Green tea may have some health benefits, but some people need to avoid caffeine or limit their intake, removing regular green tea as an option. This doesn't mean that you can't get some of the benefits of green tea, such as a small increase in metabolism. Decaf green tea still contains the same beneficial antioxidants as regular green tea, just in smaller amounts.
Green Tea and Metabolism
A study published in the Journal of Nutrition in February 2009 found that antioxidants in green tea called catechins helped improve the amount of abdominal fat lost through exercise in overweight people. The study authors noted that catechins can affect metabolism and increase energy expenditure and the breakdown of fat. There was some caffeine in the beverage given to the subjects, however, so that could be partially responsible for the benefits noted in this study.
Green Tea Plus Exercise
An animal study published in Molecular Nutrition & Food Research in May 2014 found that decaffeinated green tea extract appears to affect the genes that are involved in metabolism and fat creation when combined with exercise. Some research using human subjects had similar results.
A study published in the Journal of the International Society of Sports Nutrition in 2015 found that decaffeinated green tea extract helped increase the breakdown of fat and improve the body composition of people when combined with exercise.
Not all studies show a beneficial effect of green tea catechins on metabolism when caffeine isn't present. For example, a review article published in the American Journal of Clinical Nutrition in January 2010 found that green tea catechins along with caffeine, but not without caffeine, were associated with decreases in waist circumference, body weight and body mass index.
Another study, published in Medicine and Science in Sports and Exercise in June 2014, found no effect from decaffeinated green tea extract on fat breakdown or metabolism during exercise.
You have to drink quite a lot of green tea before experiencing much of a change in metabolism. The amount of green tea extract used in the Molecular Nutrition & Food Research study was equivalent to more green tea than most people would typically drink in a day.
Check with your doctor to make sure drinking more green tea would be safe for you because green tea can interact with certain medications, including antibiotics, sedatives, beta-blockers, blood thinners, heart medications, chemotherapy medications, medications to treat depression and nonsteroidal anti-inflammatory drugs.