The metabolic window is a term used in strength training to describe the supposed 30-minute (give or take, depending on the individual as what's referred to as "Bro Science" varies) period after exercise during which nutrition can shift the body from a catabolic state to an anabolic one. Specifically, it is during this period that the intake of protein and carbohydrates can aid in the increase of muscle mass.
Some theorists believe the metabolic window begins to close within minutes of the end of a workout. The same nutrients taken two hours later result in significantly reduced protein synthesis and muscle glycogen storage. Further research is needed into the effects of food and exercise timing, but methodology has to be considered in these experiments.
The "metabolic window", or what is also referred to as the “anabolic window”, is considered by gym goers and supplement companies to be a 45-minute opportunistic period to increase muscle size by ingesting carbohydrates and amino acid supplements.
Contrary to this belief, researchers studying the effects of carbohydrates and amino acid supplementation taken various times after a workout found no difference in a 0-3 hour period in the rate of uptake of protein into the muscles or protein balance (Rasmussen et al., 2000). More importantly, Tipton (2001) found that weight lifters may actually benefit with respect to muscle anabolism by taking in carbohydrates and amino acids before working out. This is because weight lifting increases blood flow, which will increase the transportation of amino acids to the muscles.