How much caffeine should I take before a marathon?

The research-backed dose for endurance performance is 3 to 6 mg of caffeine per kilogram of body weight, taken 45 to 60 minutes before the start. For an 70 kg (154 lb) runner, that is 210 to 420 mg. Most athletes do well at the lower end of that range, around 3 mg/kg, which reduces the risk of jitteriness, GI distress, or a mid-race crash. If you regularly consume caffeine, your habitual dose matters too: a habitual drinker of two to three cups of coffee daily may need to approach the higher end to get the same effect as a non-consumer at the lower end.