Journal Title (Medline/Pubmed accepted abbreviation): J Nutr Biochem
Page numbers: 51-60
Article type: Review: Current Topics
Obesity and associated comorbidities are a growing health problem worldwide. Obesity can result from a very small positive energy imbalance between dietary intake and energy expenditure. The actual amount of weight gained per year in the US and Australia, on average, is only 0.5 kg, regardless of ethnic or economic status and gender. Intake of green tea, brewed from unfermented Camellia sinensis leaves, has been associated with reduced body fat and waist circumference. Green tea catechins (GTC) are the putative phytonutrients responsible for the antiobesity effects observed with tea. Polyphenols, including flavonols, flavones, and flavan-3-ols (catechins) compose up to 35% of dried tea leaf mass. Fermented teas, such as oolong and black, contain only about half the catechins of green tea. The primary catechin by weight and bioactivity is epigallocatechin-3-gallate (EGCG). Typical brewed green tea (250 mL) contains 50 to 100 mg catechins and 30 to 40 mg caffeine, although these amounts can vary greatly depending on preparation. Accordingly, most research into green tea and weight loss has focused on use of standardized GTC/EGCG supplements. Several small studies and a meta-analysis have evaluated weight loss and changes in body composition with GTC. The meta-analysis described an overall benefit of ~ 1.3 kg weight loss in GTC users compared with controls over a typical 12-week intervention. Some of these studies have suggested an ethnocentric effect of GTC, whereby Asians achieve greater weight loss than Caucasian participants; however, the Caucasian participants frequently achieved positive changes in body composition, even if weight loss was not significantly improved over placebo.
Four potential antiobesity mechanisms of GTCs are reviewed:
- Increased energy expenditure
- Increased fat oxidation
- Decreased appetite
- Decreased nutrient absorption
Increased energy expenditure occurs because GTCs stimulate the sympathetic nervous system (SNS). Several trials have demonstrated modest (3% to 4%) increases in 24-hour energy expenditure with GTCs plus caffeine or with tea. Most trials of GTCs and tea did not show a significant increase in energy expenditure compared with equivalent caffeine dose alone, suggesting that the caffeine content may be responsible for increased thermogenesis. Variations in catechol-O-methyltransferase (COMT) enzyme activity, more prevalent in Asian populations, are suggested to contribute to greater energy expenditure and weight loss in these populations with GTC.
Alterations in fatty acid oxidation and metabolism may be increased with GTC intake, and appear to be independent of caffeine content. However, most trials of GTC and fatty acid oxidation are acute studies and may not adequately demonstrate the long-term benefits of chronic GTC/tea ingestion. Evidence for a role for GTC in fatty acid metabolism is sparse. Available evidence from animal studies has demonstrated increased hepatic beta-oxidation and decreased expression of fatty acid synthase with GTC intake. Human studies have identified only variable effects of GTC on respiratory quotient, a marker of relative fat and carbohydrate oxidation. Indirect evidence suggests that GTC could increase site-specific fat lipolysis, especially in the abdomen. In 2 intervention trials, each of 12-week duration, GTC (+ caffeine) decreased waist circumference in both Japanese and Caucasians, compared with caffeine-matched controls, despite no net change in body weight. The authors also discuss several trials showing an even greater effect of GTC/caffeine on fat oxidation when combined with exercise.
Appetite may also be suppressed by consumption of GTC or tea. Several studies have offered inconsistent results on whether GTC supplementation reduces appetite; however, at least 1 trial has suggested a possible decrease in ad libitum food intake (~ 95 kcal less in 1 meal) after supplementation with GTC. Lastly, the authors discuss evidence for a potential role of GTC in decreasing nutrient absorption, specifically through decreased glucose absorption in the gastrointestinal tract via reduced α-amylase and α-glucosidase activity. Moreover, cell-based studies have identified a role for GTC in reducing glucose uptake in intestinal cells and in inhibition of the sodium-dependent glucose transporter. Intake of GTC concurrently with a meal, in studies of both rats and humans, has also been found to reduce plasma glucose concentrations; however, plasma glucose and insulin levels may temporarily increase when GTC intake occurs 1 hour before a meal. Fat digestion and absorption may also be inhibited by GTC consumption, possibly through inhibition of gastric and pancreatic lipase activity.
The authors describe several studies discussing potential antiobesity mechanisms of GTCs. These mechanisms include increased energy expenditure, increased fat oxidation, decreased appetite, and decreased nutrient absorption. The evidence supporting benefit of tea consumption or supplementation with GTC extracts on body weight and/or body composition is mixed, especially in Caucasians. However, given the low cost, ready availability, and safety of moderate tea consumption, this approach may be beneficial for inclusion in a diet or weight/fat loss management plan.