Caffeine is the most common psychoactive substance in the world. It is found naturally in coffee, green tea, and dark chocolate and is added to many processed foods and drinks. Caffeine is a known metabolism booster and is often added to commercial weight loss supplements.
How it works: Short-term studies have shown that caffeine can boost metabolism by 3 to 3 percent and increase fat burning by 29 percent.
Efficacy: Some studies also show that caffeine can reduce average body weight in humans.
Side effects: In some people, too much caffeine can cause anxiety, insomnia, tremors, irritability, nausea, diarrhea, and other symptoms. Caffeine is also addictive and can reduce the quality of your sleep. There is no need to take caffeine supplements or tablets with them. The best sources of quality coffee and green teas are antioxidants and other health benefits.