Coffee can help people feel less tired and increase energy levels. That’s because it contains a stimulant called caffeine — the most commonly consumed psychoactive substance in the world. After you drink coffee, the caffeine is absorbed into your bloodstream. From there, it travels to your brain. In the brain, caffeine blocks the inhibitory neurotransmitter adenosine. 

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Caffeine is found in almost every commercial fat-burning supplement — and for good reason. It’s one of the few natural substances proven to aid fat burning. Several studies show that caffeine can boost your metabolic rate by 3–11%. Other studies indicate that caffeine can specifically increase fat burning by as much as 10% in obese individuals and 29% in lean people. However, it’s possible that these effects diminish in long-term coffee drinkers.