A main component of coffee and tea, caffeine is the most common thermogenic, showing up in most fat-burning formulas and energy drinks.
Caffeine works in several ways. It increases epinephrine (also known as adrenaline), which is a potent hormone that dismantles stored body fat. Caffeine may also act centrally by decreasing caloric intake. Another consistent effect of caffeine intake is that users perceive exercise as less strenuous, which could be due to caffeine’s role as a potent adenosine receptor antagonist. Caffeine intake also increases the use of fat during rest and exercise. In fact, moderate caffeine intake is associated with improved long-term body-weight regulation and health.
Take home: Caffeine levels peak 30–120 minutes after ingestion and boosts metabolic rate by about 13% over that time period. And despite some embellished warnings about caffeine, moderate intake is very well tolerated in healthy people. Go with 100–300 mg of caffeine in the morning and the same amount one to two hours before your workouts.