Green and black teas both contain compounds that may slightly boost weight loss, and scientists have yet to conduct a major trial that confirms a clear winner. No matter what you drink, however, the best way to lose weight is to eat a sensible diet and get plenty of exercise. Once you've made these key lifestyle changes, tea may help enhance your results.
Black and green teas are both made from leaves of the Camellia sinensis plant, so the only difference lies in processing. Green tea leaves are air-dried to wither, and then steamed or pan-fired. Black tea leaves are withered, rolled and left to fully ferment, after which firing occurs. This fermentation process reduces levels of antioxidants called catechins, which are associated with weight loss. Black tea, however, is higher in caffeine, which has also been shown to help control weight.
The Skinny on Catechins
Researchers tested the effect of catechins on weight loss in a study published in "Obesity" in 2007. During the trial, one group of subjects was given green tea with 583 milligrams of catechins, while the other group was given green tea with just 96 milligrams of catechins. At the end of the 12-week study, the high-catechin group lost more weight and body fat and also experienced a greater reduction in waist size. This indicates that higher catechin levels within the same tea variety are linked to increased weight loss but doesn't necessarily prove that green tea is more effective than black tea.
Significance of Caffeine
In a study published in the "American Journal of Clinical Nutrition" in 2006, researchers followed subjects over 12 years and analyzed their caffeine intake. They found that those who increased caffeine consumption by drinking coffee or tea tended to gain slightly less weight over time than those who decreased caffeine consumption. It's unclear whether caffeine outranks catechins as a weight-management aid, however, so even with these findings it's impossible to hand black tea the crown.
Caffeine in high doses can cause upset stomach, anxiety, heart palpitations and dizziness, so it's unwise to start guzzling green or black tea in an attempt to lose more weight. Caffeine content varies, but one major brand's black tea contains 55 milligrams per serving, and its green tea contains 35 milligrams of caffeine per serving. Consuming 200 to 300 milligrams of caffeine per day is considered moderate, but if you're sensitive to caffeine it's possible to experience side effects with much less.