Health Professionals Newsletter July '10

Welcome to the latest newsletter from the Tea Avisory Panel (TAP). This service will keep you up to date with current activities and events and inform you of the latest published research. We'll keep you updated on a quarterly basis with news and views from across the tea industry.

Research Update

Tea catechins with caffeine are associated with reduced BMI and body weight

A review of recent literature has found that consumption of green tea catechins (GTC), along with caffeine, is associated with significant reductions in body mass index (BMI) and body weight.

The global burden of overweight and obesity continues to escalate along with the associated increased risk of chronic diseases such as cardiovascular disease and type 2 diabetes. Evidence is growing of a link between GTC consumption and reduced BMI and body weight - although results have been conflicting.

The present study involved a comprehensive review of published data investigating the association between GTC consumption and anthropometric measures.

Published randomized control trials on GTC (with or without caffeine) and BMI, body weight, waist circumference (WC) or waist to hip ratio (WHR) were identified through a comprehensive search including MEDLINE, EMBASE, Cochrane CENTRAL and the Natural Medicines Comprehensive Database.

A total of 15 trials (n=1243) met the inclusion criteria. Three separate analyses were conducted to account for the possible confounding effect of caffeine:

Group A - GTC with caffeine compared with caffeine-matched controls; 7 trials (n=600)
Group B - GTC with caffeine compared with caffeine-free controls; 6 trials (n=524)
Group C - Caffeine-free GTC compared with caffeine-free controls; 2 trials (n=119)

For each analysis the weighted mean difference was calculated for the 4 anthropometric measures (BMI, body weight, WC and WHR) as the differences between the mean in the GTC and control groups.

Tests for publication bias and heterogeneity were conducted. The follow-up of subjects taking part ranged from 8 - 24 weeks (median 12 weeks). Treatment groups received GTC in a variety of forms including green tea extract capsules and green tea beverage. The dose ranged from:

576-714mg/day (median 588mg/day) for group A
141-1207mg/day (median 474mg/day) for group B
282-548mg/day (median 415mg/day) for group C

The types of populations studied included children, healthy adults and adults with co-morbidities (diabetes, hyperlipidaemia, obesity and overweight). Additional advice on exercise and diet varied between the studies; some studies encouraged participants to follow their normal diet and exercise regimen, others advised subjects to follow their normal diet but incorporate some planned exercise and in some studies subjects were advised to follow a calorie controlled diet.

The initial meta analysis indicated a statistically significant reduction in BMI, body weight and WC for those consuming GTC with caffeine compared with caffeine-match controls (group A); suggesting a direct effect of the GTC:

Weighted mean difference for BMI: -0.55(95% CI: -0.65, -0.40) kg/m²
Weighted mean difference for body weight -1.38 (95% CI: -1.70, -1.06) kg
Weighted mean difference for WC -1.93(95% CI: -2.82, -1.04) cm

Publication bias was indicated for BMI and body weight. However, using a further analysis to determine the effect of this on the results, it was found that the weighted mean differences and 95% confidence intervals still remained similar and statistically significant. Study heterogeneity was indicated for WC and WHR; suggesting individual studies included in this analysis were in some way fundamentally different from each other.

There was also a statistically significant reduction in body weight for those consuming GTC with caffeine compared with caffeine-free controls (group B);

Weighted mean difference for body weight -0.44 (95% CI: -0.72, -0.15) kg. No difference was seen for BMI, WC or WHR.

Finally, there was no difference in any anthropometric measures between those in the caffeine-free GTC group compared to caffeine-free controls (group C).

Because of the methodological differences between the studies, conclusions from this meta-analysis need to be interpreted with caution. For example different doses of GTC were used in the studies; population groups varied; some trials regulated additional tea, GTC-rich foods or caffeine consumption; some specified when GTC should be consumed whereas others did not; the advice on additional diet and exercise also varied between studies. As a result of these differences the authors suggested that the 3 groups shouldn't be cross-compared to determine which type of treatment is the best.

In conclusion, based on this meta-analysis using the currently available literature, GTC and caffeine may positively affect BMI and body weight. However the authors have suggested that the effect is small and is unlikely to be clinically relevant.

Phung, O., J., Baker, W., L., Matthews, L., J., Lanosa, M., Thorne, A. and Coleman, C., I. (2010). Effect of green tea catechins with or without caffeine on anthropometric measures: a systematic review. Am. J. Clin. Nutr. 91: 73-81.

And what about black tea?
Green tea and black tea come from the same plant Camellia sinensis. The majority of studies examining the effect of tea on body weight have focused on green tea or green tea extracts and the effectiveness of black tea has been less well studied. Given the high prevalence of black tea consumption (approximately 80% of worldwide tea consumption) further studies investigating the effect of this drink on weight regulation are warranted. In the mean time, because tea contains no calories and only 13 calories when semi-skimmed milk is added, it is an ideal drink to enjoy when watching your weight.

Tea helps with weight loss and weight maintenance

In another review of the recent literature consumption of green tea was associated with positive effects on weight loss and weight maintenance.

Studies have shown that green tea can enhance energy expenditure and fat oxidation and thereby induce weight loss (WL). Caffeine is also known to stimulate energy expenditure, but a similar amount of caffeine plus the catechin epigallocatechin gallate (EGCG) (found in green tea) simulates energy expenditure to a greater magnitude than caffeine alone. Many studies have been conducted with Asian subjects where studies have produced more favourable results on WL than those in Caucasian subjects.

The aim of the present study was to review all available long-term studies that have examined the effects of EGCG on WL and weight maintenance (WM) to establish whether catechins contribute to the regulation of body weight.

Published studies that reported the long-term (at least 12 weeks) effects of catechin consumption on WL or WM were included. Studies had to be randomized and participants blinded to the treatment (either catechins versus no catechins or high dose versus low dose catechins). Catechins could be in the form of a beverage or capsules. In the WL studies participants consumed an EGCG-caffeine mixture concurrently with a low energy diet for 12 weeks whilst in the WM studies, subjects first lost weight for 4 weeks and then consumed an EGCG-caffeine mixture for either 12 or 13 weeks.

Other variables extracted from the original studies included participant ethnicity, BMI, gender and regular caffeine intake. Low caffeine intake was considered less than 300mg per day and high was more than 300mg per day. If regular caffeine intake was not reported an assumption was made; in the case of Asian populations this would be low. The mean weight change over the study period was extracted or calculated from the data. A negative effect size indicated more WL or less weight gain than the control group.

A total of 11 studies met the inclusion criteria comprising 1226 individuals; 7 studies compared EGCG-caffeine mixture to true placebo whilst 4 studies compared a high with a low dose EGCG-caffeine mixture. The range of EGCG-caffeine dosage ranged from 270 - 1207 mg per day in the treatment groups, zero in the true placebo groups and 96-130 mg per day in the low EGCG-caffeine mixture groups. Three of the eleven studies included Caucasian subjects; the remaining 8 studies were Asian subjects - all of whom had a low habitual caffeine intake.

A test for publication bias was not reported, but the effect of study heterogeneity was examined.

The meta-analysis indicated a modest but significant positive effect of catechins on WL/WM; mean effect size (u=-1.31kg, 95% CI: -2.05 to -0.57; P<0.001). Thus subjects consuming EGCG lost on average 1.31 kg more weight (or gained 1.31kg less weight) than those consuming zero or low amounts of EGCG. However, there was clear heterogeneity suggesting individual studies included in this analysis were in some way fundamentally different from each other and the results from which should not be combined. The results were not affected by the difference in dosage of EGCG-caffeine mixture between the treatment and placebo groups.

A larger average effect size was observed in Asian populations than Caucasian populations; (u=-1.51kg, 95% CI: -2.37 to -0.65) and (u=-0.82kg, 95% CI: -2.13 to -0.50; P<0.001), respectively but this difference was not significant.

A larger average effect size was also observed in low habitual caffeine consumers compared to moderate-high caffeine consumers; (u=-1.60kg, 95% CI: -2.38 to -0.83) and (u=-0.27kg, 95% CI: -1.63 to 1.10), respectively. This difference was only significant when ethnicity was also controlled for (P=0.04).

The heterogeneity in this study may be explained by differences in confounding factors such as ethnicity and habitual caffeine intake - which in some instances was assumed.

Differences in the sensitivity to caffeine was proposed as a reason why some people appear more sensitive to the effects of caffeine than others and may explain the observed difference to treatment in this study. This is likely to occur at a genetic level and may explain why Asian subjects lost more weight because they habitually consume lower levels of caffeine and are genetically more sensitive to caffeine consumption. The authors conceded that more research with Caucasian populations would be required to examine this effect in more detail. The authors also proposed an enzymatic inhibition hypothesis as the mechanism by which EGCG induces WL/WM. Again this effect is not the same across all ethnic groups and may explain some of the differences observed.

This meta-analysis has shown that an EGCG-caffeine mixture may have a positive, albeit small, effect on WL/WM, and that the effects of ethnicity and habitual caffeine intake may have a potentially important bearing on this effect. However, given the degree of heterogeneity in this study these results should be interpreted with caution.

Hursel, R., Viechtbauer, W. and Westerterp-Plantenga, M. S. (2009). The effect of green tea on weight loss and weight maintenance: a meta-analysis. Int. J. Obesity 33: 956-61.

Tea, caffeine and weight regulation - a review

A narrative review in 2009 of green tea catechins and caffeine has presented evidence of a positive effect on weight management through effects on metabolic rate via regulation of total body fat and a sparing of fat-free mass during weight loss.

The introduction reminds the reader of the global prevalence of overweight and obesity and its threat to the health of populations in an ever-increasing number of countries. In the authors opinion obesity is caused, at the most fundamental level, by an imbalance between energy intake and energy expenditure (EE). Stimulating EE or preventing its decline during 'dieting', by means of natural products like green tea and caffeine, is the focus of this review. Studies examining the short and longer term effects of green tea (GT) and caffeine consumption are discussed followed by a review of the potential mechanisms of action and safety of GT and caffeine.

The review describes the pharmacologically active phenolic components of green tea, of which epigallocatechin gallate (EGCG) is thought to be the most active and most abundant.

A number of short term (<24 hours) human studies were described which have shown GT extracts containing caffeine resulted in a significantly increased 24 hour EE and fat oxidation compared to a control and caffeine alone. This suggests that GT has thermogenic properties over and above that explained by its caffeine content. A variety of doses and regimes of GT and caffeine intake were described, some using capsules and other beverages. Increased EE of the order of 4-8% have been described in Caucasian as well as Asian populations, in young and older adults with a BMI of 21-35 kg/m² and whose habitual caffeine intake also varied. Only one study was included that showed no significant effect of GT on EE and fat oxidation in Caucasian young men. The author concluded the 13 hour study was too short and any possible effect needs to be built up over 24 hours.

There seems little doubt from the evidence presented that administration of caffeine over the short-term increases EE and basal metabolic rate. Such effects have been reported in lean and obese subjects, although according to one study the increase in thermogenesis was smaller in obese subjects.

The majority of long-term experimental studies included in this review reported significant reductions in body weight, body fat or waist circumference and an increase in EE with varying doses of catechins and caffeine. One study found no effect on EE in subjects following a low energy diet combined with GT or placebo. In this study caffeine intake was standardized for both groups at 300mg per day. A moderate intake of caffeine was blamed for making the GT ineffective. A meta-analysis of long term studies concluded that GT and caffeine showed promising results for body weight regulation but study heterogeneity meant conclusions should be interpreted with caution.

Despite short term studies finding that caffeine seems to increase thermogenesis, long-terms studies have produced equivocal results. There was a suggestion that sensitivity to the effects of caffeine are lost over time, which may explain some of the reported findings. However one study reported that 300mg of caffeine daily reduced energy intake by 22% in men (but not women) and another study found that people who increased their caffeine consumption over a 12 year period gained less weight than those who decreased their consumption over the same period.

The mechanism of action proposed for the effects observed on EE and fat oxidation involves inhibition of the enzyme catechol O-methyltransferase (COMT) by GT catechins; COMT inactivates norepinephrine (NE). Amongst other roles, NE stimulates the sympathetic nervous system (SNS) and causes an increase in EE and fat oxidation. Thus in the presence of catechins derived from GT consumption the SNS is continuously stimulated causing an increase in EE and fat oxidation. However, this phenomenon does not occur equally in all ethnic groups and may explain some of the mixed results obtained from studies involving more than one ethnic group. Asian populations are more likely to produce a high-activity version of the enzyme whereas Caucasian populations are more likely to product a lower activity version. This may explain why in some studies with Caucasian subjects no effect was seen after green tea ingestion.

Caffeine's mechanism of action was also proposed with one of the effects being at the level of the SNS. After caffeine ingestion, SNS activity increases and hormone sensitive lipase is activated, promoting fat breakdown.

Short and long term studies have confirmed the safety of caffeine consumption. It has been suggested that the short-term lethal dose for caffeine is around 5 to 10g of caffeine (equivalent to 125 cups of tea). Longer term health effects have been observed following an intake of 400mg of caffeine a day (equivalent to 8 cups of tea) including changes in blood pressure and reported side effects such as palpitations, anxiety, headaches, restlessness and dizziness. Green tea has been widely consumed in China and Japan for many centuries and is regarded as safe - most short and long-term studies show no effect on blood pressure or heart rate.

This review has discussed the results of studies reporting short and long-term effects of GT and caffeine ingestion. Proposed mechanisms of action for the observed effects on body weight regulation, namely effects on EE and fat oxidation, were also discussed. The author suggested further research is now required on the bio-availability and bio-activity of green tea/caffeine mixtures, the effects of ethnicity and related enzyme activities and the effects of individuals' sensitivity due to regular consumption of green tea/ caffeine.

Westerterp-Plantenga, M. S. (2010). Green tea catechins, caffeine and body-weight regulation. Physiology and behavior Feb 13. [Epub ahead of print].