Severe depletion loaded no more glycogen than a moderate diet, and neither run was faster

What glycogen supercompensation is

Muscle stores carbohydrate as glycogen. After hard exercise empties those stores, the muscle can refill past where it started rather than merely back to it, and that overshoot is called supercompensation.

Carbohydrate loading is the practice of trying to produce that overshoot on purpose in the days before a race. The classical version pairs a period of very low carbohydrate eating with hard training, then floods the athlete with carbohydrate.

Drawn from background physiology, not from this paper.

Why they ran it

By 1981 the overshoot itself was not in question. What the authors set out to do was compare three different exercise and diet regimens against each other, rather than testing one on its own, and to put a performance run on the end of it instead of stopping at the biopsy.

Drawn from the stated aims of the paper.

Three regimens were run in trained runners. The classical arm ate three days at 15 percent carbohydrate then three days at 70 percent. The modified arm ate three days at 50 percent then three days at 70 percent. The third arm ate 50 percent carbohydrate for all six days. Every arm did the same five day depletion and taper running sequence.

Muscle glycogen after the classical arm was 207 millimoles of glucosyl units per kilogram of wet tissue. After the modified arm it was 203. After the taper only arm it was 159. The first two are the same number to within the precision of the method.

The 20.9 kilometre run burned more glycogen in the two loaded arms than in the taper only arm, 5.0 and 5.1 against 3.1 millimoles of glucosyl units per kilometre. There were no differences in run time between the three arms, and no differences in the glycogen left afterwards.

The numbers

Muscle glycogen, severe depletion arm (15 then 70 percent carbohydrate)	207 mmol glucosyl units/kg wet tissue
Muscle glycogen, modified arm (50 then 70 percent carbohydrate)	203 mmol glucosyl units/kg wet tissue
Muscle glycogen, taper only arm (50 percent throughout)	159 mmol glucosyl units/kg wet tissue
Glycogen used per kilometre, the two loaded arms	5.0 and 5.1 mmol glucosyl units/km
Glycogen used per kilometre, taper only arm	3.1 mmol glucosyl units/km
Difference in 20.9 km run time between arms	none reported

Why this might happen

Shown in this study The study measured this step directly.

The authors do offer an account of what the extra glycogen did, and it is not what a coach would hope. The two loaded arms burned roughly sixty percent more glycogen per kilometre than the taper only arm over the same distance in the same time.

Their conclusion is that starting glycogen influences how much glycogen is subsequently used. Having more of it changed the fuel mix, not the finishing time.

Drawn from the paper's own conclusions.

What this does not show

It does not show that carbohydrate loading is useless. Both loaded arms finished with substantially more glycogen than the taper only arm. What this rules out is the severe low carbohydrate phase being the ingredient that produced the loading, not the loading itself.
A small performance effect would have passed straight through it. Six runners can only reveal a large difference. The one or two percent that decides a race is well inside the noise of a trial this size, and the paper reports no times, only that they did not differ.
One distance, one sex, one sport. Twenty point nine kilometres in trained men on a treadmill protocol. Nothing here reaches a marathon, an ultra, a cycling event, or women.

Where this leaves us

The severe low carbohydrate phase, which is the part of the classical protocol athletes find hardest and most often skip, was not the part doing the work. A moderate diet followed by three high carbohydrate days reached the same muscle glycogen. This refines the original Scandinavian finding rather than contradicting it: the overshoot is real, the misery appears to be optional.

Six trained male runners covering 20.9 kilometres, in a protocol where every arm still did five days of depleting and tapering running.

A trial with enough runners to detect a one or two percent time difference, which is the size of effect that would actually matter and the size this one could never have seen.

Caveats worth holding

Glycogen is reported in millimoles of glucosyl units per kilogram of wet tissue, roughly a quarter of the dry weight numbers used elsewhere in this issue. The units are not interchangeable.
Six runners in a three trial design, so order effects and the effect of repeated biopsies on the same muscle cannot be excluded.
The paper reports that run times did not differ but does not print the times themselves in the abstract, so the size of the non difference cannot be judged.
Every arm did the same depletion and taper running. The comparison is between diets on top of that running, not between doing the running and not doing it.