Caffeine increases maximal fat oxidation during a graded exercise test: is there a diurnal variation?
Level 2 - randomized trial
Individual randomized crossover trial
PubMed 33413459 · doi:10.1186/s12970-020-00400-6
What was done
In a triple-blind, placebo-controlled, crossover trial, 15 active caffeine-naïve men (mean age: 32 ± 7 years) completed graded cycling exercise tests under four conditions at seven-day intervals in random order: morning (8 am) with 3 mg/kg caffeine or placebo, and afternoon (5 pm) with 3 mg/kg caffeine or placebo. Maximal fat oxidation (MFO), exercise intensity eliciting MFO (Fat max), and VO2max were measured via indirect calorimetry.
What was found
MFO, Fat max, and VO2max were significantly higher in the afternoon than in the morning (all P < 0.05). Compared to placebo, caffeine increased mean MFO by 10.7% in the morning (0.28 ± 0.10 vs. 0.31 ± 0.09 g/min, P < 0.001) and by 29.0% in the afternoon (0.31 ± 0.09 vs. 0.40 ± 0.10 g/min, P < 0.001). Caffeine also increased mean Fat max by 11.1% in the morning (36.9 ± 14.4% vs. 41.0 ± 13.1%, P = 0.005) and by 13.1% in the afternoon (42.0 ± 11.6% vs. 47.5 ± 10.8%, P = 0.008).
Why it matters
This study demonstrates that acute caffeine ingestion enhances whole-body fat oxidation and aerobic capacity during exercise across different times of day, raising morning fat oxidation to levels comparable to afternoon baseline.
Limits
The sample size was small (n = 15) and restricted entirely to healthy, caffeine-naïve active men, limiting generalizability to women, habitual caffeine users, or clinical populations. The study only measured acute responses to a single graded cycling test rather than long-term training adaptations, and the trial was retrospectively registered.
Cited by
- supports Fat oxidation during exercise is approximately 13% higher in the afternoon than in the morning, but taking caffeine 30 minutes before morning exercise increases morning fat oxidation to afternoon levels.