http://journal.diabetes.org/diabetesspectrum/00v13n3/pg132.htm
“Between 50?60% of protein becomes glucose and enters the bloodstream about 3?4 hours after it’s eaten.”
sweetlyvoiced.com/2013/04/its-cuplicated.html
(see graph)
There?s a great deal of variability across persons. Some people are in nitrogen balance at protein intake of 0.9 g/kg/day; others need as much as 1.5 g/kg/day. At 1.2 g/kg/day, half the sample was in nitrogen balance.
Various factors influence the interpretation of this data:
The sample was of endurance athletes. Endurance exercise increases protein needs, so most people would reach nitrogen balance at lower protein intakes. Resistance exercise doesn?t require as much protein: Experienced bodybuilders are typically in nitrogen balance at 1.2 g/kg/day. [2]
Most of the sample probably ate a high-carb diet. Glucose needs were met from dietary carbohydrates. Low-carb dieters would need additional protein for glucose manufacture.
As Ned states, in caloric deficit, protein needs are increased; in caloric surplus, protein needs are decreased. If you?re restricting calories for weight loss, expect to need a bit more protein to avoid muscle loss.
Supplementing leucine ?increased protein synthesis and decreased protein breakdown? [2], thus leading to nitrogen balance at lower protein intakes.
The point of nitrogen balance is dynamic: if everyone in the sample ate 0.9 g/kg/day, then they?d eventually get into nitrogen balance at 0.9 g/kg/day. The body adjusts to conserve muscle at given food availability.
The average person needs much less protein to be in nitrogen balance. The US RDA for protein, 0.8 g/kg/day, was set so that 97.5% of Americans would be in nitrogen balance. [2] But just to be conservative, and because we?re developing advice for athletes, let?s consider 1.5 g/kg/day as the protein intake that brings our athletes into nitrogen balance.
What about the protein intake that exhausts benefits? At what intake is muscle synthesis no longer promoted?
Ned, citing a review paper [2], offers the following answer: ?[P]rotein intake beyond 25 percent of what is necessary to achieve a nitrogen balance of zero would have no effect on muscle gain.?
On my reading it?s not so easy to infer a clear answer, but let?s go with this. If so, then muscle gains would be exhausted at 1.25*1.5 = 1.875 g/kg/day even for the most strenuous athletes.
What about toxicity?
We deal with this in our book (p 25). At a protein intake of 230 g/day (920 calories), the body?s ability to convert ammonia to urea is saturated. [3] This means the nitrogen from every additional gram of protein lingers in the body as ammonia, a toxin.
Clearly marginal dietary protein is toxic, via ammonia poisoning, at this intake level. A reasonable estimate for where toxicity begins is between 150 to 200 g/day.