How Much Caffeine is Too Much?
I can't live without coffee, but when should I cut myself off?
As a number of studies have demonstrated, caffeine can increase alertness, power, and endurance and lower your perceived exertion during exercise. But, according to Louise Burke, author of Caffeine for Sports Performance, those benefits appear to top out at doses higher than three milligrams per kilogram of body weight (about 2.5 cups of coffee for a 150-pound person).
After that there’s a plateau until you consume twice that amount, at which point negative effects emerge: jitters, headaches, and irregular heartbeat. In other words, stick with the Grande.