One day of carbohydrate filled the muscle, with no depletion phase at all

Why they ran it

The authors start from something already accepted: that even without a glycogen depleting bout, trained athletes reach maximal muscle glycogen on three days of a carbohydrate rich diet. What has never been examined, they write, is whether all three of those days are necessary.

Drawn from the paper's introduction.

Muscle glycogen rose from 95 to 180 millimoles per kilogram of wet mass after a single day of the diet, and then remained stable despite two more days of eating the same way.

The biochemical assay was checked against densitometry on muscle sections stained with periodic acid Schiff. That second method agreed, and added something the assay could not see on its own: type I, type IIa and type IIb fibres had all reached maximal levels after 24 hours, so no fibre population was still filling.

There was no glycogen depleting exercise anywhere in the protocol. The athletes ate and stayed inactive.

The numbers

Muscle glycogen before loading	95 (5) mmol/kg wet mass
Muscle glycogen after 1 day	180 (15) mmol/kg wet mass, p<0.05
Change from day 1 to day 3	no significant further change
Carbohydrate dose	10 g/kg body mass per day, high glycaemic index
Glycogen depleting exercise	none
Fibre types at maximal glycogen after 24 h	I, IIa and IIb

Why this might happen

Proposed by the authors This is the explanation the authors offer in their discussion. This study did not test it.

The authors put the speed down to the combination rather than to either half of it: a large dose of high glycaemic index carbohydrate arriving in a body that is spending nothing, so that none of what goes in is used again before it can be stored.

The fibre level staining is what lets them say the muscle was full rather than merely fuller. When the slow and the fast fibres have all arrived at the same ceiling, there is nowhere left for a second and third day of eating to put anything.

Drawn from the paper's conclusions.

What this does not show

Nobody raced. Every outcome in this paper is a muscle glycogen measurement. The trial cannot say whether a 24 hour protocol produces the same result on the road as a longer one, because no performance test of any kind was run.
It does not show the depletion phase is worthless. There was no depletion arm here to compare against. What it shows is that a full muscle is reachable without one, which is a narrower claim than depletion adding nothing.
The inactivity is part of the protocol, not a footnote. These eight men did no training for three days. An athlete who trains through the loading days is not doing what was tested here.
The carbohydrate was specifically high glycaemic index. The foods were chosen for a high glycaemic index, and the same dose of slower carbohydrate was not tested alongside it. The speed of the result may belong partly to the food choice.

Where this leaves us

The three day window, already the modern short version of the classical protocol, turns out to be about two days longer than the muscle needs. This refines standard practice rather than overturning it: what the muscle responds to is a dose and an absence of training, not a calendar.

Eight endurance trained men eating 10 grams of high glycaemic index carbohydrate per kilogram per day while doing no exercise at all.

The same one day protocol with a performance endpoint attached to it. That study has still not been run, which is the largest single hole in this literature.

Caveats worth holding

Wet mass units, and means reported with standard errors rather than standard deviations.
Eight men, no women, no control diet arm.
The paper establishes a ceiling in the muscle, not a ceiling in the athlete. Liver glycogen was not measured and is a separate store with separate rules.
Three days of complete inactivity is unusual in the last days before a race, and the protocol depends on it.