REF-1121 · Maunder et al., 2026 · Journal of Applied Physiology
Accelerating endogenous carbohydrate oxidation impairs durability of the moderate-to-heavy intensity transition
Maunder et al. made two hours of moderate riding burn more of the body's own carbohydrate, without changing the work done, and measured what that did to thresholds afterwards.
Eleven trained cyclists (8 men, 3 women) were tested four times in random order: rested (CON), rested after the intervention (CON+NH), after 2 h at 90% of VT1 (EX), and after the same 2 h with the intervention (EX+NH). The intervention was 500 mg of niacin (which blocks fat release from fat tissue) plus mild heat pads on the thighs (warmer muscle burns glycogen faster).
Plain water only during the ride. In EX+NH carbohydrate burned in the ride rose ~19% (250 ± 77 vs 214 ± 65 g). Power at VT1 fell ~7% after the ride alone and a further ~7% when more carbohydrate had been burned (~13% vs rested).
End-test power of the 3-min all-out test (the CP estimate) did not differ between trials; W′ was ~25% lower after the ride (EX vs CON alone P = 0.063), with no extra loss from the intervention. The intervention did nothing to rested thresholds.
Conclusion: faster use of the rider's own carbohydrate deepened the slide of the first threshold in a long ride (the authors: VT1 durability is "sensitive to" it); the CP estimate and W′ did not detectably respond to this dose.
Protocol
Randomised crossover; four experimental trials 9 ± 7 days apart after a characterisation visit. 11 endurance-trained cyclists (eight male, three female); one dropped out after three trials (injury unrelated to the study), so VT1 in EX+NH is N = 10.
Before each trial: a meal with ~1 g/kg carbohydrate ~1 h before arrival, no vigorous exercise for 24 h and no alcohol or caffeine for ~18 h (the paper also says caffeine 24 h), a replicated 3-day diet and training record.
Prolonged phase: 2 h cycling at 90% of the VT1 power from the characterisation trial (EX, EX+NH) or 2 h seated rest (CON, CON+NH). Plain water ad libitum; no carbohydrate during the 2 h.
Intervention (NH): 500 mg niacin ~15 min before cycling (on arrival in CON+NH) plus wired heat pads on the upper legs through the 2 h; the authors previously measured ~2.5°C higher estimated muscle temperature with this method. The niacin flush had resolved in all cases before the threshold tests.
Tests after the 2 h: a five-step incremental test (25 W every 4 min, from 50 W below to 50 W above the estimated VT1) for power at VT1, then ~15 min later a 3-min all-out test: end-test power (mean of the last 30 s, CP3MAOT) as the heavy-to-severe transition and work above it as W′.
Substrate oxidation from gas exchange; plasma glycerol, NEFA, glucose and lactate at 40, 80 and 120 min. Substrate data are N = 7 (a VCO2 fault in four participants).
Key Results
| Outcome | Result (full text) |
|---|---|
| Carbohydrate oxidised in the 2 h | 250 ± 77 g in EX+NH vs 214 ± 65 g in EX, ~19% more, dz = 1.00, P = 0.038 (N = 7) |
| Fat oxidised in the 2 h | 68 ± 24 vs 76 ± 18 g, ~12% less, P = 0.082 (not significant) |
| Energy expended in the 2 h | not different between EX and EX+NH (P = 0.448) |
| Plasma glucose | not different between trials (P = 0.558) |
| Plasma NEFA / lactate | NEFA lower in EX+NH at 80 min; lactate higher in EX+NH at 40 min |
| Power at VT1 | CON 233 ± 63 W; EX 218 ± 62 W (~7% lower, P = 0.002); EX+NH 195 ± 58 W (N = 10), ~7% below EX (P = 0.007) and ~13% below CON (P = 0.002); CON vs CON+NH not different |
| CP3MAOT (heavy-to-severe) | no main effect of trial (P = 0.062); no two trials differed; EX+NH vs CON ~7%, P = 0.097 |
| W′ | main effect of trial (P = 0.001); ~25% lower after the ride (authors' summary). EX+NH below CON (P = 0.032) and CON+NH (P = 0.005); EX below CON+NH (P = 0.015) but EX vs CON P = 0.063; no difference between EX and EX+NH |
| Why VT1 fell (calculated, N = 7) | metabolic power fell similarly in EX and EX+NH (−28 ± 20 vs −26 ± 14 W, P = 0.855); efficiency added +15 ± 15 W in EX but −6 ± 11 W in EX+NH (P = 0.010) — the extra loss came from efficiency |
The results text labels the VT1 values "233 ± 63 vs. 218 ± 62 W in EX vs. CON"; the surrounding text (a ~7% REDUCTION after prolonged exercise) makes CON 233 W and EX 218 W, which is how they are given here. The ordering confirms it: EX+NH is "a further ~7%" from EX but "~13%" from CON, which fits only if EX is the lower of the two.The percentages are the authors'; the raw means give −6%, −11% and −16%, presumably because the percentages are paired or from the N = 10 subset (not stated).
Mechanisms
Measured: more carbohydrate oxidised in the ride (N = 7). The fat-oxidation rate had a trial effect, but total fat was not significantly lower (P = 0.082). Energy expenditure (N = 7) and plasma glucose did not differ. Authors' reading: the extra VT1 loss is therefore not due to more total work or lower glycaemia.
Calculated (a model splitting the VT1 loss into metabolic power and efficiency; N = 7): the extra loss came through efficiency. After the plain ride efficiency improved (+15 ± 15 W) and partly offset the fall in metabolic power; after the faster-burn ride it did not (−6 ± 11 W, P = 0.010). Measured alongside: energy expenditure during the incremental test was higher in EX+NH than EX at four of five steps.
Authors' interpretation, not measured (no biopsies, no liver data): faster muscle glycogen use, plausibly in type I fibres, forcing recruitment of less efficient type II fibres; faster liver glycogen use is also possible. The study cannot say whether muscle or liver glycogen carried the effect.
Authors' interpretation: CP and W′ may respond only when glycogen falls below some level, or when the ride is harder or longer (a heavy-domain 2 h with 60 g/h in Clark 2019 REF-145; a 3-h moderate ride in Norte 2026, cited by the authors) — not tested here.
Practical Application
The first threshold slid further when the rider burned more of their own carbohydrate. The authors call this the first direct evidence that the body's own carbohydrate has a mechanistic role in VT1 durability. Same external work, similar plasma glucose, ~19% more carbohydrate burned (N = 7) → nearly twice the VT1 loss (~13% vs ~7%).Their inference, not tested here: at least part of why carbohydrate in the ride protects VT1 (REF-181) is that it spares the body's own stores.
Late in a long ride, aerobic-ceiling watts are not the same watts. The upper limit of easy riding after 2 h unfuelled was ~7% lower than fresh, and ~13% lower when the body's own carbohydrate had been burned faster (glycogen not measured). Our inference: zone caps for the later part of long sessions should come down, not stay at the rested number (SYNTH-32).
Burning the body's own carbohydrate faster cost VT1 durability here (not CP or W′). The levers were niacin, which limits fat supply, and local thigh heating in an 18°C lab. Inference, not tested here: a hot environment and a low fat-oxidation capacity could act the same way; riding unfuelled is the baseline both trials shared, and its cost is shown by REF-181.
Not from this paper: the authors suggest a greater capacity to burn fat (from endurance training) might improve durability; they did not test it, and the base does not support deliberately under-fuelled long rides as a durability tool (SYNTH-32).
Grams per hour come from the ladder (SYNTH-02), not from this study — it gave no carbohydrate in the ride.
Limitations
n = 11 trained cyclists, laboratory, one 2-h moderate ride at 90% of VT1; substrate oxidation on only 7 riders (achieved power ~57% for the carbohydrate difference), so the ~19% is imprecise.
Glycogen was never measured; "faster glycogen use" is inferred from gas exchange, lactate and the known effects of niacin and heat.
Niacin plus thigh heating is a laboratory tool, not a race situation; it may act through routes other than carbohydrate, although it changed nothing in the rested trials.
No carbohydrate in the 2 h, after a ~1 g/kg meal: the size of the slides applies to an unfuelled ride, not a fuelled race.
The CP null comes from a protocol in which no two trials differed, not even EX vs CON (main effect P = 0.062; EX+NH vs CON ~7%, P = 0.097). It does not show CP is immune to glycogen; the authors suggest a heavier or longer ride is needed. For W′ the ride did lower it, but the intervention added nothing at this ~19% dose.
No placebo or sham; the niacin flush made the condition obvious. VT1 is gas-exchange based, but the 3-min all-out test depends on effort.
Accepted manuscript: numbers and wording may change in the issued version.
How this entry was checked and changed
Why this level
randomised crossover in 11 trained cyclists; substrate data on n=7 only; read from the full text (accepted manuscript, final wording may change)
Full text (checked 2026-10-13)
Eleven endurance-trained cyclists (eight male, three female); one completed three of four trials (full text).
Carbohydrate oxidised in the prolonged phase 250 ± 77 vs. 214 ± 65 g, dz = 1.00, P = 0.038; fat 68 ± 24 vs. 76 ± 18 g, P = 0.082 (full text).
VT1 195 ± 58 W in EX+NH (N = 10), a further ~7% vs EX (P = 0.007), ~13% vs CON (P = 0.002) (full text).
Metabolic power −28 ± 20 vs. −26 ± 14 W (P = 0.855); efficiency contribution 15 ± 15 vs. -6 ± 11 W (P = 0.010); calculated, N = 7 (full text).
W′ EX+NH below CON (P = 0.032) and CON+NH (P = 0.005); EX vs CON P = 0.063; CP3MAOT no main effect (P = 0.062) (full text).