Consumption of Green Tea May Help Prevent Obesity

Grove KA, Lambert JD. Laboratory, epidemiological, and human intervention studies show that tea (Camellia sinensis) may be useful in the prevention of obesity. J Nutr. 2010;140: 446-453.

Green tea (Camellia sinensis), a popular beverage worldwide, contains large amounts of polyphenols known as catechins: (-)-epicatechin, (-)-epigallocatechin, (-)-epicatechin-3-gallate (ECG), and (-)-epigallocatechin-3-gallate (EGCG). A cup of brewed green tea contains 240-320 mg of catechins, with EGCG accounting for 30% to 50% of that amount. Green tea also contains the xanthines: caffeine, theophylline, and theobromine. It has been studied for its potential health benefits and its effects on certain chronic diseases. Obesity, which affects about 20% of U.S. adults, is a risk factor for chronic diseases (e.g., diabetes, cardiovascular diseases, and cancer). These authors summarize the literature on the potential efficacy of tea to prevent obesity.

In their review of laboratory studies, the authors focus on mechanistic data from animal model studies that have examined the effects of tea on obesity and related pathologies, including diabetes, hypercholesterolemia, and fatty liver disease. Most studies have used green tea, green tea extract (GTE), or pure EGCG in both genetic and dietary models of obesity.

One cited study reported that treatment with 7.0 µmol/g dietary EGCG for 15 weeks reduced body weight gain in male mice fed a high-fat diet compared with control mice. The treated mice had significantly lower adipose tissue weight, fasting blood glucose, fasting plasma cholesterol, and plasma alanine aminotransferase levels. The same investigators reported that short-term dietary EGCG treatment of obese mice tended to reduce body weight gain compared with high-fat-fed controls. “This treatment regimen represents a more realistic obesity-related application of EGCG or green tea supplementation, because the most likely consumers of these products would have a preexisting weight problem,” state the authors. Other cited studies, as well as one including rats fed a normal-fat diet, support these findings.

Regarding tea and genetic animal models of obesity, the authors cite a study of the efficacy of GTE in obese, leptin-deficient mice, in which there was no significant effect on body weight. The study did, however, report higher plasma high-density lipoprotein cholesterol and lower hepatic triglyceride levels in the GTE-treated mice.

Few studies have examined the interaction between tea polyphenols and exercise; however, the animal studies cited by these authors support the benefits of tea polyphenols plus exercise in reducing body weight gain.

According to the authors, the “in vivo effects of EGCG may be explained by underlying mechanisms suggested by in vitro studies of de novo lipogenesis, lipid absorption, and carbohydrate absorption and utilization.” Several studies have begun to examine the effects of tea polyphenols on fatty acid synthase. Two studies have reported that EGCG can inhibit pancreatic lipase activity, but the effective concentrations in these studies varied.

The authors cite several studies in dietary models of obesity that have examined the role of green tea-mediated modulation of glucose uptake and disposition in obesity prevention. The findings from two studies indicate that treatment with green tea can increase carbohydrate catabolism. In vitro studies have suggested that EGCG and ECG play a role in inhibiting glucose uptake in the gut.

Most studies of animal models of obesity and tea have found no significant effect of tea or tea components on energy intake. The exceptions are two studies cited by these authors: one reported a significant decrease in food intake by mice treated with Oolong tea extract compared with untreated mice, and the other study reported that 0.5% tea catechins reduced energy intake by 5.6% (the trend was not significant, however).

Regarding human studies, the authors note that there are few epidemiological studies examining the impact of tea on body weight and other obesity-related markers. More interventional studies have been conducted. In one study of healthy Japanese men, the participants consuming high-catechin tea for 12 weeks had lower body weight, waist circumference, and body fat mass compared with baseline values. In a follow-up study by the same investigators, 240 obese subjects were treated with a catechin-enriched green tea beverage or a control green tea beverage for 12 weeks. Those in the high-catechin beverage group had a significant decrease in body weight, total fat area, and visceral fat compared with baseline values. These decreases were greater than those in the low-catechin control group. The authors note that the results of studies on tea and body weight and body fat have not been “universally positive,” citing some studies that reported no significant effects of green tea consumption on body weight.

The authors further cite a study that reported a beneficial effect on weight loss and weight maintenance following weight loss with green tea consumption and a study that showed a trend toward a greater decrease in body weight with green tea and exercise, compared with exercise only. “Further studies on the interaction between tea and exercise in humans are warranted,” say the authors.

U.S. sales of green tea-based dietary supplements are increasing, partly because of the number of reports describing the potential antiobesity and other beneficial effects of tea and tea polyphenols. Green tea-based dietary supplements can potentially deliver a much higher dose of catechins than do green tea beverages. Since 1999, say the authors, 34 case studies have linked the consumption of green tea-based supplements to hepatotoxicity. Laboratory studies in animals have generally supported the supplements’ potential toxicity at high doses. “These findings suggest that caution should be exercised in the use of green tea-based dietary supplements and that further studies are needed to determine the upper limit of safety for bolus dosing with tea polyphenols as well as the underlying mechanisms of toxicity.”

The authors conclude that “laboratory studies in animals, a limited number of epidemiological studies, and small-scale human intervention studies support the hypothesis that tea and tea polyphenols have beneficial effects on weight gain, weight loss, and prevention of obesity.”

—Shari Henson