CarbohydrateCarbs

Carbohydrates are sugars and starches that the body breaks down mainly to glucose. What is not used immediately is stored as glycogen in muscle and liver, with a limited capacity of a few hundred grams in total; beyond that, sustained excess energy is stored as fat.

It matters because glycogen is the dominant fuel for anything above easy intensity. Depleted stores mean a measurable drop in high-intensity work capacity, which is why very-low-carbohydrate approaches tend to feel worst in exactly the sessions that matter most for strength and speed.

In practice, carbohydrate is the most negotiable macronutrient: protein needs are relatively fixed and a floor of dietary fat is needed for hormones and absorption, so carbohydrate largely fills the space that remains. How much you need scales with how much hard training you do.