Ask a runner or a triathlete about protein and you will usually get an answer about muscle size. That framing sells protein short, and it is the reason a lot of endurance athletes under-eat the one macronutrient that decides how well they absorb the training they are doing.

Protein is structural material. Your body builds well over a thousand distinct proteins from it: the keratin in your hair, the collagen in your skin, the actin and myosin that let a muscle contract, the haemoglobin that carries oxygen, the immunoglobulins that defend you, the enzymes that run every metabolic pathway, and hormones like insulin. Muscle is one line on a very long list.

This is part one of a two-part guide: what protein actually does, how much you need, and where to get it. Part two covers timing and supplements.