What the research found
Recent work on carbohydrate fueling during endurance exercise reveals a more nuanced picture than "more carbs = preserved glycogen = better durability." Researchers examined durability—how well your physiological capabilities hold up as exercise duration increases—through three angles.
One study deliberately accelerated internal carbohydrate burn in trained cyclists using niacin and mild muscle heating during two-hour rides. When cyclists burned through their own carbohydrate faster, their power at the ventilatory threshold (a breathing-based marker of exercise intensity capacity) declined roughly twice as much compared to normal conditions. The decline appeared linked to metabolic efficiency: the body couldn't convert expended energy into pedal power as effectively, possibly because depleted slow-twitch muscle fibers forced recruitment of less-efficient fast-twitch fibers.
A separate study had trained cyclists consume either 60 or 100 grams of carbohydrate per hour during four-hour rides with intervals, followed by a 15-minute time trial. Power output did decline in both groups, but the higher intake reduced that decline from 9.3% to 6.4%—a meaningful but incomplete preservation. Muscle biopsies showed substantial glycogen depletion in both conditions despite the different carbohydrate intakes, suggesting the performance benefit wasn't explained by "sparing" stored glycogen. Instead, whole-body carbohydrate oxidation (the total carbs being burned from all sources) was better sustained with higher intake.
Why it matters for you
If you're tracking training durability and periodising your fueling around long efforts, these findings reframe what carbohydrate availability actually does. The old model—eat carbs, preserve glycogen, maintain power—doesn't fully capture the mechanism. What appears to matter more is whether your body can sustain carbohydrate oxidation (burning available fuel) throughout the effort, which depends on both what you consume and what your body supplies.
For practical MyKine users, this suggests your fueling decision isn't simply "higher is always better." The 60 vs. 100 gram study shows a dose response, but both still caused measurable fatigue. If you're monitoring biomarkers like blood glucose stability during long sessions, the research hints that maintaining consistent carbohydrate availability may support metabolic efficiency—the conversion of effort into actual work—more than it protects internal glycogen stores.
The implication for endurance protocols: the quality of your durability may depend partly on fueling logistics that maintain steady carbohydrate oxidation rather than on absolute carbohydrate intake alone. If you're experimenting with different fueling amounts, pay attention to how your power output or pace deteriorates, not just whether you "feel" depleted.
Caveats
- Small sample sizes: 11 cyclists in the first study, 15 in the second; both well-trained, limiting generalisability to recreational athletes or other sports
- Indirect measurement: The first study estimated carbohydrate oxidation changes using an intervention (niacin + heating), not direct measurement
- Controlled conditions: laboratory settings with fixed intensities and intervals may not capture real-world variability in pacing and effort distribution
- Limited scope: findings are specific to durability at submaximal intensities; critical power (the higher boundary of severe exercise) showed no difference in one condition, suggesting effects aren't universal across all physiological measures