Mixed
Ingesting 6 mg/kg of caffeine either 1 hour before or distributed throughout endurance exercise enhances time trial performance by approximately 3%, regardless of timing.
For endurance events, taking 6 mg of caffeine per kg of body weight is effective for boosting performance. You can take this entire dose 1 hour before starting, or split it into small doses (1 mg/kg) every 20 minutes during the event. Both methods result in roughly a 3% improvement in time trial performance. If you prefer, drinking Coca-Cola (decaffeinated or regular) during the last part of your event can also provide a similar boost, likely due to the caffeine and carbohydrate content.
Overall, 1) 6 mg/kg caffeine enhanced TT performance independent of timing of intake... Improvements in TT were as follows: Precaf, 3.4%... Durcaf, 3.1%...
Why this rating
Randomized, double-blind, placebo-controlled crossover design with highly trained athletes, though sample size is small (n=12 and n=8).
Source
Effect of different protocols of caffeine intake on metabolism and endurance performance
Gregory R. Cox et al. · Journal of Applied Physiology · 2002
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