Almost everything known about fuelling and glycogen comes from muscle, because muscle is what gets biopsied. The liver is the other half of the story and is usually inferred rather than measured.
The authors set out to measure both at once during high intensity intervals, using a magnetic resonance method that reads glycogen without a needle, and to ask what drinking carbohydrate during the session actually changes in each store.
Drawn from the stated aims of the paper.
Nine male cyclists rode 8 intervals of 5 minutes with 1 minute recovery on three occasions: placebo, 60 g of maltodextrin, and the same maltodextrin plus caffeine, taurine, l theanine, l citrulline and citicoline. Liver and muscle glycogen were measured before and after with carbon 13 magnetic resonance spectroscopy, which means no biopsy.
Muscle glycogen use was around 40 percent lower with carbohydrate than placebo (p=0.006). With the multi ingredient version the sparing was not evident (p=0.073).
Liver glycogen use did not differ between any of the three trials (p=0.101), even though post exercise plasma glucagon was lower in both carbohydrate trials (p=0.001) and plasma glucose was comparable across trials (p=0.175).
Mean power output during the later intervals was 2.8 percent higher with the multi ingredient drink (p=0.046). That came with higher plasma lactate than carbohydrate alone (p=0.003), lower bicarbonate (p=0.005), a shift in base excess (p<0.001) and lower total carbon dioxide (p=0.004).
| Muscle glycogen use, carbohydrate vs placebo | around 40% lower, p=0.006 |
| Muscle glycogen use, multi ingredient vs placebo | sparing not evident, p=0.073 |
| Liver glycogen use across trials | no difference, p=0.101 |
| Mean power output, late intervals, multi ingredient | +2.8%, p=0.046 |
| Plasma lactate, multi ingredient vs carbohydrate | higher, p=0.003 |
Proposed by the authors This is the explanation the authors offer in their discussion. This study did not test it.
The authors explain the disappearing sparing as a behavioural consequence rather than a metabolic one. Their words are that the effect is not evident with the caffeine containing blend, potentially due to an increased capacity to sustain higher power outputs resulting in greater glycogen utilisation.
In other words the fuel saving was spent rather than lost. The riders converted it into 2.8 percent more power in the later intervals.
The liver result is left unexplained. The authors note the dissociation plainly, that liver glycogen use did not differ despite lower post exercise plasma glucagon in both carbohydrate trials.
None of this was tested here. It is the authors own reading of their result, offered in their concluding sentence.
Drawn from Abstract and concluding statement, full text paywalled.
Carbohydrate during high intensity intervals looks like a muscle glycogen story rather than a liver glycogen story, which refines the usual framing that in feeding carbohydrate you are protecting hepatic output.
Nine trained male cyclists doing 8 by 5 minute intervals after a standardised day of eating.
A design that gives caffeine on its own against the same carbohydrate, and holds power output fixed, would separate whether the blend removes the sparing or the rider does.
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