After a hard workout, what does it take to get your carbohydrate stores back to normal?

A new study from researchers at the University of Bath, published in the journal Medicine & Science in Sports & Exercise, takes a look at one aspect of that question. They had subjects do two runs to exhaustion, four hours apart. Between the two exercise bouts, they either drank a low-carbohydrate or high-carbohydrate drink. As you’d expect, they reached exhaustion in the second run far earlier if they didn’t refuel with carbs; and muscle biopsies showed that in both cases, exhaustion corresponded to when their muscles reached a critically low level of glycogen (the form in which carbohydrate is stored in muscles).

That’s a fairly straightforward result. But it made me think of a minor detail from another recent study, in which cyclists did a hard interval workout in the evening, either before or after dinner, then went to bed, and did another easy ride before breakfast the next morning. This study, from Stephen Lane and his colleagues at RMIT University and a few other Australian institutions (led by John Hawley), was published in the Journal of Applied Physiology, and was looking at the effects of “training low,” i.e. with low carbohydrate stores.

I’ll have more details on Lane’s study in a future post, but I wanted to emphasize one particular figure:

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This shows muscle glycogen content, determined from biopsies at various time points, for the “fed” (ate dinner after the workout) and “fasted” (went to bed after the workout without eating anything) groups. Of course, the “fed” group has higher glycogen levels after dinner and maintains higher levels the next morning before and after the ride. (This, in turn, influences things like the rate of fat-burning.)

But what jumps out at me is that even in the group that has dinner after the workout, the muscle glycogen levels are still pretty darn low! Despite eating a full meal after the workout then getting a full night of sleep, muscle glycogen levels the next morning before the ride (REST 2) are maybe 70 percent of what they were before the previous evening’s workout.

Does that matter? For an easy run or ride, probably not. In fact, as the study’s premise suggests, periodically training with low carbohydrate stores might have some benefits in terms of stimulating greater training adaptations, though they probably need to be lower than that. It matters more if you’re hoping to do back-to-back hard efforts, like racing or working out on consecutive days.

And more generally, it’s just worth being aware that it takes time and lots of carbs to fill your muscles back up with fuel. If you’ve only got 12 hours between efforts, you won’t be back to 100 percent—and you’ll be worse off if you don’t make a deliberate effort to take in plenty of carbohydrates after the workout.

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