I found this explanation on a Weight Watchers site:
Q: What is the story on caffeine? Does it affect weight loss?
A: I've read conflicting information on this topic in a wide variety of publications. Those studies based on sound science, though, seem to show little – if any – true correlation between caffeine consumption and weight loss.
Whereas caffeine can raise your heart rate, most research has shown that it doesn't cause enough of a rise in metabolism to make a difference in your body's ability to burn calories. Other research has shown a slight appetite suppression due to caffeine intake but again, the effects are deemed too insignificant to have an impact on your weight.
One other area of science to consider is the effect of caffeine on hydration status. Although caffeine is a diuretic and can cause you to lose extra water through increased urine output, it's only a temporary loss of water weight, not body fat. Plus, a dehydration effect usually only occurs when you consume more than 4 cups of caffeinated beverages in a day.
That said, how you ingest caffeine is a matter of importance. There's a big caloric difference between one lump of sugar in your coffee versus three -— not to mention fancy coffee store add-ins like caramel syrup and chocolate shavings. Indulge in too many of these extras and you can certainly affect your weight loss -— in the wrong direction.