The ritual is obvious if you walk past any trailhead at six in the morning. Stretching a calf against a bumper, runners wait for the caffeine to arrive while standing by their cars with tiny paper cups. Many will tell you the same thing when you ask why: it stores your glycogen for later.
That assertion is widespread. It is repeated by running brands, and the reasoning seems clear. Caffeine encourages the body to burn fat, leaving more carbohydrates available for the last few miles. It’s a compelling tale. Additionally, it is not entirely supported by the better evidence.
A synopsis based on research connected to McMaster suggests otherwise. Twenty men who were loaded with glycogen were given either a placebo or six milligrams of caffeine per kilogram approximately ninety minutes prior to two hours of cycling. The conclusion was that muscle glycogen was not spared by the caffeine. Instead, it appeared to alter the intensity of the effort and the nervous system’s reaction to it.
Compared to fuel savings, that benefit is quieter but perhaps more plausible. Adenosine, a chemical that aids in alerting the brain to fatigue, is blocked by caffeine. When you run long enough, the voice telling you to “stop” becomes a little softer, but the legs still hurt. It’s possible that runners are experiencing this rather than biochemistry in the muscle.
That reading is consistent with a Taiwan trial from 2024. After 90 minutes of cycling in a high-altitude chamber simulation, eight young men were pushed to exhaustion at a much faster pace. Caffeine users outlasted placebo users by about 44%, lasting 434 seconds as opposed to 246. However, there was no discernible difference in fat burning between the caffeine and placebo groups. Instead of a fuel switch, the authors themselves proposed that blunted fatigue and pain were likely the cause.
Eight individuals is a small group, and no one took a direct measurement of glycogen. The researchers acknowledged this. Therefore, there is a limit to how much one should rely on it. Even so, it’s difficult to ignore how frequently the widely accepted explanation outpaces the facts.
The other thing that is lost is dosage. The 6 milligrams per kilogram used in those studies equates to roughly 450 milligrams for a man weighing 76 kg. A shot of espresso contains about 64 milligrams. It would take seven shots to match the lab, which sounds more like a bad afternoon than a performance strategy. Approximately 3 to 6 milligrams per kilogram should be taken 30 to 60 minutes prior to the start, according to practical advice from running-industry sources.
That gap is important. Many runners who are waiting at the trailhead with a single shot are receiving, at best, a gentle prod and, at worst, a pleasant routine. Neither is a negative result. On a chilly morning, a tiny cup can help you concentrate better, and concentration is important.
There are drawbacks as well. In the middle of a run, coffee on an empty stomach can cause cramps and jitters, and a late-afternoon dose can subtly ruin the sleep that is essential for healing. This is something that many runners discover the hard way, usually around mile five.
Does the pre-run espresso conserve glycogen? Most likely not in any way that a muscle biopsy would reveal. Is it beneficial? Yes, frequently, but via a different path than the one that people give credit to. Continue drinking from the cup if it suits you. Just quit convincing yourself that it’s a fuel-saving tactic.
