The best data I have seen suggests a ~10.7g/hour limit for protein transport across the intestinal lumen into the blood. That’s for absorption. Not all foods absorb at the same rate, so you will not get the same amount per hour.
http://www.ncbi.nlm.nih.gov/pubmed/16779921 is the review.
Of course, not all foods pass through the digestive tract at the same rate either… larger solid food boluses, boluses with long chain fatty acids, and boluses with more solid food protein all tend to pass through slower, giving a prolonged absorption window. In the end, though, your maximal absorption will occur via repeated and overlapping fast-digesting protein pulses.
However, actual utilization is a lot more complicated.
For example, Alan Aragon has reviewed quite a lot of literature that suggests that sustaining prolonged elevation of blood amino acid levels beyond 4-5 hours offers no benefit and may actually suppress the anabolic response beyond that point. He believes that the research also suggests that people get their best results from eating every 4-5 hours, but I think that is only true with solid meals. I think that the truly dedicated can get better results by using smaller pulses of whey protein every 2 hours, but that is a scheduling hazard for people with a real life and Alan’s suggestion is clearly the best overall strategy for regular people in my opinion.
If that is accurate, then we are looking for pulses and there is limited use for either trying to keep a sustained peak absortion rate for more than 3-4 hours. Instead we would see better results by generating repeated peaks that drop down to near-basal levels before the next bolus is introduced. Large single doses do not appear to generate much of any additional total muscle protein synthesis vs a smaller 30g dose in the young or elderly:http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3197704/figure/F2/
Considering how rapidly whey protein appears to absorb, you could use many small doses to achieve this effect.
Overall protein utilization for muscle protein synthesis appears to hinge upon achieving a ~3g dose of leucine that is absorbed fairly rapidly. This corresponds to ~28g of whey protein, 36-38g of beef protein and ~42g of chicken protein.
Our bodies are capable of selectively absorbing more ‘important’ EAA like leucine when there is more total AA than can be absorbed in full present in the intestines.
This leucine peak appears to be the nutritional trigger for muscle protein synthesis in particular, so it is worth keeping that as a central piece of your strategy. A large, fast-absorbing bolus of protein like 30g of whey is going to maximize your actual muscle gain, because pretty much nothing in the body is perfectly linear and that includes the response to blood leucine concentration.
Boirie Y, Dangin M, Gachon P, Vasson MP, Maubois JL, Beaufrère B. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc Natl Acad Sci U S A. 1997 Dec 23;94(26):14930-5.
There is also more recent research that continues to find this to be true.
The effect is much more pronounced in the elderly, as their muscle tissue appears to become resistant to lower doses of leucine but nearly identically sensitive to the 3g dose that comes with 30g of whey when compared to 25 year olds. This suggests that older individuals should make an effort to consume 30g of protein at one time, instead of many small servings. The net effect is much larger when they get 30g, even though they absorb just fine.
Consuming proteolytic enzymes such as Aminogen (pancreatic enzymes) appears to raise the peak blood amino acid levels as well as how long the peak remains. This may cause downregulation of endogenous pancreatic enzyme production over time, and I do not believe there is any clinical data to suggest what a suggested on-off cycle of this strategy should look like.
Additionally, you need to provide the energy to support the use of the protein you absorb for anabolic processes to occur and exercise so that gene transcription related to increased protein anabolism is upregulated, triggering the additional post-exercise increase in muscle protein synthesis after feeding. Young and old have nearly identical responses.
http://www.ncbi.nlm.nih.gov/pubmed/8368290 There is a lot of newer replication.
Together this appears to allow us to run the following game plan:
1) Eat 40-50g of protein every 4-5 hours with a meal. You could also do 30g every 3 hours. That’s what I do most of the time, but I would really just match the intake to your schedule. The longer the gap between meals, the more important it will be to use real meats that are not super lean in order to sustain the gastric emptying so that the protein actually sustains you through the entire gap.
2) Engage in resistance training to maximize response to each meal.
3) If you are adventurous perhaps do 1 day on, 1 day off supplementation with pancreatic enzymes or brand-name Aminogen at a dose of 2.5g per meal. You could do 5g, but the benefit is only ~10% for doubling your costs and is more likely to cause issues, so I don’t recommend that approach personally. It is probably wise to also take 1-2 weeks off every 2-3 weeks, as a precaution.
It’s simply impossible to say whether #3 would actually result in additional muscle mass gain… I don’t know if there is data out there on that, and I can’t guarantee safety, so I would consider this to be a risk that you would choose to take knowing that there are no guarantees, as opposed to an evidence-based recommendation. There were no indications of the potential for harm in the available research, but that doesn’t mean all the bases were covered and there is a very small body of research.