CarbohydrateCarbs
The body's primary fast fuel, stored in muscle and liver as glycogen.
Carbohydrates are sugars and starches: chains of glucose and related molecules that the body breaks down for energy, stores in muscle and liver as glycogen, and burns preferentially as exercise gets harder.
They are not essential in the strict nutritional sense, since the body can make the glucose the brain needs from other sources, but they are the fuel that makes hard training feel possible. As intensity rises the body leans progressively harder on carbohydrate, because it produces energy faster per unit of oxygen than fat does.
Type matters less than the internet suggests and more than nothing. What separates most whole food carbohydrate from most refined carbohydrate is fibre, water and the rest of the food matrix, which change how fast it arrives and how full it leaves you, rather than any difference in the glucose itself.
Fibre is the part with the strongest outcome evidence. A series of systematic reviews pooling 185 prospective studies and 58 trials found 15 to 30% lower all cause and cardiovascular mortality comparing the highest fibre eaters with the lowest, with benefit greatest between 25 and 29 g a day.
For training, the useful rules are simple. Total daily intake matters most, availability around hard sessions matters for the sessions themselves, and during long efforts the ceiling is absorption rather than appetite: around 60 g an hour from a single sugar, rising to roughly 90 g an hour when glucose and fructose are combined.
Low carbohydrate diets work for weight loss the way any diet works, by reducing intake, and the rapid first week of loss is largely the water that glycogen carries with it. They tend to impair high intensity performance, which is a fair trade for some people and a poor one for others.
In practice, set protein and a fat floor, then let carbohydrate take the remainder with our macro calculator, scaling it to how much hard training you actually do.