Effect of Acute and Chronic Dietary Supplementation with Green Tea Catechins on Resting Metabolic Rate, Energy Expenditure and Respiratory Quotient: A Systematic Review

The consumption of green tea catechins (GTC) is associated with modulations of fat metabolism and consequent weight loss. The aim of this systematic review was to investigate the effect of GTC on resting metabolic rate (RMR), energy expenditure (EE), and respiratory quotient (RQ). Eligible studies considered both the chronic and acute intake of GTC-based supplements, with epigallocatechin gallate (EGCG) doses ranging between 100–800 mg. Findings from 15 studies (n = 499 participants) lasting 8–12 weeks (for chronic consumption) or 1–3 days (for acute intake) are summarized. This review reveals the positive effects of GTC supplementation on RQ values (272 subjects). Regarding the effects of acute and chronic GTC supplementation on RMR (244 subjects) and EE (255 subjects), the results did not allow for a definitive conclusion, even though they were promising, because some reported a positive improvement (two studies revealed an increase in RMR: one demonstrated an RMR increase of 43.82 kcal/day and another demonstrated an increase of 260.8 kcal/day, mainly when subjects were also engaged in resistance training exercise). Considering GTC daily dose supplementation, studies in which modifications of energetic parameters occurred, in particular RQ reduction, considered GTC low doses (100–300 mg). GTC may be useful for improving metabolic profiles. Further investigations are needed to better define adequate doses of supplementation.


Introduction
Obesity is a common disorder with complex causes; it is manifested by a chronic energy imbalance characterized by the excessive accumulation of fat and the excessive conservation of triacylglycerol in the cells that form adipose tissue [1,2].
The current treatment of obesity includes reducing the calorie intake in the diet and increasing physical activity in order to increase energy expenditure (EE), but this is still not enough to reduce the trend and prevalence of obesity worldwide [3].
Recently, dietary supplements have been proposed for the management of body weight in order to counteract obesity [4].
Research in this area is expanding, with particular emphasis in the area of lipid mobilization [5]. An example of bioactive compounds present in food, which can increase the efficiency of weight loss by lipid mobilization, is green tea catechins.
Epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG) are the four most abundant flavonoids in green tea; in particular, EGCG is the most active and most abundant polyphenol, as it represents about 35% of total catechins and has received attention as a potential anti-obesigenic agent [6].
In animal models, it has been demonstrated that the consumption of green tea or its main components, green tea catechins, is associated with weight loss through the modulation of fat metabolism and EE [7][8][9].
In humans (specifically obese subjects), various randomized controlled trial(RCT) have demonstrated a positive effect of green tea on weight loss or body composition [10][11][12][13][14]; green tea taken with co-supplements (capsaicin and ginger) has also been found to have beneficial effects on weight loss [15].
A 2012 Cochrane review, while not recent, claimed that green tea preparations appear to induce a small, statistically non-significant weight loss in overweight or obese adults [16], and more recent a systematic review in 2017 reported that the daily consumption of green tea with doses of EGCG between 100 and 460 mg/day shows a greater effectiveness on body fat and body weight reduction in intervention periods of 12 weeks or more [17].
These conclusions were confirmed by a more recent meta-analysis published in 2019, which suggested that the consumption of green tea supplementation in obese subjects with metabolic syndrome has beneficial effects on the improvement of lipid and glucose metabolism, as well as in the facilitation of weight loss [18].
There are several proposed mechanisms whereby green tea catechins (GTC) may influence body weight and composition. The predominant hypothesis is that GTC influence sympathetic nervous system (SNS) activity, increasing EE, and promoting the oxidation of fat. Other potential mechanisms include modifications of appetite, the up-regulation of enzymes involved in hepatic fat oxidation, and decreased nutrient absorption [19,20].
In vitro studies have shown how EGCG and ECG can be involved in the inhibition of lipogenesis, interacting with the acid-fat-synthetase enzyme [21][22][23]. A further mechanism of action studied for GTC is the inhibition of the activation of the transcription nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), which leads to the overexpression of an enzyme involved in the β-oxidation of lipids as a final response [24,25], a mechanism that, among other things, explains the fact that the increase in fat metabolism continues over time, even after the cessation of the intake of tea catechins [26].
It was also highlighted that green tea can be involved in the processes that regulate glucose absorption [27][28][29][30].
In in vivo studies conducted on animal models, in which obesity was induced by diet or caused by a genetic origin, have shown that treatment with green tea or its polyphenolic compounds is effective in aiding the loss or maintenance of body weight [31][32][33][34][35][36].
The catechins in green tea may stimulate thermogenesis and fat oxidation through inhibition of COMT, an enzyme that degrades norepinephrine (NE) [45]. Studies in rats and mice have shown an EGCG-induced reduction in food intake and/or an increase in EE [46].
In humans, there has only been one meta-analysis on this topic that supports the findings that EGCG has an effect on metabolic parameters [47].
Given this background, this systematic review was aimed at investigating the effects of acute and chronic GTC supplementation on metabolism, in particular on resting metabolic rate (RMR), EE, and respiratory quotient (RQ).

Materials and Methods
The present systematic review was conducted in accordance with the preferred reporting items for systematic review and meta-analyses (PRISMA) statement [48]. It was carried out through the following steps: (1) the formulation of the review question: "what are the benefits on metabolism associated with the consumption of green tea?"; (2) the defining of subjects: humans; (3) the formulation of a search strategy for the identification of relevant intervention studies that included the effect of green tea on metabolism; and (4) the analysis of the data through the systematic review.

Search Strategy
Articles written in English were identified by searching Scopus (https://www.scopus. com/home.uri) and Google Scholar (https://scholar.google.it/). The search strategy was based on the following search terms: "green tea" AND "green tea extract" AND "resting metabolic rate" AND "basal metabolic rate" AND "energy expenditure" AND "respiratory quotient" and "substrate oxidation" AND "substrate utilization." The literature search was conducted in September 2020, and this search retrieved 715 studies that were entered in the flowchart process ( Figure 1). Eligible studies were required to report baseline values, follow-up values, and the daily dosage of green tea used to detect effects on energy metabolism parameters, in particular RMR, EE and RQ. Eligible studies were required to consider green tea extracts with no more than 50 mg of caffeine content. Dietary supplementation was considered in both capsule and liquid forms.   Table 1 shows the effects of green tea supplement on RMR values, summarizing evidence from seven studies (244 subjects). Four of these seven studies considered the chronic intake of green tea, lasting from 8 to 12 weeks of intervention. The remaining three studies referred to an acute intake of green tea extract during the test days.

Chronic Consumption
Regarding chronic consumption, two studies revealed an increase in RMR values after green tea extract supplementation. In particular, in the randomized controlled trial by Auvichayapat et al., at the eight week of intervention, the difference in fasting RMR was 43.82 kcal/day (183.38 kJ/day) (p < 0.001); the author assumed that the registered weight loss was due to the increase in EE and fat oxidation [42]. Cardoso et al. showed that the increase in RMR values of 260.8 kcal/day (1091.92 kJ/day) occurred when subjects who consumed green tea were also engaged in resistance training exercise. This was due to the significant increase in lean body mass, which is more metabolically active, and the body consequently expends more energy to maintain it. In contrast, the subjects who supplemented with green tea alone, without physical activity, showed a decrease in RMR (-270.4 kcal/day-1130.44 kJ/day) due to decreased body mass, which lowers the calorie expenditure necessary for body mass maintenance [49].
The other two studies included in the present review considering chronic assumption did not find any significant changes in fasting RMR and substrate oxidation after 8 and 12 weeks of supplementation with green tea, respectively, compared to baseline values [14,50] 3.1.2. Acute Intake Concerning acute intake, just one study showed that the tea supplement increased fasting RMR over two hours measured as the area under curve (AUC). The greatest difference between supplement and placebo was seen at one hour [51]. The other studies reported that the short-term consumption of a commercially available EGCG supplement did not increase fasting RMR [52,53]. A base diet started 4 weeks prior to the green tea protocol was continued for all the duration of the treatment.
The RMR decreased significantly in groups 1 and 2 but increased significantly in groups 3 and 4 (p < 0.05). In group 1, the RMR for body mass showed no change, while in group 2, there was a measurable but not significant decline. In groups 3 and 4, the RMR for body mass increased significantly, and it was highest in group 3.
Indirect calorimetry with a pneumotachograph connected to a gas analyzer (Aerosport VO2000; Medical Graphics Corporation).
Usual diet. Macronutrient composition not specified.   Figure 2a,b shows that there was an overall positive correlation between posology and RMR: the coefficient was r = 0.797 and r 2 = 0.636, The association was positive: the increase in posology was related to an increase in RMR. As shown in Figure 2a, this trend was correlated for acute and chronic intake. Between acute and chronic intake, there was not any statistically significant difference for the trend of 0.273.

Acute Intake
Concerning acute intake, just one study showed that the tea supplement increased fasting RMR over two hours measured as the area under curve (AUC). The greatest difference between supplement and placebo was seen at one hour [51]. The other studies reported that the short-term consumption of a commercially available EGCG supplement did not increase fasting RMR [52,53]. Figure 2a,b shows that there was an overall positive correlation between posology and RMR: the coefficient was r = 0.797 and r 2 = 0.636, The association was positive: the increase in posology was related to an increase in RMR. As shown in Figure 2a, this trend was correlated for acute and chronic intake. Between acute and chronic intake, there was not any statistically significant difference for the trend of 0.273.  Table 2 shows the effects of green tea supplement on EE values, summarizing evidence from eight studies (255 subjects). Four of these eight studies considered the chronic intake of green tea lasting from 8 to 12 weeks of intervention. The remaining four studies referred to an acute intake of green tea extract during the test days.  Table 2 shows the effects of green tea supplement on EE values, summarizing evidence from eight studies (255 subjects). Four of these eight studies considered the chronic intake of green tea lasting from 8 to 12 weeks of intervention. The remaining four studies referred to an acute intake of green tea extract during the test days.    Abbreviations: EGCG, epigallocatechin gallate; VO2, oxygen consumption; VCO2, carbon dioxide production; ANOVA, analysis of variance; EE, energy expenditure; RQ, respiratory quotient.

Chronic Intake
Regarding chronic intake, most of the studies (three studies) included in this review revealed no statistically significant changes in EE after supplementation with green tea extract when compared to baseline values [54][55][56]. On the contrary, in the study by Chantre et al., the subjects consuming a daily dose of 375 mg of catechins (of which 270 mg were epigallocatechin gallate) showed a significant weight loss at the end of the treatment. The green tea extract stimulated thermogenesis and fat oxidation, and these results indicated the potential of GTC to influence body weight and body composition via changes in both EE and substrate utilization [57].

Acute Intake
The same trend was observed in studies about acute intake. Only one study reported a significant increase in 24-h EE in the green tea-treated group, with a mean increase of 330 KJ; the total 24-h EE with the green tea extract was significantly higher than both the placebo and caffeine groups by 3.5% and 2.8%, respectively [26]. The other three studies reported no statistical significant changes in EE values after assuming green tea extract [58][59][60]. Table 3 reports the effects of green tea supplement on RQ values, summarizing evidence from eight studies (202 subjects). RQ values were detected in both chronic and acute studies; three of these eight studies considered the chronic intake of green tea, and the intervention was 12 weeks. The remaining five studies referred to an acute intake of green tea extract during the test days.

Chronic Intake
Concerning chronic intake, only two studies considered the evaluation of RQ. One of these revealed a significant reduction in fasting RQ during supplementation with the green tea extract, suggesting an increase of fat oxidation and a decrease in carbohydrate oxidation [42,57]. This finding suggested that EGCG alone has the potential to increase fat oxidation and may thereby contribute to the anti-obesity effects of green tea. On the contrary, the study by Mielgo-Ayuso detected no significant changes in non-protein RQ and whole-body fat oxidation between the EGCG and control groups.

Acute Intake
Of the five studies focusing on acute intake, two revealed a significant reduction of RQ values after green tea extract administration [26,60], whereas the remaining three studies did not show statistically significant differences between the supplement and placebo groups in RQ [51,53,59].
This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, and the experimental conclusions that can be drawn. Figure 3a,b shows that there was an overall positive correlation between the posology and RQ: the coefficient was r = 0.392, and r 2 = 0.154, The association was positive, with an increase of posology related an increase of the RQ. As shown in Figure 3a, this trend was correlated for acute and chronic intake. Between acute and chronic intake, as shown in Figure 3b, there was not any statistically significant difference for the trend of 0.690. Thai diet (65% carbohydrates, 15% protein, and 20% fat).
The reduction in RQ during treatment with the green tea extract suggested that fat oxidation was higher.

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Group 1: tablets of 500 mg of green tea extract (where 125 mg were catechins).   ogy and RQ: the coefficient was r = 0.392, and r = 0.154, The association was positive, with an increase of posology related an increase of the RQ. As shown in Figure 3a, this trend was correlated for acute and chronic intake. Between acute and chronic intake, as shown in Figure 3b, there was not any statistically significant difference for the trend of 0.690.

Sensitivity Analysis
To assess the impact of study quality on effect sizes, sensitivity analyses were conducted for each study included in the present systematic review. For each scale, studies were divided into two groups (low quality and high quality) based on total quality score. When the scale developer suggested a cut-off point for low versus high quality, this was

Sensitivity Analysis
To assess the impact of study quality on effect sizes, sensitivity analyses were conducted for each study included in the present systematic review. For each scale, studies were divided into two groups (low quality and high quality) based on total quality score. When the scale developer suggested a cut-off point for low versus high quality, this was used.
Methodological quality and sensitivity analysis showed that fifteen studies were evaluated (Table 4). Inter-rater reliability was over 3 for the Jadad scale and the quality index. The selected studies (RCT) had a median quality score of 3.67 (range 1-5) out of a maximum score of 5 on the Jadad scale. The cohort or intervention studies that used the Chalmers scale had a median quality score of 60% (range 0-10).

Discussion
The present systematic review revealed the positive effects of the supplementation of GTC on fasting and postprandial RQ values.
As a reflection of carbohydrate and fat oxidation, RQ may be a metabolic index that predicts subsequent weight gain [63]. In general, RQ is in the range from 0.70 (complete fat oxidation) to 1.0 (complete carbohydrate oxidation), and anywhere within this range indicates that a mixture of energy substrates are simultaneously oxidized [64].
RQ represents a key metabolic predictor of weight gain and obesity [64]. The relationship between obesity and RQ is dynamic in response to weight changes. A high fasting RQ could be a predictor of increased body weight and fat mass over a 12-month period among young adults when compared with individuals with a low/moderate or low-RQ value [65].
Individuals with high levels of cardiorespiratory fitness and low levels of fat mass have been shown to be 'metabolically flexible,' so they are able to switch between glucose and fat oxidation in response to homeostatic signals, such as in postprandial or fasting condition [66].
RQ is affected by different factors, such as diet composition (high levels of carbohydrates results in elevated RQ levels), body composition (an increases in fat mass lead to an elevations of RQ) [65], and even genetic trim [67].
Given the importance of knowing the metabolic aspect of each subject, the RQ evaluation through indirect calorimetry should be the basis for personalized dietary interventions.
The observed reduction in RQ values indicates high fat oxidation and low carbohydrate oxidation, and this finding represents a promising strategy for weight loss. In a previous study by Rondanelli et al., the consumption of a combination of bioactive food ingredients (containing epigallocatechin gallate, besides capsaicins, piperine, and L-carnitine) produced a significant increase in RMR and a significant decrease in RQ, with a consequent reduction in body mass index (BMI) and fat mass values, as assessed by dual-energy X-ray absorptiometry (DXA) [68].
Regarding the effects of acute and chronic GTC supplementation on RMR and EE, the results did not allow for a definitive conclusion because some included studies reported a positive improvement but others reported no changes. Moreover, the dose-response analysis revealed that a dose-dependent association existed only for RMR, while there was no dose-effect association for RQ.
A strength of this systematic review is the fact that the included studies considered green tea extracts or, at least, very small amounts of caffeine (<50 mg of the total content of green tea extract). Recent interesting studies have investigated the effects of green tea extract supplementation on metabolic parameters and body composition, but these supplement had high caffeine contents [69][70][71]. The same observation was first carried out by Dullo et al., who revealed that the effects of the green tea extract in enhancing thermogenesis and fat oxidation could not solely be explained on the basis of its caffeine content, because treatment with an amount of caffeine equivalent to that in the extract failed to alter EE, RQ, or substrate oxidation [26]. The implication of this result is that these metabolic effects resulted from compounds other than caffeine in the green tea extract. The most likely explanation for the lack of a thermogenic effect of caffeine is that the dosage (50 mg at 3 times/day) was below the threshold for stimulating thermogenesis. Cardoso et al. considered a daily dose of 20 g of green tea with 40 mg of caffeine; the authors declared that, at these levels, neither caffeine nor theobromide presented any effect on the appraised parameters, and only the catechins presented some effects in this case. Even a meta-analysis showed that catechin-caffeine mixtures, like caffeine-only supplementation, stimulate daily energy expenditure dose-dependently when 0.4-0.5 kJ mg −1 was administered. However, compared with placebo, daily fat-oxidation was only significantly increased after catechincaffeine mixture consumption [72]. This is an important finding regarding green tea extracts containing different kinds of catechins.
The present review confirmed the results obtained from a recent systematic review and meta-analysis, revealing that EGCG intake moderately reduces RQ [47]. The analyses also showed that the EGCG resulted in metabolic and EE differences, but the effects on the other measures of energy metabolism, such as fat oxidation, were relatively mild. Overall, these outcomes, together with our results, support the findings that EGCG has an effect on metabolic parameters, as previously reported by Kapoor [47].
A further consideration could be the daily dose of consumed green tea required for detecting effects on energy metabolism. The present systematic review included studies considering a wide range of EGCG daily dose supplementations, from 100 to 800 mg. Interestingly, the studies in which modifications of energetic parameters were detected, in particular RQ reduction, used low doses of EGCG, from around 100 to 300 mg. These findings were in accordance with the study of Kapoor et al., in which it emerged that EGCG alone has the potential to increase metabolic rate at a 300 mg dose [47].
This review had some limitations, such as the heterogeneity of population considered in each study and the different prescriptions allowed in parallel to the treatment (physical activities, abstention from caffeinated food, etc.). Moreover, nutritional supplements were not homogeneous, as some green tea extracts were provided as capsules and others were administered diluted in water.
Overall, the current findings support the fact that EGCG has an effect on metabolic parameters. Even though the observed effects were mild and data were limited, an improvement in energy metabolism parameters was revealed by some studies included in the current review. Thus, the intake of GTC, which contain EGCG, represents a valid plant-based and safe dietary supplementation; when administered in a proper dose, GTC supplements could be useful tools during weight loss programs because of their effects on thermogenesis stimulation and fat oxidation. The observed findings are encouraging, but further investigations are needed in order to better understand the potentiality of green tea extract on human metabolism and the adequate dose of supplementation that is useful for significative changes on metabolic parameters.

Conflicts of Interest:
The authors declare no conflict of interest.