In my opinion, this is a loaded question. The answer simply put is: it depends.
In theory, consuming the proper ratio of nutrients during this immediate post training time period not only initiates the rebuilding of damaged muscle tissue and restoration of energy reserves, but enhances both body composition and exercise performance. Several researchers have made reference to an anabolic “window of opportunity” whereby a limited time exists after training to optimize training-related muscular adaptations. (2)
The reference to this "window" is based largely on the presumption that resistance training is carried out in a fasted state. During which, muscle protein breakdown (MPB) is increased. So it would make sense to provide immediate nutritional intervention in the form of protein and carbohydrate to initiate a rise in muscle protein synthesis (MPS) and decrease in MPB to shift from a catabolic state to anabolic.
This of course begs the question of what influence the pre training nutrition has on the urgency of post training nutrition. If the pre exercise meal is consumed within 1-2 hours prior to training in an attempt to maximize training performance, depending on its size and composition; this meal can possibly function as both a pre- and an immediate post-exercise meal, since the time course of its digestion/absorption can persist well into the recovery period. (2)
On the other hand, there are others who might train before lunch or after work, where the previous meal was finished 4–6 hours prior to commencing exercise. This gap in nutrient consumption can be considered significant enough to warrant post-exercise intervention if muscle retention or growth is the primary goal.
In light of these findings, when training is initiated more than ~3–4 hours after the preceding meal, the classical recommendation to consume protein (at least 25 g) as soon as possible seems warranted in order to reverse the catabolic state, which in turn could expedite muscular recovery and growth. (2)
On the subject of carbohydrate- it's availability during and after exercise is of greater concern for endurance as opposed to strength or hypertrophy goals. The importance of co-ingesting post-exercise protein and carbohydrate has recently been challenged by studies examining the early recovery period, particularly when sufficient protein is provided. (2) For the goal of maximizing rates of muscle gain, these findings support the broader objective of meeting total daily carbohydrate needs instead of specifically timing its doses, unless in the case of elite athletes with multiple training sessions/day.
Factors determining the optimal post training timing are almost certainly more than one and likely depend on a combination of protein type, dose of protein available and content of the essential amino acids (in particular leucine content), and other nutrients ingested at the same time. It is argued that the ‘anabolic window of opportunity’ is not as critical or narrow as reported. (1)
(1)
However from a practical standpoint, protein ingestion within one hour post exercise is likely to be a convenient time to ingest protein. There certainly is no evidence that protein ingested immediately post exercise will impede the muscle anabolic response.
KEY TAKEAWAYS:
If resistance training session is in a fasted state, immediate nutrient replenishment is recommended
If pre training meal was 4+ hours prior to exercise session, immediate nutrient replenishment is recommended
Post (+intra) training carbohydrate ingestion is a greater priority for endurance sport as opposed to strength and hypertrophy training. In which case, total dietary carbohydrate intake is the focus- unless one of the above scenarios is applicable; or in the case of multiple training sessions/day.
Combination of protein type, dose of protein available and content of the essential amino acids (in particular leucine content), and other nutrients ingested at the same time play a role in determining the optimal post training timing
The ‘anabolic window of opportunity’ may not be as critical or narrow as reported, however there is certainly no evidence that protein ingested immediately post exercise will impede the muscle anabolic response.
References:
1. Witard, O. and K. Tipton. “Defining the anabolic window of opportunity. Is protein intake immediately post resistance exercise critically important for muscle growth.” Agro Food Industry Hi-tech (2014): 10-13.
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