Green tea, derived from the leaves of Camellia sinensis, is a globally popular beverage renowned for its potential health benefits. Beyond its traditional use, green tea has garnered attention for its purported effects on weight loss and related metabolic conditions. This article explores the scientific evidence surrounding green tea consumption and its impact on weight, body composition, and associated health markers.
The Rising Tide of Type 2 Diabetes and the Search for Natural Solutions
The occurrence of type 2 diabetes mellitus (T2DM) is increasing worldwide, particularly in developing countries, where it was previously much lower. It is estimated that by 2030, about 439 million adults will be diabetic. In the United States, the prevalence of T2DM has doubled during the past 15 years. Also, diabetes is associated with an increased risk of cardiovascular diseases (CVD) accounting for approximately 80% of all diabetic deaths. Abnormal lipid metabolism, glycation, and oxidative stress may play some role in the development of long-term diabetic complications. Obesity also complicates the management of T2DM by increasing insulin resistance and blood glucose concentrations. It is an independent risk factor for dyslipidemia, hypertension, and CVD. Obesity at all ages increases diabetes prevalence and doubles it in Iranian women. In T2DM patients, weight loss is significantly correlated with glycemic control. A rapidly growing area of using natural herbs to control metabolic problems has, therefore, attracted the researchers’ attention.
Green Tea: A Profile of Beneficial Compounds
Tea is the most popular beverage in the world after water. It is generally consumed in forms of green, oolong, and black tea, and all of them originate from the leaves of Camellia sinensis. Among them green tea (GT) is an excellent source of phenolic antioxidants including catechins and epigallocatechin 3-gallate (EGCG). A great deal of scientific interest has been focused on the beneficial health effects of green tea. Green tea's potential benefits are often attributed to its bioactive compounds, including catechins and caffeine.
Catechins and EGCG
Catechins are a type of flavonoid, which is a type of plant compound called polyphenols. In particular, epigallocatechin-3-gallate (EGCG) is the most abundant and well-studied catechin in green tea. Some research suggests that EGCG may help reduce markers associated with obesity, such as: total body weight, body fat percentage, waist-to-hip ratio, waist circumference, body mass index (BMI). These compounds are believed to influence various physiological processes related to weight management.
Caffeine
Green tea also contains caffeine, a known stimulant that can affect metabolism and energy expenditure.
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How Green Tea May Affect Weight Loss
Several mechanisms have been proposed to explain how green tea might contribute to weight loss:
Boosting Metabolism and Energy Expenditure
Some studies suggest that green tea extract may help boost resting metabolic rate (RMR), which could help you burn more calories at rest by increasing your total energy expenditure. Previous studies show that EGCG, the most effective ingredient of GT, could increase thermogenesis and 24-hour energy expenditure by about 4%.
Enhancing Fat Oxidation and Lipolysis
Fat is stored in cells as triglycerides. To burn body fat, your body must first break these down into fatty acids and glycerol to move it into your bloodstream. This process is known as fat lipolysis. When you exercise, hormones like epinephrine and glucagon are secreted to help facilitate fat oxidation and the release of fatty acids. These fats are then used as energy and, effectively, burned. This is known as fat oxidation. Some research suggests that catechins in green tea, especially EGCG, could aid fat oxidation and lipolysis by boosting the effects of some fat-burning hormones like catecholamine. EGCG may also help inhibit an enzyme that breaks down the hormone norepinephrine. When this enzyme is inhibited, the amount of norepinephrine increases, promoting fat breakdown. Caffeine may also help boost fat metabolism, especially when consumed at rest instead of during exercise. Ultimately, green tea may help your fat cells break down more fat, which is released into your bloodstream for use as energy by cells like muscle cells.
Potential Effects on Appetite
Some people suggest that drinking green tea may help you consume fewer calories, which may contribute to maintaining a calorie deficit and, therefore, losing weight.
Clinical Evidence: Studies on Green Tea and Weight Loss
The effects of green tea consumption on the anthropometric and biochemical parameters of patients with type 2 diabetes were investigated in an interventional trial, without changing the usual lifestyle of the participants.
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Human Studies
A randomized clinical trial included 63 patients with type 2 diabetes (30 males and 33 females). After a two-week run-in period without green tea, they were randomly assigned into one of the three groups, with a different daily intake of green tea; four cups of green tea per day (n = 24), two cups of green tea per day (n = 25), and the control group (n = 14) with no green tea intake for two months. At baseline and after the intervention, blood tests, dietary, and anthropometric variables were assessed. Consumption of four cups of GT per day caused a significant decrease in body weight (73.2 to 71.9) (P < 0.001), body mass index (27.4 to 26.9) (P < 0.001), waist circumference (95.8 to 91.5) (P < 0.001), and systolic blood pressure (126.2 to 118.6) (P < 0.05) in this group. No significant change was seen in the other groups and between group comparisons.
Another 2020 review found that consuming at least 500 milligrams (mg) of green tea extract daily for 12 weeks helped reduce body weight and BMI. The authors concluded that green tea may help promote weight loss when combined with eating a balanced diet and exercising regularly.
However, it’s important to note that not all studies support the weight loss effects of EGCG, caffeine, and green tea. A 2023 review concluded that green tea extract supplementation had no significant effects on the two appetite-regulating hormones, leptin and ghrelin.
Animal Studies
Recent studies funded by FAPESP have deepened our understanding of the mechanisms of action of this infusion and revealed that green tea treatment reduced weight and significantly improved glucose sensitivity and insulin resistance in obese mice. A previous study published in August 2022 in the European Journal of Nutrition found that obese mice treated with green tea experienced a reduction of up to 30% in body weight.
Dosage and Consumption Methods
The dose of green tea supplementation researchers use for weight loss varies across studies. For example, the 2023 meta-analysis suggests that 1,000 mg or less of green tea daily over 8 weeks or less may offer weight loss benefits. However, the 2020 review suggests that results were more significant when people consumed less than 500 mg daily for 12 weeks.
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Types of Green Tea
Green tea comes in several varieties, but for weight loss, there are unlikely to be significant differences between them. Plain, minimally processed green teas are likely to retain the richest nutritional content. Ready-made tea bags do not always guarantee the quantity or quality of the compounds. The ideal for consumption would be to use standardized green tea extract, like those found in compounding pharmacies.
Safety and Potential Side Effects
According to the NCCIH, green tea is safe in doses of up to 8 cups daily. This limit is smaller in people who are pregnant or breastfeeding. The NCCIH recommends consuming no more than 6 cups, or around 300 mg of caffeine, daily. In rare cases, green tea can cause liver problems, but this may be more likely when consuming green tea extract in pill form. Green tea may also interact with certain medications. For example, the NCCIH states that high doses of this tea can reduce blood levels, which can affect the beta-blocker nadolol (Corgard).