REF-1749 · Villalba et al., 2026 · International Journal of Sports Physiology and Performance, ahead of print: 1-10 · Paper · PubMed

In 14 endurance-trained runners, 3 mg/kg of caffeine taken 60 min before a 1-h outdoor run at their Fatmax pace raised fat oxidised over the hour (40.46 vs 33.62 g with placebo) and lowered carbohydrate oxidised (77.32 vs 93.16 g) at unchanged energy expenditure and heart rate, and RPE was a point lower (8 vs 9 on the Borg scale; its range is not given in the abstract); one small double-blind crossover read from its abstract, with fuel use a marker and no performance measured

Main caveat: One crossover of 14 (3 women); fat and carbohydrate oxidation estimated from gas exchange are markers, not outcomes.
limited, mixed evidenceMeasured on trained athletesRead from the abstract only

Villalba and colleagues (Del Coso group) asked whether caffeine's increase in fat oxidation, shown mostly in laboratory studies, also appears during a field run.

Fourteen endurance-trained runners (11 men, 3 women; VO2max 58.6 ± 6.3 mL/kg/min) took 3 mg/kg of caffeine or a placebo, double-blind and in random order, 60 min before running for 1 h at their individual Fatmax intensity on a 1.97-km outdoor course while carrying a portable gas analyser.

With caffeine, total fat oxidised over the hour was higher (40.46 vs 33.62 g; P = .018; d = 0.76) and total carbohydrate oxidised lower (77.32 vs 93.16 g; P = .036; d = 0.66), with no change in total energy expenditure (P = .936). RPE was lower with caffeine (8 vs 9; P = .024; d = 0.75), and heart rate did not differ (P = .140).

The authors conclude that 3 mg/kg of caffeine increases fat oxidation during an hour of running at Fatmax in field conditions.

Participants

n = 14 healthy endurance-trained runners, 11 men and 3 women; VO2max 58.6 (6.3) mL/kg/min.
Age, habitual caffeine intake, training volume and competition level: not in the abstract.

Protocol

Preliminary trial: VO2max and the intensity of maximal fat oxidation (Fatmax).
Two experimental trials, randomised, double-blind: caffeine 3 mg/kg or placebo, 60 min before a 1-h run at the individual Fatmax intensity on a 1.97-km outdoor course.
Measures: substrate oxidation and energy expenditure by portable indirect calorimetry and stoichiometric equations; heart rate throughout; RPE on the Borg scale.
Not in the abstract: how pace was held at Fatmax outdoors, weather, pre-trial diet and fasting, habitual caffeine use, washout, the Fatmax test protocol, and whether participants guessed their condition.

Key Results

Outcome over the 1-h runPlaceboCaffeinePd
Total fat oxidised, g (mean [SD])33.62 [9.23]40.46 [11.32].0180.76
Total carbohydrate oxidised, g93.16 [37.84]77.32 [30.35].0360.66
Total energy expenditurenot givennot given.936—
RPE, Borg scale (range not stated)9 [2]8 [2].0240.75
Heart ratenot givennot given.140—
Our arithmetic: about 6.8 g more fat (roughly a fifth more) and about 15.8 g less carbohydrate over the hour.

Practical Application

A usual dose of caffeine (3 mg/kg, an hour before) shifted an easy outdoor run toward fat and away from carbohydrate and made it feel about a point easier on the Borg scale, at the same energy cost and heart rate. One small crossover read from its abstract; fuel use is a marker, and the trial measured no performance.
For [SYNTH-39]: caffeine is an ordinary reason for a session's RPE to sit below its power- or pace-based score; here heart rate did not move while RPE fell. The guide's list of causes for a low sRPE names caffeine beforehand among favourable conditions, citing this trial; the effect measured was in-session Borg RPE, not session RPE.
For [SYNTH-67]: nothing here changes the guide's caffeine advice, which rests on performance; this trial adds a substrate effect on an easy run, not a reason to take caffeine before easy sessions.
Inference, not tested here: a shift of about 7 g of fat in an hour does not show a training benefit, a body-composition change, or a sparing of glycogen that later improves a race; and an athlete who logs a low RPE on a caffeinated easy run is not necessarily fitter.

Limitations

One crossover of 14 (3 women); fat and carbohydrate oxidation estimated from gas exchange are markers, not outcomes.
No performance test; the 1-point RPE difference is on a Borg scale whose range the abstract does not give, with SDs of 2 points.
Field conditions (weather, course, pacing control) and pre-trial diet are not in the abstract.
Habitual caffeine intake and whether blinding held are not in the abstract.
Written from the abstract and the PubMed record; full text not read.
How this entry was checked and changed
Why this level

a crossover of 14 (the rubric's B band) taken one step down because its main outcome, substrate oxidation, is a marker; it is also read from the abstract only, so the cap for an abstract-only small study is met either way