so called building block for our bodies. Many very important structures in our bodies are made up of protein, such as enzymes, cellular receptors, and of course muscle fibers.

However, I am clueless about how our bodies use protein as a fuel. So here we go with the questions:

1. When does the body use protein as a fuel? Is it during normal activity or more vigorous exercise?
2. Does it convert the protein to energy, such as how carbos are converted to ATP for energy?
3. Or, is the protein used just to repair that protein that is lost from the body due to damage from exercise?
4. That being said, does said "damage" actually occur, i.e. micro tears in muscles and such? I do know that enzymes and other cellular structures are constantly being torn down and replaced, is that true with muscles?
5. Where does the body get its source of protein? From the body then diet replaces that or can it utilize protein directly from our GI system?

I'm sure I will have more questions later, but that's enough to hopefully get some good knowledgable responses in the meantime.

Enthalpic

1/2) Protein supplies between 0 and 10% of your energy demands. At rest it is close to 0%, during times of extreme stress and starvation close to 10%.

Your body will deaminate certain amino acids to produce glutamine with the remaining carbon skeleton being metabolized as fat (acetyl coA). The glutamine / glutamic acid can enter krebs like a carb.




3) Amino acids are used to form all sorts of products and some of these are lost at greater rates following exercise. However, your kidneys are always working at retaining as much as possible.

4)Yes damage does occur and proteins are always being created and destroyed. Your body will not work at retaining a protein it doesn't need. However, exercise does not effect muscle protein turnover as much as many would imagine.
Protein and amino acid metabolism in human muscle.
https://www.ncbi.nlm.nih.gov/pubmed/9...ubmed_RVDocSum

5)Your body has a large protein / amino acid pool that it is constantly being drawn from and fed at the same time.

The_Spaniard

Im no expert but i know one reason for people with low fat percentages the body will use muscle for fuel in a attempt to keep what little body fat handy for survival since you need a certein amount of body fat to actually be alive. I believe your body can burn protein for full from both the muscle thats already there and muscle that you consumed in your diet. Yes when your workout you make little micro tears , and when your body uses the muscle you already have as fuel its pretty minimal form what i hear as long as your not doin a super long endurance event. So yeah muscles are constantly being damged a little and rebuilt stronger. I really dont know whent eh body uses protein for fuel for people with higher body fat percentages, im not sure if it happens at all, iw ould think the body would use the body fat first. I have also read research saying that taking protein before a workout will help you later in post workout because the protein is already floating in your blood stream to be used to rebuild right after workout and you dont get the delay of digeting protein afterwards tillt he muscles get to rebuild. I am not completely 100% on all this info but im pretty sure on it, details might be a little fof correct me fi im wrong. Hope i helped in some way.

The_Spaniard

oops he beat me to most of it.

Tabagas_Ru

Here is a good scientific review of protein from the Journal of the International Society of sports nutrition.

To look at the signaling process and how proteins are marked to be broken down search for info on the Ubiquitin Proteasome System, sometimes referred to as the UPS. This will tell you which amino acids will prevent protein from being broken down as well as the pathway itself.

TurboTurtle

What 'size' gets transported from the digestive system to the blood stream? Amino acids, dipeptides, larger peptides, proteins? - TF

Nickel

amino acids, di- and tri-peptides.