Coffee and tea are the two important dietary sources of caffeine, thus can increase consciousness and work productivity(Reference van Dam, Hu and Willett1). Both coffee(Reference Ludwig, Clifford and Lean2) and tea(Reference Khan and Mukhtar3) contain several beneficial components including polyphenols such as lignans and chlorogenic acid and epigallocatechin gallate, which are believed to be associated with a lower risk of site-specific cancers, CVD and premature death(Reference Poole, Kennedy and Roderick4,Reference Yi, Wu and Zhuang5) .
There is evidence that coffee and tea, especially green tea, have weight-loss effects. Caffeine can induce noradrenaline and dopamine release, thus stimulating neuronal activity in the brain and decreasing body weight(Reference Zheng and Hasegawa6). Caffeine can also inhibit phosphodiesterase and suppresses the negative effects of adenosine on noradrenaline release, thus can induce fat oxidation(Reference Dulloo, Seydoux and Girardier7). Green tea decreases ghrelin secretion, nutrient absorption and adipogenesis, in contrast, increases adiponectin levels and substrate oxidation(Reference Huang, Wang and Xie8). Intervention studies suggested that green tea supplementation, especially in the form of green tea extract, can result in a significant reduction in body weight in individuals with overweight or obesity(Reference Lin, Shi and Su9). Supplementations with caffeine(Reference Tabrizi, Saneei and Lankarani10) and green coffee extract(Reference Asbaghi, Sadeghian and Rahmani11) was also associated with a significant decrease in body weight in adults. A systematic review and meta-analysis concluded that green tea has no significant effect on prostate-specific antigen level(Reference Sharifi-Zahabi, Hajizadeh-Sharafabad and Abdollahzad12). Their geographical subgroup analysis revealed that green tea significantly reduced prostate specific antigen level in the USA population compared with non-USA populations.
However, the potential weight loss effects of other beverages, including coffee, decaffeinated coffee, sour tea and black tea, have not yet been evaluated. In addition, previous reviews have mainly focused on supplementation with green tea, green coffee and caffeine as an herbal extract. Indeed, the potential weight loss effects of drinking tea or coffee, in the form of a beverage, have not been investigated in a systematic review. Therefore, we aimed to perform a systematic review and network meta-analysis of randomised trials to compare and rank the effects of drinking different tea and coffee on body weight in adults.
Materials and methods
The meta-analysis was conducted per instructions outlined in the Cochrane Handbook for Systematic Reviews of Interventions(Reference Higgins, Thomas and Chandler13) and the Grading of Recommendations Assessment, Development, and Evaluation Handbook(Reference Schunemann14). The protocol of the study was approved by the ethic committee of Tehran University of Medical Sciences (IR.TUMS.EMRI.REC.1401.075) and was registered with PROSPERO (CRD42022372417)(Reference Jayedi and Shab-Bidar15).
Systematic search
PubMed, Scopus and CENTRAL were searched from inception until July 2024. Grey literature sources, including clinicaltrial.gov and ProQuest, were also searched to find potential additional trials. As per our a priori search strategy (online Supplementary Table 1), two authors (AMR and RN) independently screened the titles and abstracts. These reviewers independently read the full texts of all potentially relevant articles and screened the reference lists of relevant meta-analyses. There was no restriction on language, date, or publication status.
Study selection and eligibility criteria
Randomised controlled trials were selected if (1) had either parallel or cross-over design and lasted at least 2 weeks(Reference Schwingshackl, Nitschke and Zähringer16), (2) were conducted in participants aged ≥ 18 years, (3) compared the effect of different types of tea or coffee including green tea, black tea, sour tea, coffee, decaffeinated coffee or green coffee on body weight, or compared one of these beverages against a control group and (4) reported change in body weight (kg) during the intervention period in both intervention and control groups in each trial as an outcome, or reported sufficient information to estimate those values. An eligible control group was defined as a placebo, water, no intervention or intake of other tea or coffee.
Exclusion criteria
Exclusion criteria were (1) cross-over trials without a washout period, (2) trials conducted in critically ill patients (e.g. trials conducted in patients with beta thalassemia), (3) trials that were conducted in pregnant or breast-feeding women, (4) trials that implemented a multicomponent intervention in the intervention arm and (5) trials that had an active control group rather than any type of tea or coffee.
Screening and data extraction
Two reviewers (RN and AMR) independently extracted the author’s name, study design (parallel or cross-over), year of publication, number of participants, baseline BMI, % female, mean age, intervention and comparator characteristics, duration and the dosage of intervention, dropouts, degree of adherence to the intervention and mean and corresponding sd of change from baseline weight in each study arm. Disagreements were resolved by consulting the first author (AJ).
Risk of bias (quality) assessment
Two authors (AJ and SS-B) independently assessed the risk of bias of the trials according to version 2.0 of the Cochrane tool for risk of bias assessment(Reference Higgins, Thomas and Chandler13).
Data analysis
Mean differences (MD) and their 95 % credible intervals (CrIs) were considered as the effect size for reporting the results. To perform the analyses, the mean and s d of change in body weight (kg) from baseline in each study arm were calculated. If these values were not reported in the trials, we followed the Cochrane Handbook guidance to calculate these values using baseline and endpoint measures(Reference Higgins, Thomas and Chandler13).
First, a random-effects pairwise meta-analysis with a Bayesian framework was performed to estimate direct estimates(Reference Ades, Sculpher and Sutton17,Reference Lumley18) . Then, a random-effects network meta-analysis with a Bayesian framework was conducted to calculate the network estimates(Reference Ades, Sculpher and Sutton17,Reference Lumley18) . The node-splitting approach was used to calculate indirect estimates and to evaluate incoherence between direct and indirect estimates. Ranking probabilities were calculated, and the surface under the cumulative ranking curves was obtained. Both pairwise and network meta-analyses were performed under the gemtc package of R version 3.4.3 (R Studio, Boston, MA).
A pre-specified sensitivity analysis was performed with trials conducted on participants with overweight or obesity. Two post hoc sensitivity analyses were also done with trials conducted in healthy and unhealthy participants. Due to the low number of trials with a low risk of bias and trials that implemented energy restriction alongside their intervention, we did not perform pre-specified sensitivity analyses with trials with a low risk of bias and energy restriction. To evaluate the potential for transitivity, the distribution of the potential effect modifiers across the available direct comparisons was assessed. Mean age, baseline BMI and percentage of female participants were considered as the potential effect modifiers (online Supplementary Figures 1–3). A comparison-adjusted funnel plot was created to assess the potential for publication bias (online Supplementary Fig. 4)(Reference Chaimani, Higgins and Mavridis19).
Grading of the evidence
The certainty of the evidence was evaluated using the Grading of Recommendations Assessment, Development, and Evaluation approach developed for network meta-analyses(Reference Brignardello-Petersen, Bonner and Alexander20). The minimal clinically important difference threshold for weight loss in adults was considered as 5 % weight loss(Reference Shi, Wang and Hao21), equal to 4·5 kg(Reference Shi, Wang and Hao22).
Results
Literature search and study selection process
Figure 1 shows the systematic search and study selection process. After the exclusion of 931 duplicates and an additional 1474 records that were not eligible according to our inclusion criteria, we read the full text of 201 records; of those, twenty-two trials were considered eligible for inclusion. Online Supplementary Table 2 presents the list of excluded studies based on the review of the full texts with reasons for exclusions.
Fig. 1. Flow diagram.
Characteristics of the included trials
In total, twenty-two randomised trials with 1710 participants proved eligible for inclusion in the present meta-analysis(Reference Ahmed Al-Naggar, Osman and Abdulghani23–Reference Wedick, Brennan and Sun44) (Table 1). All but three trials(Reference Lecoultre, Carrel and Egli32,Reference Martínez-López, Sarriá and Mateos33,Reference Revuelta-Iniesta and Al-Dujaili39) had a parallel design. Three trials had a cross-over design, in which the length of the washout period was 1 week(Reference Revuelta-Iniesta and Al-Dujaili39), > 4 weeks(Reference Lecoultre, Carrel and Egli32) and 8 weeks(Reference Martínez-López, Sarriá and Mateos33). Three trials were conducted in men(Reference Lecoultre, Carrel and Egli32,Reference Ohnaka, Ikeda and Maki38,Reference Superko, Bortz and Williams40) , three in women(Reference Ahmed Al-Naggar, Osman and Abdulghani23,Reference Cardoso, Salgado and Cesar28,Reference Mortazavi, Paknahad and Hasanzadeh34) and the remainder in either sex. Nine trials were conducted in participants with overweight and/or obesity(Reference Ahmed Al-Naggar, Osman and Abdulghani23,Reference Alperet, Rebello and Khoo24,Reference Basu, Betts and Mulugeta26,Reference Cardoso, Salgado and Cesar28,Reference Kobayashi, Kawano and Ukawa31,Reference Mousavi, Vafa and Neyestani35,Reference Nabi, Sedighinejad and Haghighi37,Reference Ohnaka, Ikeda and Maki38,Reference Wedick, Brennan and Sun44) and nine in healthy participants(Reference Ahmed Al-Naggar, Osman and Abdulghani23,Reference Hochkogler, Schweiger and Rust29–Reference Lecoultre, Carrel and Egli32,Reference Revuelta-Iniesta and Al-Dujaili39,Reference Superko, Bortz and Williams40,Reference van Dusseldorp, Katan and van Vliet42,Reference Wedick, Brennan and Sun44) . The intervention duration was 2 weeks in one trial(Reference Revuelta-Iniesta and Al-Dujaili39), 4 weeks in three trials(Reference Ahmed Al-Naggar, Osman and Abdulghani23,Reference Lecoultre, Carrel and Egli32,Reference Mortazavi, Paknahad and Hasanzadeh34) , 8 weeks in nine trials(Reference Basu, Betts and Mulugeta26,Reference Cardoso, Salgado and Cesar28,Reference Hochkogler, Schweiger and Rust29,Reference Martínez-López, Sarriá and Mateos33–Reference Mousavi, Vafa and Neyestani35,Reference Superko, Bortz and Williams40,Reference Vieira Senger, Schwanke and Gomes43,Reference Wedick, Brennan and Sun44) , 12 weeks in four trials(Reference Bak and Grobbee25,Reference Kajimoto, Kajimoto and Yabune30,Reference Kobayashi, Kawano and Ukawa31,Reference Nabi, Sedighinejad and Haghighi37) and longer than 12 weeks in the other five trials(Reference Alperet, Rebello and Khoo24,Reference Bøhn, Croft and Burrows27,Reference Ohnaka, Ikeda and Maki38,Reference Toolsee, Aruoma and Gunness41,Reference van Dusseldorp, Katan and van Vliet42) . Four trials were rated to have a low risk of bias(Reference Alperet, Rebello and Khoo24,Reference Hochkogler, Schweiger and Rust29,Reference Ohnaka, Ikeda and Maki38,Reference Wedick, Brennan and Sun44) , five trials were rated to have some concerns(Reference Bak and Grobbee25,Reference Bøhn, Croft and Burrows27,Reference Kobayashi, Kawano and Ukawa31,Reference Lecoultre, Carrel and Egli32,Reference Mortazavi, Paknahad and Hasanzadeh34) and the other twelve trials were rated to have a high risk of bias(Reference Ahmed Al-Naggar, Osman and Abdulghani23,Reference Basu, Betts and Mulugeta26,Reference Cardoso, Salgado and Cesar28,Reference Kajimoto, Kajimoto and Yabune30,Reference Martínez-López, Sarriá and Mateos33,Reference Mousavi, Vafa and Neyestani35–Reference Nabi, Sedighinejad and Haghighi37,Reference Revuelta-Iniesta and Al-Dujaili39,Reference Superko, Bortz and Williams40,Reference van Dusseldorp, Katan and van Vliet42,Reference Vieira Senger, Schwanke and Gomes43) (online Supplementary Table 3).
Table 1. Characteristics of the trials included in the network meta-analysis of tea or coffee drinking on weight loss in adults
Cont, control; Int, intervention.
Network meta-analysis
Comparative effects of different beverages on body weight are presented in Fig. 2 and Table 2. In the main analysis incorporating all trials, green tea drinking was effective for weight loss compared with placebo (MD: –1·23 kg, 95 % CrI: –2·45, –0·02; low certainty evidence) and water (MD: –1·61 kg, 95 % CrI: –2·90, –0·35; very low certainty evidence). Other beverages, including black tea, sour tea, coffee, decaffeinated coffee and green coffee, were not effective for weight loss compared with either placebo or other beverages (very low- to low certainty evidence). Based on surface under the cumulative ranking curves values, green tea was the most effective beverage for weight loss (75 %) (Table 3).
Fig. 2. Network diagram for weight loss. The size of the nodes is proportional to the total number of participants allocated to each beverage, and the thickness of the lines is proportional to the number of studies evaluating each direct comparison.
Table 2. Comparative effects of different beverages on body weight (Mean difference (kg) and 95 % credible interval)*
* The results indicate the effects of other columns in comparison to the first column. For example, the effect of green tea v. black tea is −0·27 kg (–3·40, 2·87). Statistically significant results are indicated in bold text.
Table 3. Beverages relative ranking for weight loss
SUCRA, surface under the cumulative ranking curves.
Sensitivity analyses
In a pre-specified sensitivity analysis with trials conducted on participants with overweight and/or obesity, none of the beverages were effective for weight loss (online Supplementary Table 4). We also performed two post hoc sensitivity analyses on healthy and unhealthy participants. In the analyses of healthy individuals, green tea drinking was effective for weight loss compared with no coffee drinking (MD: −2·67 kg, 95 % CrI: −5·25, −0·26) or water (MD: −3·31 kg, 95 % CrI: −5·83, −1·04) (Table 4). No significant effect was seen in the analysis of unhealthy individuals (online Supplementary Table 5).
Table 4. Comparative effects of different beverages on body weight in healthy individuals (Mean difference (kg) and 95 % credible interval)
Boldface used for the significancy of the values.
Grading the evidence
The certainty of the evidence for direct, indirect and network estimates is presented in online Supplementary Tables 6–8. The certainty of the evidence was rated low for the effects of green tea v. placebo, coffee v. sour tea, coffee v. placebo, sour tea v. decaffeinated coffee and water v. placebo (online Supplementary Table 8). The certainty of the evidence was rated very low for other comparisons. The magnitude of the impacts for all comparisons was lower than the minimal clinically important difference threshold (4·5 kg), suggesting that tea or coffee drinking may result in a small weight loss in adults.
Discussion
In this network meta-analysis incorporating data from twenty-two randomised trials, we compared the weight loss effects of drinking tea and coffee in adults. The results suggest that green tea drinking may result in a small weight loss in adults compared with either placebo or water with very low to low certainty of evidence. A sensitivity analysis in participants with overweight and/or obesity indicated null findings, suggesting that tea or coffee drinking was not effective for weight loss in this population. We found a larger effect of green tea on weight loss in healthy individuals; however, the results should be interpreted with caution due to the low number of trials that included healthy participants.
Previous pairwise meta-analyses addressed the potential weight loss effects of tea and coffee in different populations. The results indicated that supplementation with green tea, mainly in the form of green tea extract, reduced body weight by 0·40 kg in patients with type 2 diabetes(Reference Asbaghi, Fouladvand and Gonzalez45), by 2·80 kg in women with polycystic ovary syndrome(Reference Colonetti, Grande and Toreti46) and by 1·07 kg and 2·53 kg in adults with overweight and obesity, respectively(Reference Lin, Shi and Su9). A similar finding was seen for green coffee, where supplementation with green coffee extract reduced body weight by 1·23 kg in adults(Reference Asbaghi, Sadeghian and Rahmani11). Another meta-analysis of randomised trials suggested that supplementation with caffeine may reduce body weight in adults(Reference Tabrizi, Saneei and Lankarani10).
Most of the trials included in the previous meta-analyses assessed the weight loss effects of green tea or green coffee extracts. Indeed, the potential weight loss effects of drinking different tea or coffee in a real-world setting have not been ascertained. Drinking tea and coffee is an essential part of social life. People drink coffee and tea in various ceremonies. Tea can bring the family together and provide a platform for sharing and caring. Coffee is generally consumed outside of the home and has some social and emotional roles(Reference Verma47).
In contrast to the main analysis, our sensitivity analysis indicated that green tea drinking was not effective for weight loss in participants with overweight and/or obesity. A previous meta-analysis of randomised trials demonstrated that supplementation with green tea extract resulted in a larger reduction in body weight in participants with obesity (MD: −2·53 kg) as compared to participants who were overweight (MD: −1·07 kg) and normal weight (MD: −0·28 kg)(Reference Lin, Shi and Su9). The inconsistent findings in our network meta-analysis may be due to the low number of trials (n 9) included in that sensitivity analysis of participants with overweight and obesity.
This is also the case for the sensitivity analysis of healthy individuals, where only nine trials were available. In addition, of the nine trials included in the sensitivity analysis of healthy individuals, three trials implemented a energy-restricted diet or exercise program in their intervention program. Therefore, the larger reduction in body weight due to green tea drinking that was found in healthy participants should be interpreted with caution.
The absence of a significant effect in unhealthy individuals might be attributed to several factors. First, the metabolic and physiological differences in these populations could influence the efficacy of tea and coffee on weight loss. Individuals with metabolic conditions often have altered metabolism, insulin resistance and hormonal imbalances(Reference Roberts, Hevener and Barnard48), which might reduce the impact of tea and coffee constituents on weight reduction. Additionally, the interventions’ duration and dosage might not have been sufficient to elicit a significant weight loss effect in these populations. Variability in the baseline characteristics of participants, such as diet, physical activity levels and adherence to the intervention, could also contribute to the null findings. Moreover, the small number of trials and the potential presence of confounding factors further complicate the interpretation of these results.
The potential weight loss effects of green tea and coffee may be partly due to the high caffeine content in these beverages. A typical brewed green tea beverage, consisting of 2·5 g tea leaves in 250 ml of hot water, contains 20–50 mg caffeine(Reference Balentine, Wiseman and Bouwens49,Reference Sang, Lambert and Ho50) , compared with 235 mg caffeine in 12 fluid oz. of coffee shop brewed coffee and ∼ 50 mg caffeine in 250 ml of brewed black tea(Reference van Dam, Hu and Willett1). Caffeine can induce noradrenaline and dopamine release, thus can stimulate neuronal activity in the brain, leading to a decrease in body weight(Reference Zheng and Hasegawa6). However, considering the null effects of coffee and black tea on body weight, it seems that other components found in green tea may explain the weight loss effects of green tea. Green tea polyphenols can decrease the absorption of lipids and proteins in the intestine and thus can reduce calorie intake(Reference Yang, Zhang and Zhang51). In addition, green tea polyphenols can activate AMP-activated protein kinase in the liver, skeletal muscle, and adipose tissues and thereby can induce substrate oxidation(Reference Yang, Zhang and Zhang51).
Different types of tea, including black tea and especially green tea, contain several healthful polyphenols entitled catechins, including epicatechin, epicatechin gallate, epigallocatechin and epigallocatechin gallate(Reference Yang, Zhang and Zhang51). Green tea is a rich dietary source of cathechins, in a way that about 30–42 % of the dry weight of brewed green tea consists of catechins, of which, epigallocatechin gallate is the principal constituent(Reference Yang, Zhang and Zhang51). Every 250 ml of green tea beverage contains 240–320 mg of catechins, most of which (60–65 %) is epigallocatechin gallate(Reference Balentine, Wiseman and Bouwens49,Reference Sang, Lambert and Ho50) . The primary polyphenols in black tea are thearubigins, catechins and theaflavins(Reference Yang, Zhang and Zhang51). The polyphenols found in tea, especially green tea, can reduce digestion and absorption of carbohydrates, lipids and proteins and can affect gut microbiota and possibly the gut–brain–liver axis(Reference Yang, Zhang and Zhang51). Epigallocatechin gallate can activate AMP-activated protein kinase, thus can reduce fatty acid synthesis and deposition of fats in muscle and liver(Reference Yang, Zhang and Zhang51).
Although our findings suggest that drinking black tea and different types of coffee was ineffective for weight loss, several healthful components exist in these beverages. Black tea and coffee are among the richest dietary sources of various polyphenols that have antioxidant, anti-inflammatory and cardioprotective properties(Reference Tomas-Barberan and Andres-Lacueva52). Epidemiologic studies suggest that drinking coffee may be associated with a lower risk of all-cause and cause-specific mortality, CVD, site-specific cancers and several metabolic and neurological disorders(Reference Poole, Kennedy and Roderick4).
Strengths and limitations
The present network meta-analysis is the first work to compare and rank the comparative effects of different tea and coffee on weight loss in adults. Previous pairwise meta-analyses have mainly assessed the effects of green tea or green coffee extracts on body weight, and the potential weight loss effects of different beverages have not been ascertained. We compared and ranked the weight loss effects of these beverages and rated the certainty of evidence using the Grading of Recommendations Assessment, Development, and Evaluation approach. We compared the magnitude of the impacts with the minimal clinically important difference threshold for weight loss, indicating that green tea drinking may result in a small reduction in body weight compared with water or placebo. There are also some limitations that should be considered. First, most of the trials included in the present network meta-analysis investigated the weight loss effects of green tea and coffee. Indeed, limited data are available about green coffee, black tea and sour tea. Second, only five trials had an intervention duration longer than 12 weeks, thus the potential weight loss effects of tea and coffee drinking, especially green tea drinking, in the long term should be investigated in future research. Third, it is important to discuss the potential impact of the risk of bias on the results of this meta-analysis. Most of the trials were rated to have a high risk of bias or some concerns. These biases could potentially affect the reliability and validity of the findings. To mitigate these concerns, we conducted sensitivity analyses excluding studies with high risk of bias; however, the overall certainty of evidence remains very low to low. Future studies with more rigorous designs are needed to confirm these findings and provide more definitive conclusions regarding the weight loss effects of tea and coffee.
Conclusions
Based on very low to low certainty evidence obtained from short-term trials, green tea drinking may result in a small reduction in body weight compared with water or a placebo. These findings should be confirmed in trials with better methodological quality and longer intervention duration, especially in adults with overweight and/or obesity.
Acknowledgements
This study was supported by Evidence Based Medicine Research Center, Endocrinology and Metabolism Clinical Sciences Institute, Tehran University of Medical Sciences, Tehran, Iran (grant number: 63681).
A. J. contributed to the study conception, literature search, data extraction, data analysis and manuscript drafting. R. N. and A. M. R. contributed to the literature search, data extraction and manuscript drafting. S. S-B. contributed to study conception, data analysis and manuscript drafting. A. A. and A. S. contributed to the manuscript drafting and approving the final manuscript. All authors acknowledge full responsibility for the analyses and interpretation of the report. All authors have read and approved the final manuscript. S. S-B. is the guarantor. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted.
There are no conflicts of interest.
This study was conducted according to the guidelines laid down in the Declaration of Helsinki, and all procedures involving research study participants were approved by the ethics committee of the Tehran University of Medical Sciences (Ethic code: IR.TUMS.EMRI.REC.1401.075).
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
Supplementary material
For supplementary material/s referred to in this article, please visit https://doi.org/10.1017/S0007114524001867
