Glycogen Dethroned …?

A couple of weeks ago I wrote Running on Fat in which I said that glycogen was king. The current paradigm can be summed up as things go better with carbs. That is before during and after. Muscle glycogen depletion during  exercise is the main factor in the onset of fatigue.  If you want to exercise again in a hurry you need to get some carbs down in a hurry. The amount and type vary from paper to paper and there is an unresolved debate about the addition of protein. Overall though it is suggested that 6 to 12 grams of carbohydrate per kilogram within 30 minutes of completing an exhausting workout should have you ready to go again the next day.

Since my muscles are not being rewarded for their efforts with a jar of marmalade after every session I have been wondering how much of a disadvantage I’m putting them at. And it’s not that easy to find out.

Some short term research has been done putting normal (high carb) athletes on low carb diets for three weeks and watching their performance suffer. Hardly surprising that it goes down hill it takes a few weeks to sort out your fluid and electrolyte balance and adjust to ketosis.

The body can make it’s own glucose from fat and if it’s starving it can turn to protein. In the absence of ingested carbohydrate does glycogen replacement grind to a halt or does gluconeogenesis step into the breach?

I was pleased to come across a paper by Volek et al describing research with 20 well matched elite athletes 10 of whom were regular high carb guys and 10 were low carbers (and had been for at least 9 months). Naturally, when you get your hands on such a group, you take muscle biopsies and put them on a treadmill for three hours, take more muscle biopsies and measure everything you can think of. Then you give them two hours to recover before taking another muscle biopsy!

The rate of fat oxidation was two to three times higher in the low carb athletes and it peaked at a higher level of effort. Glycogen stores, usage and replacement were very much the same in both groups.

Their conclusion …

Compared to highly trained ultra-endurance athletes consuming an HC (High Carb) diet, long-term keto-adaptation results in extraordinarily high rates of fat oxidation, whereas muscle glycogen utilization and repletion patterns during and after a 3 hour run are similar.

At the ultra marathon level the benefits of using fat as fuel are appealing to more and more competitors, you just don’t run out.

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