Caffeine
Evidence: strong
The most reliable legal performance aid: 3-6 mg/kg about an hour before, working chiefly by lowering perceived effort. AIS Group A.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
Caffeine is the most reliably effective legal performance aid in endurance sport, classed Group A by the Australian Institute of Sport and supported by an ISSN position stand (Guest et al. 2021). It works chiefly through the brain: by blocking adenosine receptors it lowers perceived exertion, so a given pace feels easier (Guest et al. 2021).
The effective dose is 3 to 6 mg per kg of body mass, roughly 200 to 400 mg, taken about an hour before exercise; caffeinated gum absorbs faster and can be taken closer to or during the effort (Guest et al. 2021). The benefit plateaus by around 3 to 6 mg/kg; higher doses, up to 9 mg/kg and beyond, add side-effects without adding performance, so there is no reason to exceed the usual range (Guest et al. 2021).
The effect size depends on how performance is measured. Caffeine reliably extends open-ended time to exhaustion, by around 17% in one meta-analysis of running (Wang et al. 2022), and improves time-trial performance by roughly 2 to 3% across endurance modes (Southward et al. 2018). Its effect on fixed-distance running time-trials specifically is smaller and less certain.
Some questions remain unsettled. Whether habitual users lose the benefit is genuinely mixed in the literature, and whether CYP1A2 genotype determines who responds, with fast metabolisers gaining more, has support in some trials and none in others, so it is contested rather than established.
Practical use
Getting the dose right
For a 70 kg runner, 3 to 6 mg/kg is roughly 200 to 400 mg, about two cups of coffee or a caffeine tablet, taken around 60 minutes before the start. Caffeinated gum or gels absorb faster and suit topping up late in a long race. More is not better: above about 6 mg/kg you mostly add jitters, a racing heart and gut upset without extra benefit. Trial your dose and timing in training, as sensitivity and gut tolerance vary widely between people.