Answer
A few studies indicate that caffeine may slightly enhance weight loss in people who exercise and maintain a low-fat diet. But there's no evidence that increased caffeine intake results in significant or permanent weight loss.
Marketers of fad diets and weight-loss supplements often exaggerate the benefits of caffeine, claiming that caffeine will significantly curb your appetite and help you drop pounds quickly. However, clinical studies on the relationship between caffeine and weight loss don't support these claims.
Here's what researchers have found:

• Caffeine may act as an appetite suppressant. However, while studies have shown that some people eat less after consuming caffeine, this effect is brief — not long enough to lead to significant weight loss.
• Caffeine may increase your body's ability to burn calories. Caffeine is thought to stimulate thermogenesis — one way your body generates heat and energy from digesting food. However, caffeine's role in thermogenesis probably isn't enough to result in significant weight loss.
• Caffeine acts as a diuretic, which means it causes an increase in the amount of urine you excrete. This water loss may temporarily decrease your body weight.

It's important to keep in mind that caffeine is a stimulant that can increase your heart rate and blood pressure, interrupt your sleep, and cause nervousness and irritability. Also, many caffeinated beverages are high in calories, which can contribute to unwanted weight gain.