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Originally Posted by Justin Jor
Carbs are, for instance, protein sparing. This isn't a real concern if you're not training fairly hard, but if you ARE training hard, and you reduce carbs, you might need to kick up the protein.
And so on. You need a certain amount of glucose for your brain, for instance. Your body will produce this, mostly from protein, so if you're at like zero carbs and trying to, for instance, gain muscle, you'd need more protein to do it than you might if you were eating more carbs.
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Without going on a tangent, I feel I have to correct these two points based on how I see it.
More appropriately, carbs are
believed to be protein-sparing, but with little evidence to confirm the belief. This belief is due to the fact that insulin stimulates proteosynthesis (creation of new protein), and also inhibits proteolysis (destruction of existing protein), therefore more insulin should do more of that. However, carbs also cause glycation - the creation of advanced glycation end-products (AGE's) - which is the binding of glucose to protein, thus rendering these protein inert or making them work differently than normal.
By comparison, ketones stimulate a different process called chaperone-mediated autophagy (CMA) which is the degradation of AGE's, so that the useless protein that's bound to glucose is now released, free to be used where needed, thus genuinely sparing existing protein, and in effect reducing one's dietary protein requirement.
Furthermore, hyperglycemia inhibits growth hormone, which is the single most powerful growth factor for muscle, therefore carbs stifle muscle growth through this inhibition mechanism rather than stimulate growth through insulin/proteosynthesis/proteolysis. Fat doesn't do that, growth hormone works normally.
Finally, protein stimulates insulin, in exactly the amount required to stimulate the exact amount of proteosynthesis and the exact amount of inhibition of proteolysis. Excess insulin (from dietary carbs) is superfluous, it will not cause any more muscle growth than already caused by the protein itself. However, excess insulin will cause growth - in fat tissue. This is helped by the fact that glucose is converted to glycerol in fat cells, thereby giving fat cells the ability to create triglycerides for storage, so the more carbs, the more glycerol, the more triglycerides, the more fat inside fat cells.
For the glucose-from-protein point, that's mostly incorrect for our purpose. Fat is made from fatty acids and glycerol, and glycerol is used to make glucose, and we have a huge amount of fat reserve so we have a huge glycerol reserve too. In order for glucose-from-protein to be true, there's gotta be no fat left, i.e. we'd have to be literally starving and emaciated. Incidentally, fasting causes insulin to drop to its lowest level, and ketogenesis to increase to its highest possible level, thus stimulating CMA to a higher degree, thus sparing existing protein to a higher degree as well. Granted, if we fast for a significant amount of time, then we get into losing protein due to lack of dietary protein itself, but that's a different cause of protein loss altogether.
As for dietary protein requirement depending on training level, I agree, but not to the point about a different protein requirement depending on carbs intake level. Muscle is made of protein, not glucose, therefore protein requirement does not change just because we eat more or less carbs. However, based on what I wrote above especially with regards to AGE/CMA, we could argue that protein requirement would actually drop as we eat less carbs, because less of it would end up glycated/useless so more of it would be used for growth.
Again, this is just how I see it, not necessarily how it actually works.