Nine trained male cyclists and triathletes rode 160 minutes at 50% of peak power twice, once drinking plenty and once drinking very little, taking in an identical 60 g per hour of labelled glucose in both trials. Finishing the ride down 2.8% of body mass, mean exogenous glucose oxidation across the 40 to 160 minute window fell from 0.61 to 0.51 g per minute, a drop of about 16% (p=0.010, dz=1.12).
Peak exogenous glucose oxidation was 0.79 ± 0.12 g/min euhydrated versus 0.70 ± 0.14 g/min dehydrated (p=0.041, dz=0.81). The gap was widest at the 60 minute mark, 0.49 ± 0.11 versus 0.33 ± 0.13 g/min (p=0.015, dz=1.52), where oxidation was a third lower in the dehydrated trial.
Dehydration was confirmed physiologically: body mass loss 2.8 ± 0.4% versus 0.2 ± 0.4% (p<0.001), post exercise plasma osmolality 302 ± 5 versus 290 ± 3 mosmol/kg (p<0.001), and a decrease in plasma volume (p=0.034).
The surprise is what did not change: gut discomfort, gastrointestinal and skin temperature, thermal sensation and intestinal fatty acid binding protein, a marker of gut barrier stress, were all unchanged between trials (all p≥0.05). The fuel was going in and simply was not being used at the same rate, without the athlete feeling anything different in the stomach.
The numbers
| Mean exogenous glucose oxidation, 40 to 160 min, euhydrated | 0.61 ± 0.09 g/min |
| Mean exogenous glucose oxidation, 40 to 160 min, dehydrated | 0.51 ± 0.11 g/min, p=0.010, dz=1.12, roughly a 16% reduction |
| Peak exogenous glucose oxidation | 0.79 ± 0.12 vs 0.70 ± 0.14 g/min, p=0.041, dz=0.81 |
| Largest single timepoint gap, 60 min | 0.49 ± 0.11 vs 0.33 ± 0.13 g/min, p=0.015, dz=1.52 |
| Body mass loss | 2.8 ± 0.4% dehydrated vs 0.2 ± 0.4% euhydrated, p<0.001 |
| Post exercise plasma osmolality | 302 ± 5 vs 290 ± 3 mosmol/kg, p<0.001 |
| Gut comfort, temperatures, I-FABP | no between trial difference, all p≥0.05 |
What this does not show
- That anyone went slower. Oxidation was measured, performance was not. The study shows less of the ingested glucose was burned, not that athletes produced less power or fatigued sooner.
- That glucose plus fructose blends behave the same. Only glucose was fed, which uses a single intestinal transporter. Whether multi transporter blends show the same penalty under dehydration is untested, and any answer is a hypothesis.
- That the penalty holds at higher intakes. 60 g per hour is a moderate dose by current practice. Whether the same proportional loss appears at 90 or 120 g per hour is unknown.
- Why it happens. Reduced splanchnic blood flow, slower gastric emptying and reduced intestinal absorption are all plausible, and the study cannot separate them. Unchanged intestinal fatty acid binding protein argues against frank gut damage but is not a mechanism.
- That this applies in heat or at race intensity. The trial ran at 50% of peak power in temperate conditions by design. It complements rather than replaces the heat stress literature, and race intensities are higher.
Caveats worth holding
- n=9, all male, all young and trained. Effect sizes are large (dz above 1.0 on the primary outcome) but this is a small mechanistic study, and confidence intervals were not reported in the abstract.
- Glucose only, single transporter. Nothing here says what happens to a glucose plus fructose blend under the same dehydration.
- 60 g per hour is a moderate dose. Whether the same proportional loss holds at 90 or 120 g per hour is untested.
- No performance outcome. Oxidation fell; nobody measured whether anyone went slower.
- The mechanism is not established. Reduced splanchnic blood flow, slower gastric emptying and reduced intestinal absorption are all plausible and the study cannot separate them.
- 50% of peak power is a steady endurance intensity. Race intensities are higher and dehydration effects may differ.
- Temperate conditions by design. This complements rather than replaces the heat stress literature, and the two should not be collapsed into one claim.
- The study did not test any drinking prescription, and individual sweat rates make any single fluid intake rule wrong for most people.