Objectives: To investigate whether coinciding peak serum caffeine concentration with the onset of exercise enhances subsequent endurance performance.

Design: Randomised, double-blind, crossover.

Methods: In this randomised, placebo-controlled, double-blind crossover study, 14 male trained cyclists and triathletes (age 31 ± 5 year, body mass 75.4 ± 5.7 kg, VO2max 69.5 ± 6.1 mL kg−1 min−1 and peak power output 417 ± 35 W, mean ± SD) consumed 6 mg kg−1 caffeine or a placebo either 1 h (C1 h) prior to completing a 40 km time trial or when the start of exercise coincided with individual peak serum caffeine concentrations (Cpeak). Cpeak was determined from a separate ‘caffeine profiling’ session that involved monitoring caffeine concentrations in the blood every 30 min over a 4 h period.

Results: Following caffeine ingestion, peak serum caffeine occurred 120 min in 12 participants and 150 min in 2 participants. Time to complete the 40 km time trial was significantly faster (2.0%; p = 0.002) in C1 h compared to placebo. No statistically significant improvement in performance was noted in the Cpeak trial versus placebo (1.1%; p = 0.240). Whilst no differences in metabolic markers were found between Cpeak and placebo conditions, plasma concentrations of glucose (p = 0.005), norepinephrine and epinephrine (p ≤ 0.002) were higher in the C1 h trial 6 min post-exercise versus placebo.

Conclusions: In contrast to coinciding peak serum caffeine concentration with exercise onset, caffeine consumed 60 min prior to exercise resulted in significant improvements in 40 km time trial performance. The ergogenic effect of caffeine was not found to be related to peak caffeine concentration in the blood at the onset of endurance exercise.