Fitness is rife with falsehoods. It's a well-known fact that heavy lifting is the only method to gain muscle and that discomfort is a sign of a solid workout.
The “anabolic window”—the concept that you must ingest protein (ideally in a smoothie) shortly after exercise—is possibly the most obstinate. If you miss the post-workout window, you might as well not have worked out.
To be honest, the anabolic window seems logical. Muscles suffer microtrauma (microscopic rips and injury) when you lift weights (or do HIIT, or run kilometres, or swim laps).
Not only is it a natural aspect of working out, but it also stimulates muscular growth. So your muscles become bigger and stronger so they can bear the stress of working out better next time.
This process requires raw materials, in this instance amino acids (the building blocks of protein). That's where a “anabolic window” comes in.
It is believed that consuming protein within 30-60 minutes of an exercise can increase muscle building.
So long as you eat a protein-rich meal within two hours following an exercise, your muscles will have all the nutrition they need to adapt and grow.