Hypertrophy arises because of a sustained excess of muscle protein synthesis (MPS) over-and-above muscle protein breakdown, usually because of performing strength training and/or eating protein.
When studying the effects of protein consumption, it has been found that:
1️⃣ Protein consumption produces dose-response effects on MPS up to around 20g of protein; and
2️⃣ Post-workout protein consumption only increases MPS rates for a couple of hours before returning to fasting levels, even when protein is continually supplied.
These observations have led to researchers referring to the “muscle-full” effect. This is the idea that when amino acid delivery reaches a certain level, which corresponds to around 20g of protein, muscle cells cease to use them as a substrate for MPS, but instead divert them toward oxidation.
This first study was done to confirm the results of previous investigations. And it did in fact show that after a workout involving only the leg muscles, 20g of post-workout protein was just as effective as 40g for increasing MPS in the 4 hours after exercise.
However, this study does not tell us what would happen after a whole body workout.
The question of what happens after a whole body workout was addressed in a follow-up study from the same group of researchers. As you can see from the infographic, the greater amount of muscle mass involved in whole body training led to an increased requirement for post-workout protein.
This suggests that a post-workout protein shake may need to be bigger when doing whole body workouts, compared to when doing split routines.
In contrast, the amount of lean body mass of each subject did not have an effect on the MPS response, which was surprising.
This may indicate that larger individuals require less protein on a pound-for-pound basis, but this seems to contradict the idea that whole body workouts lead to greater protein requirements than split routines, so this may need to be confirmed by future research.
Interestingly, other research indicates that MPS during an energy deficit (as when trying to achieve weight loss through a diet) is lower than when in energy balance. And strength training while in an energy deficit only increases MPS to the same resting level as in energy balance.
This suggests that post-workout protein may be more important for stimulating MPS while in an energy deficit, than during energy balance.
Taken together, these studies suggest that greater post-workout protein shakes may be helpful either when performing whole body training (compared to split training) and when in an energy deficit (compared to in energy balance).
Even so, given that these studies did not assess the effects of MPS over the course of the whole day in the context of matched daily protein intakes, the effect of post-workout protein in isolation from daily protein consumption is hard to assess.