A few key studies have shown the potential of green tea to increase fat oxidation, but this has not been tested during exercise. A group of prestigious researchers from United Kingdom well known for their extensive work in studying the effects of dietary manipulations on exercise metabolism undertook a study to test whether green tea had any effect on fat oxidation during exercise. In addition they tested the effects of green tea on glucose tolerance in a separate study. In the first study, subjects performed 30 min of exercise and fat oxidation was measured. Subjects consumed green tea extract over a 24 hr period before exercise standardized to provide 890 mg total polyphenols and 13366 mg EGCG. Rates of fat oxidation during exercise were 17% higher during the green tea extract trial. In the second study the effects of green tea extract on a glucose tolerance test were evaluated. Although glucose levels were not affected, insulin responses to the glucose tolerance test were 15% lower during the green tea trial and improved insulin sensitivity. The study is important because it shows that green tea not only increases fat oxidation at rest, but also during exercise when fat breakdown and utilization are already increased. While this affect is generally attributed to a stimulatory effect on cAMP in cells, the results of this study show that lowering insulin may also contribute to the greater fat oxidation.
Venables MC, Hulston CJ, Cox HR, Jeukendrup AE. Green tea extract ingestion, fat oxidation, and glucose tolerance in healthy humans. Am J Clin Nutr. 2008 Mar;87(3):778-84.