Do you remember the huge "scandal" about green tea supplements a couple of years ago?
Dr. Oz promoted Green Tea supplements on his show for "fat loss", only to be immediately attacked by the media and scientists alike: "green tea supplements do NOT significantly increase fat burn".
This is actually true: studies have shown that drinking a lot of green tea will not have a dramatic effect on fat burning or losing weight (green tea still has many other health benefits and remains a healthy beverage), unless you go with REALLY high dosages.
BUT, this is not the end of the story!
This new study was published in the American Journal of Clinical Nutrition, and looked at the effect of combining a green tea-like beverage with cold exposure on the activity of "Brown Adipose Tissue (BAT)" in humans.
BAT is fat tissue which is "metabolically active". This means that when we are exposed to the cold, these tissues burn energy to keep us warm. This is why people living in cold climates (e.g. Alaska) have more BAT than others.
So... the study showed the following: combining the green tea-like beverage with exposure to cold (19C, so not crazy cold) led to a significant increase in total energy (calories) burned by those individuals.
This even happened from the first drink/session, and the effects increased over the 5-week duration of the study.
IMPORTANT NOTE: the beverage used in the study contained over 600mg of catechins (polyphenols) per cup. This is substantially higher than most teas available in the market, so supplementation may be needed to get enough catechins to reach that effect.
Bottom Line: Green Tea (extract in high enough doses or 3-4 cups/day) does seem to trigger a significant increase in metabolic activity and calorie burning, but only when combined with exposure to cold temperatures (19C, so you don't need to jump into an ice bath!).