There is a strong case to be made for making sure you consume plenty of carbohydrate before endurance training, and also during longer workouts. You will feel better and perform better, especially in harder sessions and in sessions that are begun in a prefatigued state during heavy training periods.

But there is also a strong case to be made for withholding carbohydrate before and during endurance training. These is mounting evidence that exercising with low levels of glycogen in the muscles—which is what happens when carb restriction and prolonged exertion are combined—triggers specific physiological adaptations that enhance subsequent performance.

So, then, what should endurance athletes do: consume carbs before and during workouts or withhold them? Why not both? More and more elite-level coaches and athletes and sports scientists are thinking along these lines. But the devil is in the details. Precisely howshould athletes balance high-carb and low-carb training? A new scientific paper by researchers at Liverpool John Moores University takes us a step closer to answering this question. Titled “Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis,” the paper was published on the online journal Sports Medicine in February and you can access the full text for free here.

In it, the authors propose that there exists a certain critical range of muscle glycogen concentration—specifically, 100–300 mmol/kg dw—that enables athletes to have it both ways in the specific sense that it is low enough to stimulate the above-mentioned physiological adaptations yet high also enough not to impair performance. By manipulating their carb intake before and during workouts in such a way that muscle glycogen levels end up in this range, athletes can gain the maximum benefit from every session. This requires that they consume plenty of carbs before and during their most challenging workouts and go low-carb before and no-carb during the lightest ones.

This approach differs from other “train low” protocols in a couple of respects. First, there is no distinction between fueled workouts and depletion workouts. The fueling objective for all workouts is the same: to provide the muscles with just enough carbohydrate to get the job done. Second, workouts themselves are not manipulated for the sake of achieving some particular metabolic objective. Rather, athletes who follow this approach simply train the same way they’ve always trained and tailor their fueling to the workouts.

The authors of the paper offer a hypothetical example of how their proposed “fuel for the work required” approach might work in progress. It consists of four days of training and fueling in the life of a professional cyclist, summarized in the table below.

It bears mentioning that fuel for the work required will look quite different when applied to a typical low-volume recreational endurance athlete. For this person, nearly every day would feature low to moderate carbohydrate intake, with only one or two key sessions per week requiring high carb intake before and during.

Whether rigorous application of this approach yields superior results in terms of fitness and performance for either elite or everyday athletes remains to be seen. The authors of the study cite the need for future research to rigorously quantify the “glycogen cost” of different workouts, so that the amount of carbohydrate required can be accurately calculated, and to determine how much inter-individual variation there is in the glycogen levels that elicit the desired benefits.

For my part, I’m not 100 percent convinced that athletes need to reach this glycogen level in every single workout to maximize these benefits. It won’t surprise me if it is eventually discovered that interspersing a few low-glycogen sessions into an otherwise high-glycogen regimen does the job. On the other hand, some of the research on low-glycogen training has shown beneficial effects on body composition, so it’s possible that the fuel for the work required approach could benefit athletes mainly by reducing body fat (as low-carb diets do) without compromising fitness and performance (as low-carb diets also do).

Stay tuned.