Protein

Protein is a macronutrient built from chains of amino acids. Beyond muscle, proteins make up enzymes, antibodies, transporters and much of the body's structural material. Nine of the twenty amino acids are essential — the body cannot make them, so they must come from food.

It matters more than any other macronutrient for body composition. Adequate protein preserves lean mass in a calorie deficit, supplies the substrate for muscle protein synthesis after training, and is the most satiating and most thermogenic of the three macros, so it does useful work on both sides of the energy equation.

In practice, sports-nutrition bodies place the useful range for people training regularly well above the basic requirement for avoiding deficiency, and total daily intake matters considerably more than the timing of any individual serving. Distributing it reasonably evenly across meals is a sensible default rather than a strict requirement.