Surprisingly, green tea is one of a select few supplements that actually has a significant body of clinical evidence validating its effects for dieters.
For instance, one study showed it helped aid weight loss by increasing the metabolic rate, causing those who use it to experience greater calorie burn (1). Other similar studies (2) concurred with this conclusion.
Another (3) showed that green tea appears to inhibit the activity of alpha amalyze, an enzyme required for the breakdown of starches (carbs), that can cause blood sugar levels to soar following a meal.
It also appears to inhibit fatty acid synthase (4), an enzymatic system that is involved in the process of turning carbohydrates into fat (early animal studies suggest the inhibition of fatty acid synthase can lead to dramatic weight loss).
If that weren't enough, further data indicates consuming green teas rich in catechins lowers cardiovascular risks in addition to fighting obesity (5).
So does this mean we're on the cusp of discovering the newest weight loss miracle?
Hardly. As always, the devil is in the details, especially when it comes to studies like these.
For instance, you were probably excited to hear that green tea does indeed elevate the metabolic rate, but you are probably going to be less excited to learn that the amount it does so (4% according to the study) equates to about 100 or so calories per day for the average individual.
When you consider that a pound of stored fat is the equivalent of 3500 calories and that you'd need to drink green tea for a minimum of 35 days to burn the equivalent amount of calories in a single pound, you begin to realize the reality...
... green tea is helpful, but it's not helpful, if you know what I mean. Especially if you've got 20 pounds or more to lose.
In other words, it does offer, real, documented benefits, but those benefits are subtle at best; it is not going to make a dramatic difference to the success of your weight loss plan. If you're not counting your calories and making an effort to exercise and add some lean muscle, don't expect green tea to be your saviour.
Of course, that doesn't mean green tea isn't a fantastic addition to your diet; with its potent antioxidant action and numerous documented benefits, you really can't go wrong drinking a couple of cups of this stuff a day.
But it's not a weight loss miracle, nor a "lose 10 pounds in 10 days" solution.
1) Am J Clin Nutr. 2000 Nov;72(5):1232-4.
2) Am J Clin Nutr. Vol. 81, No. 1, 122-129, January 2005
3) Am J Clin Nutr. 2007 Apr;85(4):1164
4) Int J Cancer. 2003 Oct 10;106(6):856-62
5) Obesity (Silver Spring). 2007 Jun;15(6):1473-83.