Green tea, a beverage celebrated worldwide for its health benefits, is rich in antioxidants like catechins and polyphenols. These compounds, particularly epigallocatechin gallate (EGCG), have shown promise in supporting weight loss by boosting metabolism, enhancing fat oxidation, and improving overall body composition. With a rich history rooted in ancient traditions, green tea is now gaining modern recognition for its potential to aid in combating obesity and improving metabolic health.
1. Introduction
According to reviews, tea polyphenols have been widely used as natural antioxidants in the food and cosmetic industries. They possess important pharmacological properties that can be effective in the prevention and treatment of chronic diseases. Tea, mostly black, green, and white tea, is one of the most consumed beverages throughout the world containing a high concentration of polyphenols, catechins, and flavonoids which significantly contribute to the flavors and health promoting potential of green and white tea. Excessive fat accumulation describes obesity, a chronic multifactorial disorder. It leads to type two diabetes, metabolic syndrome, pulmonary diseases, polycystic ovary syndrome, various cancers, osteoarthritis, social stigmatization, and discrimination. Obesity is a serious public health challenge in both developing and developed countries. Weight loss, weight maintenance, and waist size reduction are obesity treatment strategies [1].
White tea, green tea, and green tea extracts, as well as their catechins, may block the development of obesity, and some investigators have claimed a relationship between green tea or green tea consumption and obesity. Daily intake of daily green tea for 12 weeks, particularly for longer durations than 12 weeks in doses of 800 mg or less, was found to improve body weight or body mass index. The extracts, catechins or polyphenols in tea, reduced levels of low-density lipoproteins in the blood, and catechins inhibited α-amylase and lipase, interfering with digestion. Moreover, caffeine in tea, when consumed in doses greater than 300 mg per day, altered thermogenesis and metabolism in white adipose tissues in a role for weight loss. Tea, particularly white tea extracts derived from epigallocatechin gallate and epicatechin gallate, affected genes involved in cholesterol metabolism [2].
2. Historical Background of Green Tea
Green tea comes from a plant known as Camellia sinensis. It is said to have been discovered not because of the taste but because of its health benefits. Chinese medical texts from as early as 2737 B.C.E. describe the medicinal properties of tea. Everyone seems to love tea, and green tea has been the favorite for most people since the beginning of the research. In almost all studies bothered to compare green tea with other types of tea; it is always found that the health benefits of green tea far outweigh black tea and other teas. [1]
Epidemiological surveys and clinical studies have shown that drinking green tea is associated with a lower risk of certain types of cancers. It has, therefore, been widely researched. All the studies seem to strongly support the hypothesis that catechins in green tea, especially epigallocatechin gallate (EGCG), have antioxidant properties. It protects the following from free-radical damage: (a) LDL, the bad cholesterol that causes atherosclerosis and heart problems; (b) DNA, so it prevents genetic damage leading to cancer; and (c) most surprisingly, the brain, protecting neurons against neurodegenerative diseases such as Alzheimer’s disease. Because of the cumulative effects of the catechins, other antioxidants in the body can exert their antioxidant activity better. For instance, drinking green tea can preserve vitamin C levels in the body better [2].
3. Chemical Composition of Green Tea
Green tea derives from processed young green leaves, strongly influenced by the climate and the geography of the vicinities, which produce a complex composition of chemical constituents. The principal chemical constituents of green tea, including catechins, amino acids, caffeine, polysaccharides, alkaloids, and proanthocyanidins, contribute to the potential health benefits of green tea [3]. Tea polyphenols, catechins, and polyphenolic compounds in green tea extracts are the primary constituents for its capability to function as antioxidants. Green tea mainly contains catechins or polyphenolic compounds, accounting for approximately 30% of the dry weight; hence, green tea extracts are rich in catechins. The four major catechins present in green tea are (−)-epigallocatechin-3-gallate (EGCG), (−)-epicatechin gallate (ECG), (−)-epigallocatechin (EGC), and (−)-epicatechin (EC), all possessing antioxidative activities [1]. In addition, green tea contains some important amino acids with unique structures, the predominant one being L-theanine, accounting for approximately 1.5–2.5% of the dry weight. This compound has been reported to be advantageous for the neuro-developmental trajectories of the young.
Eight types of alkaloids have been identified in green tea. Among them, caffeine (1.3–4.0% expressed as dry weight), theobromine (0.1%), and theophylline (0.03%) are more abundant. Interestingly, caffeine has been reported to enhance the thermogenic effect and lipid mobilization of catechins, augmenting the weight-reducing action of green tea extracts. Green tea polysaccharides, mainly consisting of glucose, galactose, and arabinose, have also been determined and exhibited antioxidative actions comparable to those of tea catechins. Proanthocyanidins, a type of tannin, are composed of a polymerization of catechins. Upon the fermentation of green tea leaves, some of the catechins are oxidized and polymerized to produce theaflavins and thearubigins.
3.1. Catechins
Catechins are a type of polyphenol, a large group of chemical compounds found naturally in plants that contain antioxidant properties. There are four main types of catechins found in green tea: epicatechins (EC), epicatechin gallate (ECG), catechin (C), and epigallocatechin gallate (EGCG). Epigallocatechin gallate is a catechin derivative abundant in green tea, accounting for nearly one third of green tea’s composition. It has gained large interest and recognition as the catechin derivative that possesses the most bioactive properties [4].
Each of the catechins has different but overlapping antioxidant properties. The amount of catechins present in a cup of green tea, in general, depends on the type of green tea, where it was grown, as well as how the leaves were processed. Catechins are largely stable in their monomer form at higher pH (> 7.0) environments and become more unstable (i.e., they polymerize into dimers, trimers, and so on) when in an acidic environment. A number of studies have shown that catechins are most stable in an environment with a pH that varies between 4 to 6. In the stomach, the pH of gastric juice is approximately 2, which is quite acidic. As a result, in the stomach, catechins are predicted to polymerize, and some catechins are known to be completely degraded under such conditions. Nevertheless, when catechins are administered through their infusion method (i.e., their drinking method), catechins in green tea are present in a concentration that is greater than 1 mM at the time of entering the duodenum area, where the pH is alkaline (8.6 and thus catechins are highly stable). From the duodenum, catechins, at least in part, are absorbed into the human circulation. Because there is little degradation of catechins in the proximal small intestine, catechins are predicted to predominantly exist in their monomer form when absorbed. Overall, catechins could exert their physiological activities at least partly as their monomer form. Understanding more about catechins would inform and create a better comprehension of the overall physiological effects of green tea.
3.2. Polyphenols
Several studies have explored the chemical composition of tea, revealing a variety of bioactive compounds. Polyphenols, particularly catechins, may be plasmid-disseminating antibiotics [5]. The most abundant catechin in green tea is (-)-epigallocatechin-3-gallate (EGCG), accounting for 50-75% of total catechins. Polyphenols are of central importance in exploring the health benefits of green tea, especially its antioxidant properties. Supplementation with green tea extract can attenuate body weight gain, hepatic steatosis, dyslipidemia, and inflammation in diet-induced obese mice [1]. Tea extracts can also ameliorate oxidative stress in rats fed high-fructose diets. Moreover, tea extracts can modulate polyphenol and microbial metabolites concentrations in mouse cecum and serum.
Polyphenols possess a high redox potential, allowing them to transform a phenolic hydroxyl (ArOH) group into a phenoxyl radical (ArO•). The radical property of phenoxyl can facilitate further radical polymerization with different radicals such as pyrrolyl or the amine radicals during the process of co-pyrolysis of polyphenols. Polyphenol-derived oxygen-containing heterocycles (OC-heterocycles) such as phenol, guaiacol, catechol, and catechin, have a distinctive paramount ring-opening polymerization (ROP) mechanism. The cleavage of the hydroxymethyl substituent on the aromatic ring of catechol generates an alkyl radical capable of exerting reactivity. The catechin polymerization by ROP would produce a plethora of polycatechin oligomers destructively co-pyrolized with other co-feeds, forming complex cups occupied by various bioactive compounds.
4. Antioxidant Properties of Green Tea
This section delves into the antioxidant properties of green tea, shedding light on its capacity to combat oxidative stress and its potential impact on overall health and weight loss. Packed with antioxidants known as catechins, green tea is adored by many as the healthiest beverage on the planet. Antioxidants are substances that scavenge harmful oxidative agents in our body, called free radicals, thereby inhibiting oxidation and, as a result, combating heart disease, cancers, skin damage, aging, and a host of other illnesses [5]. Surprisingly, free radicals are not all bad, as they are also needed by cells, and normally exist in balance with antioxidants. Such balance is important for a number of reasons, as excess free radicals can cause oxidative stress (OS) and tissue injury. Sadly, this delicate equilibrium is easily disturbed in favor of free radicals by aging, pollution, smoking, heavy metal, excess alcohol intake, drug overdoses, and pesticide residues. When this happens, injuries can occur due to peroxidation of lipids, proteins, and DNA. Epidemiological and clinical trials suggest that excessive free radicals and OS are definitely implicated in the etiology of many dis-eases and conditions. These include cancers, atherosclerosis, dyslipidemia, hypertension, neurodegenerative diseases (e.g., Alzheimer’s and Parkinson’s), and lung injuries. A systematic review on the antioxidant properties of green tea concluded that, irrespective of differences among individuals in terms of studies population, type, brewing and consumption style, etc., green tea as commonly consumed in the Eastern world is superior to other beverages presently known with respect to antioxidant capacity and OS inhibition. This has sparked great interest in green tea (and its catechins) as a potential therapeutic option for these diseases. However, there is growing realization that, despite exciting results in vitro or in animal models, many botanical or herbal remedies, including tea polyphenols, fail to translate into success in human clinical trials. At least part of the reasons appears to be flawed methodology inherent in the studies, rather than direct defects of the herbs. In spite of these difficulties, catechins remain of great interest while the anti-OS properties of green tea deserve more attention.
5. Mechanisms of Action in Weight Loss
Thermogenesis and fat oxidation are physiological processes that promote the utilization of body fat as an energy source, leading to body weight regulation. They differ in terms of site, site, mode, and physiological significance. The body provides energy by metabolizing substrates, including glucose, fatty acids, and amino acids. The metabolic processes of these substrates include glycolysis, β-oxidation, TCA cycle, and oxidative phosphorylation, which yield energy in the form of ATP and dissipate heat. Heat dissipation is also carried out through thermogenic processes, such as mitochondrial uncoupling, futile cycling, and non-ATP energy-consuming processes in other organelles [5]. Brown and brite adipocytes have a unique mitochondrial uncoupling with uncoupling protein 1 (UCP1), converting the stored triglycerides into heat energy, which is first activated by noradrenaline.
Fatty acid oxidation is a process of breaking down long-chain fatty acids into acetyl-CoA. It reduces energy storage in the form of triglycerides and promotes energy utilization. Fatty acid oxidation starts with the activation of fatty acids in the cytoplasm, forming acyl-CoA, which is catalyzed by acyl-CoA synthetase. Then acyl-CoA is translocated into mitochondria, which is catalyzed by the carnitine acyltransferases (CPT) I and II. The β-oxidation occurs in mitochondria, generating acetyl-CoA, NADH, and FADH2. Acetyl-CoA is then fed into the TCA cycle [3].
5.1. Thermogenesis
[5]. There are positive thermogenic effects of tea extract on mitochondrial biogenesis and activity in brown adipose tissue of diet induced obese mice. Green tea and tea extracts ingestion augment energy expenditures and fat oxidation in humans. The purpose of this study was to determine the effects of tea extracts on thermogenesis [6]. The administration of tea extracts did not alter heart rate or blood pressure, the daily amounts of tea infusion in this study were not hypertensive. The acute ingestion of tea extracts could increase energy expenditure, fat oxidation, and glucose metabolism in humans. The 24h energy expenditures gained during cocoa and green tea intake were accounted for 3.29% of total energy expenditure and expressed 136.26 kcal/day which is similar to the supplements of green tea catechins. Tea extracts suppressed Simpson’s metabolic index while catechin supplementation tended to increase it significant catechin intake did not significantly increase daily energy expediture (EE). The tea extracts augmented fat oxidation (FO) during the post-ingestion period but did not alter the 24h overall fat oxidation.
5.2. Fat Oxidation
β-oxidation of fatty acids occurs in mitochondria and is the major mode of fat oxidations. During this process, fatty acids are degraded to acetyl-CoA via dehydrogenation (by acyl-CoA dehydrogenase), hydration (by enoyl-CoA hydratase), β-hydroxyacyl-CoA dehydrogenation (by β-hydroxyacyl-CoA dehydrogenase) and thiolytic release (by thiolase). Then the acetyl-CoA produced from β-oxidation of fatty acids enters into the tricarboxylic acid cycle. Fatty acyl-CoA molecules act as substrates of mitochondrial β-oxidation. Long-chain acyl-CoA must be transported into mitochondria via the carnitine-palmityl transferase (CPT) system. Carnitine acylcarnitine translocase transports acylcarnitine into mitochondria and releases carnitine. Inside the mitochondrion, carnitine acyltransferase 2 transfers long-chain fatty acids back to acyl-CoA. All the above proteins that catalyze the steps of mitochondrial β-oxidation need to be upregulated to stimulate this pathway. Carnitine palmitoyltransferase 1 (CPT1) and acyl-CoA oxidase are rate-limiting steps of fatty acids and their acyl-CoAs, respectively, entering into mitochondrial β-oxidation [7]. The goal of this experiment was to demonstrate whether polyphenols in green tea extracts could directly stimulate fat oxidation pathways in skeletal muscles by using the experiments involving extensor digitorum longus (EDL) muscle from C57BL/6J mice and Fraser Valley special green tea produced by a unique process to maintain higher polyphenol contents than other common teas [5]. It was found that green tea polyphenols stimulated fat oxidation via activation of proteins in fat oxidation pathways including long-chain fatty acyl-CoA synthetase, CPT1, acyl-CoA oxidase and fatty acyl-CoA hydratase in skeletal muscle.
6. Research Studies on Green Tea and Weight Loss
There have been a number of studies measuring the impact of drinking green tea on weight loss and other body composition variables. Some studies have shown little in the way of statistically significant effects, while other studies have shown a significant decrease in body weight, body mass index, waist circumference, and percent body fat, as well as an increase in fat-free mass or fat-free mass index [2]. Most studies suggest that drinking green tea increases energy expenditure. One study showing no effect of green tea on weight loss also recorded no effect on daily energy expenditure. In contrast, multiple studies demonstrating decreases in weight and waist circumference also recorded increases in energy expenditure, total fat oxidation, and/or 24-hour fat excretion [1]. The suggested mechanism through which drinking green tea affects weight loss is by partially augmenting the thermogenic effect of exercise.
Green tea drinkers appear to have a distinct advantage over non-drinkers in terms of energy balance and body fat. Nine studies have been included in this analysis, and numerous other studies measuring the effect of drinking green tea on related variables have been conducted. While some studies, predominantly those funded and published by the Japanese Tea Association, appear to have been unsuccessful in demonstrating a statistically significant effect, the overwhelming majority of studies point toward a conclusion that green tea is effective for weight loss and reducing other measurements of body fat. While many of the studies had substantially low sample sizes (i.e., < 16), the robustness of the majority of studies supporting the green tea hypothesis indicates that tea should be further evaluated as a dietary supplement and/or in pharmacological trials with other substances.
7. Optimal Dosage and Consumption Patterns
To derive the weight loss and antioxidant benefits of green tea, it is essential to know the optimal dosage and consumption patterns. The recommended dosage and consumption patterns of green tea should be briefly discussed, outlining the specifics of this for individuals looking to achieve weight loss through green tea, as well as how green tea should fit into a regular routine.
In a study involving 26 obese individuals, the consumption of tea was randomized to 2 doses: 470 mg EGCG/day (5 cups) or 860 mg EGCG/day (9 cups) over 6 weeks. A third group was also tested in parallel, which only tested caffeine effects at 320 mg/day counterpart (3 cups). An initial group of 53 who were drinking <5 cups daily was shown to have a significant decrease in weight. After 6 weeks, body fat, waist, waist-to-hip ratio, rate of fat loss, and body mass index (BMI) were all found to decrease significantly when drinking the 470 mg EGCG/day. The107-day intervention with 856 mg of EGCG resulted in trend-wise decreases in BMI, waist and hip, and fat mass. In another study involving 93 participants, after 12 weeks, a daily intake of <500 mg of green tea significantly decreased body weight and BMI. However, for a significant change at the waist circumference and %BF over the 12 weeks, a daily intake of <800 mg of green tea must be consumed [1].
8. Combining Green Tea with Other Weight Loss Strategies
There is increasing evidence that the combination of lifestyle factors (e.g., regular exercise or physical activity (PA), low glycaemic index (GI) diets, total dietary carbohydrate restriction, increasing protein or fibre intake, or multiple lifestyle interventions) produces a larger weight-loss effect than single lifestyle modifications alone. Some studies have explored the possibility of synergistic effects of GT with other treatments for weight management. GT and exercise do not have a predictable additive or synergistic effect [3]. However, habitual GT drinkers had a greater fat reduction (lower percent body fat) during exercise, whereas non-drinkers experienced a more pronounced fat reduction in resting conditions. GT and energy-restricted diets prevent the increase in body weight and adipose mass during over-feeding and obesity, with a decrease in appetite also being observed in GT-treated rats. Nevertheless, these food intake and weight suppression effects from GT consumption are independent of diet-induced changes, energy expenditure, and alteration of leptin and insulin levels. Information regarding the possible synergies of pairing GT consumption with other weight management strategies should be further investigated [5].
8.1. Exercise
Teas, the second most consumed beverage in the world, are produced by the infusion of Camellia sinensis leaves. Teas are classified into six categories: white, yellow, green, oolong, black, and pu-erh tea, depending on the length of leaves’ oxidation time. All teas generally contain polyphenols, caffeine, and other bioactive compounds. Tea polyphenols have been found to ameliorate a number of diseases, including obesity, metabolic syndrome, diabetes, and CVDs. Obesity is characterized by excess or abnormal fat accumulation and a chronic low-grade inflammatory state [3]. Metabolic syndrome, usually related to obesity, describes a constellation of cardiovascular risk factors including diet, physical inactivity, age, and gender. However, weight loss is not always associated with improvement in metabolic syndrome. Therefore, there is a need for strategies that will not only prevent but also treat diet-induced metabolic syndrome. Since China started to drink tea five thousand years ago, a large number of studies have been conducted on its medicinal effects.
Weight gain, obesity, and associated metabolic diseases have become increasingly serious worldwide due to an imbalance between energy intake and expenditure. Energy intake from a high-fat diet and low levels of physical activity increase the risk of developing obesity and metabolic diseases. Obesity is also a risk factor for obesity-related diseases, including hypertension, hyperlipidemia, type 2 diabetes, insulin resistance, and nonalcoholic fatty liver [5]. After consumption, tea leaves are fully oxidized in oolong tea, black tea, and pu-erh tea, but are either not oxidized (white tea and yellow tea) or partially oxidized (green tea). Prior studies have demonstrated that both decaffeinated and caffeinated green tea and black tea polyphenols significantly decrease weight gain in a dose-dependent manner. It was proposed that teas alter microbiome populations and functional pathways related to inflammation and metabolism.
8.2. Dietary Modifications
The section concentrates on dietary modifications that may synergistically affect weight loss when combined with the consumption of green tea. Studies in diet-induced obese rodents supplemented with green tea extract had changes in gut flora and increased symptomatic production of hypolipidemic bacteria, which could represent a green tea epiphenomenon worth investigating. Goishi tea addition reduced weight gain in HFD-fed rats, and the weight-loss effects of tea polyphenols were reviewed in rats and mice. The analysis of recent articles using various model organisms is presented along with suggestions for further studies to validate tea’s beneficial effects on human dietary habits [5].
A meta-analysis of lipid profile studies found significant reductions in body weight, BMI, waist Circumference, triglycerides, total cholesterol, and LDL-cholesterol. Accumulating evidence suggests that herbal supplement usage could alleviate the rising obesity trend, similar to pharmaceutical drugs. Tea consumption can reduce obesity and metabolic syndrome [3].
9. Potential Side Effects and Precautions
The side effects of green tea consumption in any form are rare. Some frequent side effects are usually mild gastrointestinal distress (such as nausea) and headache [3]. GC consumption crossed to 3 or 4 cups, still has no severe adverse reaction. This is indeed the strong and natural herb medicine for body losing weight control.
Should pregnant or breast-feeding women consume green tea? There is an insufficient reliable information about green tea’s safety during pregnancy and breast-feeding [1]. Although green tea contains useful vitamins and nutrients, it also contains caffeine. Too much caffeine may cause side effects and may not be safe because caffeine can easily cross the placenta and reach the fetus. It is suggested that people should be consumed with carefulness.
Should people undergo surgery consume green tea? There is not enough reliable information to know if it is safe to consume green tea for 2 weeks before or after surgery. Caffeine in green tea could cause stomach upset, diarrhea, and upset thinking. It also might increase heart rate and blood pressure.
10. Conclusion and Future Research Directions
For thousands of years, tea has been consumed around the world for comfort and enjoyment. The cultivation, preparation, and drinking of tea is an ancient cultural tradition in many Asian countries. In modern day, tea has also emerged as a widely consumed beverage in non-Asian nations, with some nations consuming more than eight billion cups per day. Tea polyphenols have declared many beneficial health effects in scientific studies and various epidemiological studies. These studies indicate the promise of tea consumption as part of a broader public health strategy to reduce the risk of cardiovascular diseases, cancers, liver disease, diabetes, obesity, and neurodegenerative diseases. Many studies demonstrated the bactericidal and virucidal effects of tea polyphenols. Tea polyphenols remarkably decreased the concentration of pathogens in teas and foods. Tea polyphenols, especially the major catechins in green tea, showed an inhibition against H1N1, and a recombinant E. coli O157:H7. Water extracted polyphenol fraction of green tea inhibited DNA and RNA viruses such as herpes simplex virus type 1, vesicular stomatitis virus, enterovirus type 71, rotavirus, and norovirus. Tea polyphenols are also promising compounds for the treatment of obesity and obesity-related metabolic diseases. Although many studies have demonstrated the great potential of tea consumption in ameliorating the development of human diseases, further studies are necessary [5].
To pursue this fully, green tea metabolism studied with humans from the perspective of pharmacokinetic characteristics. Caffeine, epicatechin gallate, and 1-methyl-, 3-methylxanthine were identified as the major metabolites of green tea polyphenols. The current understanding of tea and tea polyphenols as functional foods and bioactive compounds, with a particular emphasis on elucidating how tea benefits health, should provide useful foundational knowledge for studying a broader institution of similarly complex food mixtures. Results from recent promising epidemiological studies should promote further initiation of large-scale, multi-national studies on diverse human populations. Caffeine and dietary polyphenols exhibit similar effects in terms of neurotransmitter and gut hormone regulation and controlling energy metabolism. Tea drinking may prevent obesity and associated metabolic syndromes through a multifactorial mechanism. BMI, body fat percentage, and weight circumference are significantly decreased in the group consuming caffeine plus tea catechins. Daily consumption of green tea prior to exercise improved fat oxidation during training sessions. Tea polyphenols are expected to be new drug candidates for treating metabolic disorders [1].
References:
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[7] S. Zhang, J. Takano, N. Murayama, M. Tominaga et al., “Subacute Ingestion of Caffeine and Oolong Tea Increases Fat Oxidation without Affecting Energy Expenditure and Sleep Architecture: A Randomized, Placebo-Controlled, Double-Blinded Cross-Over Trial,” 2020. ncbi.nlm.nih.gov