Endurance competitors commonly shift toward carbohydrate in the days before a long event. The practice targets a specific storage limit rather than general energy availability.
Stored carbohydrate is finite and intensity-dependent
Muscle stores carbohydrate as glycogen, with a further store in the liver. The total is enough for a few hours of hard exercise and considerably less at higher intensities.
Fat is available in far larger quantities but is converted to usable energy more slowly. Above a certain intensity, the rate of energy demand exceeds what fat oxidation can supply.
Carbohydrate therefore sets the ceiling for how long a given pace can be held, and the point at which stores run low often coincides with the point where pace collapses. The drop tends to be abrupt rather than gradual.
Storage capacity is not fixed
Glycogen is stored with water, and muscle can hold substantially more than habitual eating produces if intake is raised while training volume falls.
The classical approach paired a depletion phase with a subsequent high-carbohydrate phase, on the reasoning that emptying the store increased the enzyme activity that refills it.
Later approaches drop the depletion phase, since raising intake during a taper achieves comparable storage without the difficulty of training hard on very little carbohydrate. The older protocol left athletes flat during the days it was applied.
Taper and intake work together
Reduced training in the final days lowers the amount of glycogen being consumed daily, so more of what is eaten remains in storage.
This is why loading is tied to a taper rather than performed during heavy training weeks, when the same intake would largely be used rather than stored.
It also explains the weight gain that accompanies loading, since stored glycogen carries water with it. The gain is expected rather than a sign that intake was excessive. That water remains available to the athlete during the event.
The strategy suits some events and not others
Loading matters where the event is long enough for stores to become limiting. For shorter efforts the store was never going to be exhausted.
The added mass is a cost in sports where body weight is carried against gravity, so the trade-off differs between a flat road event and a mountain one.
Gut comfort is a further constraint, since high intake in the final days can leave residue that causes problems during the event itself. Fibre content is usually reduced for the same reason.
Fuelling during the event is a separate question
Stored glycogen sets the starting position; intake during the event determines how quickly it is drawn down, and the two are planned together.
Both depend on tolerance that varies widely between individuals, and dietary changes around competition are best planned with a qualified practitioner rather than trialled at an event. Rehearsing the plan in training is the standard way to find that tolerance.
