Though the many benefits of green tea have been known in Asian cultures for centuries, most research on tea by Western scientists didn’t begin until the mid-1980s. Research is by nature time-consuming and expensive, with many false starts and dead ends. Some research projects take years, publishing the results can take many months and follow-up research can stretch into decades. By Western scientific measure, research on green tea is in its infancy.

The first mention of tea as an aid to losing weight was recorded centuries ago by the Chinese, usually referring to Pu-erh tea. Studies published in the mid-80s garnered a little attention, but it wasn’t until the late 1990s that Western scientists started to seriously look at green tea.

The University of Geneva Study

A 1999 article published in the American Society for Clinical Nutrition began to generate interest in green tea as a possible aid in losing weight. Though tea has been widely consumed throughout the world for centuries, little scientific study had been reported on its effects in humans. A team of scientists from the University of Geneva (Switzerland), interested in plant ingredients that might affect weight loss in humans, wanted to see if the caffeine and catechin polyphenols (natural compounds in green tea) would make a difference in the energy expenditure (metabolism) and fat oxidation (fat burning) in humans. Since the Geneva study was the major catalyst for interest in green tea and continues to generate additional studies, let’s look at how the research was done and what was learned.

Researchers recruited ten healthy men who were classified as lean to mildly obese (8-30% body fat) to participate in the research study. They did not include anyone who smoked, who was a competitive athlete or who had a history of weight loss. They wanted men who regularly consumed a typical Western diet, one with fat contributing 35-40% of dietary energy. Each man’s physical characteristics were noted, including height, weight and percentage of body fat.

On three separate occasions each subject spent 24 hours in a respiratory chamber. They were randomly assigned one of three treatments: 1) two capsules of green tea extract which contained 50 mg of caffeine and 90 mg of epigallocatechin gallate (EGCG), a powerful antioxidant found in tea leaves, as well as other ingredients naturally found in green tea; 2) two placebo capsules filled with cellulose, which has no effect on the body; or 3) two capsules with 50 mg of caffeine. The caffeine capsules contained no additional ingredients.

The dietary energy intake (number of calories) and diet composition for each man were identical and were categorized as weight-maintenance. The food-energy content was about 13% protein, 40% fat and 47% carbohydrates. No other foods or drinks with caffeine were allowed for 24 hours before or during the time in the respiratory chamber. The men’s sedentary lifestyles, physical activity, meal patterns and sleep times were similar during the testing periods. The men read, watched television or listened to the radio while in the respiratory chamber. They did not exercise. There were three 24-hour testing periods for each man over a period of 5-6 weeks.

The results

Using respiratory and urinary testing, the scientists found that the men who had been given the green tea capsules had a four-percent increase in energy expenditure in 24 hours. Caffeine is known to affect metabolism but in this study the caffeine alone had no effect on the subjects, perhaps because the dosage wasn’t high enough to stimulate metabolism. The subjects who got the caffeine capsules consumed a total of 100 mg of caffeine at each meal, less than the caffeine in an average cup of brewed coffee. The same amount of caffeine was in the green tea capsules but the caffeine was not responsible for the increased metabolic effects. The scientists speculated that the caffeine and the components of the green tea, working together, caused the increased metabolism and fat oxidation. Decaffeinated tea also creates weight loss but scientists cannot agree on whether it would have as much of an effect on metabolism and fat oxidation as would the caffeine form. However, research continues.

The scientists also learned that the men given the capsules that contained green tea extract and caffeine did not experience increased heart rates, making it a safer alternative than other drugs used to help lose weight, especially in people with high blood pressure or other cardiovascular problems.

So, to simplify and translate, the men who got the green tea capsules burned an extra 80 or so calories in a 24-hour period without exercising and without changing their normal diets.

Other studies

Another study reported in the online edition of the January 2005 American Journal of Physiology: Regulatory, Integrative and Comparative Physiology showed that mice that had been given green tea extract over a ten-week period showed increased exercise endurance of 8-24% more than mice who exercised but were not given the extract. What’s more, they found the mice given the extract burned fat more efficiently. Scientists reduced the amount of caffeine in the extract for the experiments and concluded that caffeine was not a factor in the increased endurance and fat oxidation. Additional experiments found that the catechin epigallocatechin gallate (EGCG), a polyphenolic (antioxidant) compound found in green tea, improved endurance, though the evidence was weak, suggesting that the other catechins in green tea were also necessary.

A single dose was not effective in increasing metabolism, leading to the conclusion that long-term use of green tea was necessary to replicate the results. It should be noted that the research was done at the Biological Sciences Laboratories of Kao Corporation in Tochigi, Japan, which makes health care products, including green tea beverages. That doesn’t mean the results were skewed, but is mentioned in the interest of full disclosure.

Researchers in another study, published in the journal Phytomedicine in January of 2002, confirmed that green tea extracts did not increase heart rates during weight loss. Seventy moderately obese subjects were given two green tea extract capsules daily, containing 270 mg of ECGC, one-third more than in the University of Geneva study. The results after three months: body weights decreased by a mean of 4.6% and waist circumferences by 4.48%.

A 12-week Japanese study on the effects of catechins on body fat reduction involved men of similar size (based on body mass index, an indicator of fat, and their waist circumferences) who were given different amounts of catechins. One group was given tea fortified with green tea extract that contained 690 mg of catechins, while the other group was given tea that had only 22 mg of catechins. During the three-month test the men ate identical breakfasts and dinners and were told to control their calorie and fat intake at other times so that overall diets were similar. At the end of the 12 weeks, waist circumference, body weight and body mass index were all significantly lower in the men who were given the tea containing the green tea extract. The men in that group lost an average of 5.3 pounds versus an average weight loss of 2.9 pounds for the other group.

A Chinese test was done on 75 people who were considered to be obese due to overeating and/or lack of physical exercise. The people, between the ages of 22 and 69, were asked to drink oolong tea twice a day for six weeks. They were asked to eat normally, not to try to diet, and to refrain from exercising.

Using echography sensors to measure the thickness of subcutaneous fat near their navels before and after the six weeks, researchers found an average decrease of four millimeters. In addition, the tests showed serum, total cholesterol and serum triglyceride levels were lower after the six-week test. Sixty-seven percent of the participants lost weight during the testing. Remember, they had no dietary guidelines and there was no documentation of what they ate during the testing period.

Another study done in Taiwan studied the tea-drinking habits of 1,103 people and analyzed body fat differences and waist-to-hip ratios. Results: those who had been drinking tea for more than ten years had 19.6% less body fat on average and 2.1% smaller waist-to-hip ratios than those who did not regularly drink tea. They concluded that tea could be a weight-loss beverage.

Penn State Study

Led by Joshua Lambert, Ph. D., scientists at Penn State analyzed several studies on green tea to determine its health and weight loss benefits. They learned that green tea promotes weight loss by preventing the body from absorbing calories from fat. The weight loss drug Alli prevents calorie absorption the same way, but green tea provides the same benefits without the drug’s risks and side effects.

The researchers discovered that green tea helps burn fat, too. It increases the body’s levels of fat-burning enzymes. Other studies have pointed to green tea as a way to increase metabolism and reduce hunger and fat-storage hormones. Green tea is also known to help prevent cancer.

Lambert suggests drinking 6-8 cups of green tea a day to get the most from its fat-fighting properties.

The researchers discovered that green tea helps burn fat, too. It increases the body’s levels of fat-burning enzymes. Other studies have pointed to green tea as a way to increase metabolism and reduce hunger and fat-storage hormones.

Scientists suggests drinking 6-8 cups of green tea a day to get the most from its fat-fighting properties.