Ori Hofmekler, the author of The Warrior Diet, is a fan of having coffee instead of food prior to a workout. From the chapter on exercising he writes:
Good, fresh coffee is a wonderful natural stimulator before a workout. Caffeine, which is a strong alkaloid, may boost your metabolism up to 20%, and therefore may help the fat-burning effects. Caffeine boosts dopamine (a major brain neurotransmitter, giving you a feeling of alertness and well-being). In general, a shot or two of good espresso will do it; or a cup of, say, English Breakfast tea.
He goes on to recommend avoiding adding sugar, to prevent an insulin spike. For those that sensitive to coffee, Ori mentions substituting green tea or guarana instead.
The Warrior Diet: Switch on Your Biological Powerhouse For High Energy, Explosive Strength, and a Leaner, Harder Body by Ori Hofmekler
Guarana – Wikipedia