While coffee gets a lot of attention in the US, tea is the most widely-consumed beverage in the world after water. Recently, scientists have begun to examine the supposed health benefits of green tea—and not all are real. Is drinking green tea truly a way to shed fat, lower inflammation, and optimize cardiovascular health? We separate fact from fiction on the ways in which this ancient drink might impact your biomarkers and your body, and share the truth of how diet marketing might mislead you.
What Is Tea Biologically?
Tea is a beverage formed by brewing dried leaves of the plant Camellia sinesis. It can be categorized into three main forms depending on oxidation levels: green (non-oxidized), oolong (partially oxidized), and black (fully oxidized). Green tea is composed of a class of antioxidant molecules called catechins. They are a form of flavonoids, compounds that regulate cell communication and help prevent oxidation. Catechins are the main substances responsible for green tea’s unique properties.
Typically, green tea is prepared by brewing 1 gram of tea leaves with 100 ml of boiling water for three minutes. It usually contains 300 mg of dry materials comprised of 30-45% catechins and 3-6% caffeine.1 A cup of high-quality green tea usually contains 150–200 mg of catechins. 2 Green tea can also be consumed in supplement form, which contains catechins and sometimes a small amount of caffeine.
Green tea appeared in China over 4,000 years ago and spread through other eastern civilizations. Western cultures typically consume high levels of black tea. However, as interest in alternative medicine practices from China and India has increased, so has interest in green tea. As a result, green tea consumption is 60% higher than ten years ago. 3 While the US is still primarily a nation of black tea drinkers, green tea consumption is currently estimated at 14% of all tea. 3
Is this surge in green tea really warranted? We distill green tea research in five main areas: body composition, lipids, glucose, inflammation, and iron.
The Research on Body Weight and Waist Circumference
Green tea may help you shed some fat and lose weight—but only to a modest degree. A recent meta-analysis assessed 11 randomized control studies examining the impact of daily green tea consumption on weight loss and weight maintenance. All studies were 12 to 13 weeks in duration. The baseline body mass index of subjects in these studies ranged from 18.5 to 35 kg/m2. Their ages ranged from 16 to 65 and both males and females were represented. Most participants were Asian (Chinese, Japanese, Korean); however, four studies were conducted among Caucasians. The amount of tea ranged on average from 250 mg to 1207 mg, or from 1 to 6 cups.
Researchers noted that subjects who consumed green tea extract or the beverage daily lost more weight compared to the placebo group—about 1.31 kg. Interestingly, the amount of weight loss didn’t depend on how much green tea was consumed. However, ethnicity did impact weight loss: Caucasians lost an average of 0.82 kg while Asians lost an average of 1.51 kg. The researchers believe that differences in caffeine consumption led to these differences. 4 As a result of these findings, the researchers stated that “catechins in green tea have a small positive effect on weight-loss and weight-maintenance.” 4
Green tea is more effective for weight loss if you are overweight or obese. In a study earlier this year, researchers recruited 102 women with a body mass index greater than 27 and a waist circumference greater than 80 cm and divided them into two groups. The first group took a high-dose, 1000-mg green tea supplement per day, while the second group consumed a cellulose supplement as a placebo. Both interventions lasted 12 weeks.
About 39 subjects in the green tea group and 38 subjects in the cellulose/placebo group completed the trial. Researchers noted that the group consuming green tea went from an average body weight of 76.8 kg to 75.7 kg, representing a 1.1 kg loss.5 Furthermore, the average waist circumference decreased from 95.1 cm to 92.8 cm.  In contrast, the placebo group had no statistically significant loss.5 This suggests a more favorable body fat distribution than before. The clinicians stated that “12 weeks of treatment with high-dose green tea extract resulted in significant weight loss, reduced waist circumference . . . without any side effects or adverse effects in women with central obesity.” 5
Key Takeaways: Daily green tea supplementation and beverage consumption (at least one cup) may increase weight loss and decrease waist circumference, key indicators of healthy fat distribution. Research indicates that these effects are more pronounced for people of from an Asian ethnic background (i.e. Chinese, Japanese) but it is unsure why.
Recommendations: If you are trying to lose weight and shrink your waist, feel free to supplement with green tea. Remember, though, the bulk of your weight loss will come from cutting calories and increasing exercise levels.
Does Green Tea Reduce Bad Cholesterol?
At InsideTracker, we measure two main sets of fat molecules called lipids in your body that are indicative of cardiovascular health. Triglycerides store energy for later use. Higher levels in the blood are associated with a higher risk of cardiovascular illnesses. Cholesterol, which serves as the building block of hormones and cell membranes, is carried through the blood by molecules called lipoproteins. There are five lipoproteins, but the two most indicative of cardiovascular health are low-density lipoprotein (LDL) and high-density lipoprotein (HDL). Elevated levels of cholesterol attached to LDL are associated with increased levels of cardiovascular disease. In contrast, higher levels of cholesterol attached to HDL are associated with decreased levels of cardiovascular disease.
How does green tea consumption affect these critical indicators of cardiovascular health? In 2011, Chinese researchers conducted a comprehensive meta-analysis including 14 randomized controlled trials involving 1,136 subjects. The studies ranged from 3 weeks to 12 weeks and included between 20 and 240 subjects. Of the 14 trials, 5 were conducted in healthy subjects, 5 in overweight and obese subjects, and 4 in subjects with a risk of cardiovascular illnesses. Doses of green tea (supplements and beverage) ranged from 150 mg/dl to 2500 mg/d, the equivalent of 1–16 cups.
"Overall, green tea consumption lowered total cholesterol concentration by 7.20 mg/dL and significantly decreased LDL-cholesterol concentration by 2.19 mg/dL. 6 These values represent a 3-5% decrease in total cholesterol. On average, HDL cholesterol remained significantly unchanged. An intricate analysis revealed that these favorable changes were not affected by the type of intervention, dosage of green tea catechins, study length, or health status. 6 Additionally, the subjects showed a fairly consistent decrease in total cholesterol and LDL cholesterol. The researchers stated that “The analysis of the eligible studies showed that the administration of green tea beverages or extracts resulted in significant reductions in serum TC and LDL-cholesterol concentrations, but no effect on HDL cholesterol was observed.” 6
The highest-quality study from this meta-analysis was conducted in 2003. Researchers recruited 240 patients from urban hospitals in China with cholesterol and triglyceride levels above the optimal zone and divided them into two groups. Both groups had similar key demographics. None of the subjects had cardiovascular complications, type 2 diabetes, or took medication to lower their triglycerides or cholesterol. All subjects were asked to maintain their normal diets.
The first group—47 males and 73 females—consumed a placebo for 12 weeks. They had an average age of 55, a mean BMI of 24.4, and about 19% were cigarette smokers. They had an average total cholesterol concentration of 239 mg/dL, average LDL cholesterol concentration of 159 mg/dL, and triglyceride concentration of 189 mg/dL. These levels are moderately high. They also had an average HDL cholesterol concentration of 55 mg/dL—a normal, healthy level.
The second group consumed one supplement per day containing 150 mg of green tea catechins—the same amount as one cup of tea. They had an average age of 54.4, an average BMI of 24.0, and about 26% were cigarette smokers. They also had an average total cholesterol concentration of 244 mg/dL, LDL cholesterol concentration of 159 mg/dL, and triglyceride concentration of 189 mg/dL—all of which are moderately elevated. They had an HDL concentration of 55 mg/dL—also a normal, healthy level.
The researchers periodically measured levels of cholesterol and triglycerides in the blood. Then they compared the baseline values with the concentration after 12 weeks. In the placebo group, triglycerides, total cholesterol, HDL cholesterol, and LDL cholesterol changed insignificantly. However, in the green tea group, total cholesterol levels decreased by 11.3%, LDL cholesterol by 16.4%, and HDL cholesterol increased by 2.3%.7 In both groups, triglyceride levels did not change significantly.
From Maron et. al (2003)
The researchers remarked that “In observational studies as well as clinical trials with statin drugs, each 1% reduction in LDL-C results in approximately a 1.0% to 1.5% reduction in the relative risk of major cardiovascular events.” 7 Thus, green tea may be a useful alternative therapy to prevent cardiovascular ailments.
Key Takeaways: Several intervention studies suggest that consuming at least one cup of green tea daily or an extract containing 150 mg of green catechins may be effective in optimizing levels of total cholesterol and LDL cholesterol. Green tea has a mild effect on increasing HDL cholesterol and no demonstrated impact on lowering triglycerides.
Recommendations: If your levels of total cholesterol and LDL cholesterol are elevated, consuming moderate amounts of green tea, green tea extract, and black tea may be useful in optimizing their levels. If your triglycerides and HDL cholesterol are out of whack, consider a diet high in omega-3 fatty acids.
Green Tea and Its Effects on Fasting Glucose
Elevated blood glucose can put you at risk for a variety of chronic conditions such as type 2 diabetes and cardiovascular diseases. Do catechins in green tea provide a useful way of managing this biomarker in InsideTracker’s metabolic panel?
In a 2013 meta-analysis, researchers examined 17 randomized, controlled studies that assessed the effects of green tea on fasting glucose levels in a total of 1,249 subjects. Fourteen were randomized, placebo-controlled studies—the highest-quality category of clinical studies. The other three were crossover studies, also a high-quality category. About 627 of these subjects took green tea during the studies while the other 624 served as controls. In 10 studies, subjects ingested green tea beverage. In the other 7, they consumed extract. None of the studies included green tea consumed as part of a multicomponent supplement. The studies lasted a minimum of two weeks to a maximum of 24 weeks. Daily catechin doses ranged from 208 mg/d to 1207 mg/d with a median of 457 mg/d. Generally, one cup of green tea has about 150–200 mg.
Fifteen studies included subjects with risk factors for cardiovascular disease, such as elevated fasting glucose levels, high triglycerides, or being overweight or obese. The other two used healthy subjects. The subjects ranged in age from 18 to 80. Additionally, the number of subjects included in each study ranged from 34 to 240. With one exception, the studies asked subjects to maintain normal levels of diet and physical activity.
The researchers noted that fasting glucose in the subjects decreased an average of 0.09 mmol/L—a small but significant amount. 8 However, there was a greater beneficial impact in individuals who had elevated levels of blood glucose or were at risk for “metabolic syndrome,” a condition with high lipid levels and blood pressure. On average, subjects had a 6% lower risk of metabolic syndrome as a result of the various green interventions. Interestingly, “the wide range of green tea doses did not lead to significant heterogeneity and affect the overall outcome of the analyses.” 8 Thus, the researchers could not determine an optimal range of green tea to take to lower fasting glucose levels.
The conclusion that green tea may help maintain optimal levels of blood glucose is supported by observational, epidemiological studies. In a 2009 meta-analysis, researchers reviewed nine cohort studies that looked at the association between green tea consumption and incidence of type 2 diabetes—a condition characterized by blood glucose levels greater than 125 mmol/L. Collectively, the studies contained 324,141 subjects and had durations of 5 to 18 years.
Throughout the studies, researchers measured a collective total of 11,400 cases of type 2 diabetes. The studies did not show an association between green tea consumption and a lower risk of developing type 2 diabetes. However, data-analysis after adjustment for other risk factors showed a significant association between consuming at least 1-3 cups of green tea per day (about 200–400 mg of catechins) and a 20% lower risk of developing type 2 diabetes. 9
Key Takeaways: Some intervention and epidemiological studies suggest that green tea beverage and extract may be useful in maintaining healthy levels of blood glucose.
Recommendations: If your blood glucose levels are elevated and you are at risk for metabolic syndrome, consider regularly consuming a moderate dosage of green tea beverage or green tea extract (equivalent of 1-4 cups). Use InsideTracker or other technology to regularly monitor your glucose and determine how much green tea you should consume.
Body Inflammation: Fact-Checking Green Tea’s Impact
Some people claim that green tea can lower inflammation and boost recovery. Are these claims true?
In a 2010 study, researchers examined the association between green tea consumption and levels of the inflammation marker C-reactive protein (CRP), a key feature in our InnerAge and Ultimate Panels. They examined 10,325 men and women aged 49 to 76 in Fukuoka City, Japan who were part of a cohort study on preventable chronic diseases. Scientists surveyed these subjects about coffee, alcohol, and green tea consumption and behavioral health characteristics. Throughout the study, researchers performed external measurements (such as body weight and body fat) and extracted blood samples.
The researchers noted that CRP concentrations in men were inversely associated with increased consumption of coffee after adjustment for lifestyle habits such as smoking. They did not observe this relationship in women. 10 However, green tea consumption showed no notable relationship with CRP concentration in both women and men. 10
Another study suggests that green tea is not an effective method in controlling CRP. In a five-day randomized crossover study, researchers assessed the impact of green tea and cocoa beverages on indicators of aging including CRP. Both green tea and cocoa contain a set of compounds called “flavanols.” Researchers provided 20 adults with controlled diets. One group consumed four cocoa beverages containing 30–900 mg of cocoa flavanols per day, and the other group consumed four tea beverages containing 30–900 mg of green tea flavanols per day. Scientists observed that as subjects consumed higher doses of cocoa flavanols, their levels of CRP decreased. 11 In contrast, green tea flavanols did not influence blood CRP levels. 11
While research doesn’t suggest a strong relationship between green tea consumption and inflammation, another form of tea may help curb an overactive immune system. In a 2007 study lasting 10 weeks, researchers divided 75 healthy, non-smoking men aged 18–55 with an average BMI of 33.2 into two groups. Both groups ate the same diet for four weeks. Then the first group of 37 men consumed a daily beverage containing 1050 mg of black tea extract for six weeks. The extract contained 6.4% flavonols, equivalent to four cups of black tea. The second group of 38 men served as controls and consumed a similar beverage without any tea extract.
After these interventions, researchers examined key measures of inflammation. The control group had similar levels of CRP before and after the intervention. In contrast, the group that consumed black tea decreased their CRP levels of CRP by 21.6% (0.97 mg/L to 0.76 mg/L). 12 Additionally, the black tea group had lower levels of white-blood cell aggregates—another indicator of inflammation.
As a result of these findings, the researchers stated that “Chronic tea consumption reduced platelet activation and plasma C-reactive protein in healthy men. Effects cannot be attributed to observable bias or lifestyle confounders. These effects of tea may contribute to sustained cardiovascular health.” 12
Key Takeaways: Epidemiological and intervention studies do not support green tea as an effective intervention for lowering markers of inflammation such as C-reactive protein. However, research suggests that similar beverages, including black tea and cocoa, may help lower CRP.
Recommendations: If you have inflammation as indicated by elevated levels of C-reactive protein (CRP), consider taking cocoa flavonols and black tea. Green tea is unlikely to make a difference.
Beware of Iron Absorption Problems
Most studies suggest that consuming a moderate amount of green tea beverage or supplement per day may benefit the body in a variety of ways. On the other hand, could this healthy, ancient beverage harm you?
One study suggests that you need to watch your iron levels if you drink green tea. In 2003, researchers recruited 27 women aged 19–39. They were healthy, had a median BMI of 22.1 kg/m2, had not been pregnant in the past two months, and took no medication affecting iron absorption. Ten consumed a test meal containing high amounts of non-heme iron—one of two forms of iron absorbed by the body. (The other form is heme; you can learn more about iron absorption in our blog “The Myth of the Nutrition Facts Label—Iron Absorption Debunked.”) The same group consumed a small amount of green tea extract and the same iron-rich meal the following day.
The researchers took blood samples after each meal. The group that took green tea extract experienced a reduction in non-heme iron absorption from an average of 12.1% to 8.9%—a 3.2% decrease. 13 As a result of these findings, they stated that “extracts used as antioxidants in foods reduce the utilization of dietary iron.” 13
Key Takeaways: Consuming green tea beverage and supplementation daily may lower your absorption of non-heme iron, thereby reducing your total iron levels.
Recommendations: If your iron levels are low or you are at risk for anemia, abstain from green tea. Check out our blogs “The Myth of the Nutrition Facts Label—Iron Absorption Debunked” and “Tired of Being Tired: How I Optimized my Iron Levels” for more detailed information on maximizing iron absorption.
Green Tea: Putting It in Perspective
An abundant amount of research suggests that green tea is good for your body and your biomarkers. It indicates that consuming at least one cup per day (or at least 150 mg in supplement form) can help you lose a little weight, optimize levels of total cholesterol and LDL cholesterol, and modestly control blood sugar. However, green tea will not help you lower triglycerides nor increase levels of heart-healthy HDL cholesterol. If you want to lower markers of inflammation such as C-reactive protein (CRP), you might want to look to alternatives such as cocoa or black tea. Additionally, if you have low levels of iron, drinking green tea or taking green tea extract may lower iron levels even more.
What impact will green tea have on your biomarkers? The only way to know for sure is to use your InsideTracker recommendation to see where you are right now. If your glucose or total cholesterol is high, consider drinking a daily cup of green tea. If your iron levels are low, consider abstaining from green tea and find out which foods are suitable for you. At the end of the day, paying close attention to your personal biochemical data and rigorously reviewed research will yield the best health and performance outcomes for you.
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List of References
1. Velayutham, Pon, Anandh Babu, and Dongmin Liu. “Green tea catechins and cardiovascular health: an update.” Current Medicinal Chemistry 15.18 (2008): 1840.
2. ConsumerLab.com. (2014). “Product review of green tea supplements, drinks, and brewable teas.” Retrieved from https://www.consumerlab.com/reviews/Green Tea Review Supplements and Bottled/Green Tea/
4. Hursel R, Viechtbauer W, Westerterp-Plantenga MS. “The effects of green tea on weight loss and weight maintenance: a meta-analysis.” International Journal of Obesity 2009; 33(9): 956-961.
5. Chen, I-Ju, et al. “Therapeutic effect of high-dose green tea extract on weight reduction: A randomized, double-blind, placebo-controlled clinical trial.” Clinical Nutrition (2015).
6. Zheng, Xin-Xin, et al. “Green tea intake lowers fasting serum total and LDL cholesterol in adults: a meta-analysis of 14 randomized controlled trials.” The American Journal of Clinical Nutrition 94.2 (2011): 601-610.
7. Maron, David J., et al. “Cholesterol-lowering effect of a theaflavin-enriched green tea extract: a randomized controlled trial.” Archives of Internal Medicine 163.12 (2003): 1448-1453.
8. Liu, Kai, et al. “Effect of green tea on glucose control and insulin sensitivity: a meta-analysis of 17 randomized controlled trials.” The American Journal of Clinical Nutrition 98.2 (2013): 340-348.
9. Jing, Yali, et al. “Tea consumption and risk of type 2 diabetes: a meta-analysis of cohort studies.” Journal of General Internal Medicine 24.5 (2009): 557-562.
10. Maki, Takako, et al. “The relationship of coffee and green tea consumption with high-sensitivity C-reactive protein in Japanese men and women.” Clinical Chemistry and Laboratory Medicine 48.6 (2010): 849-854.
11. Stote, K. S., et al. “Effect of cocoa and green tea on biomarkers of glucose regulation, oxidative stress, inflammation and hemostasis in obese adults at risk for insulin resistance.” European Journal of Clinical Nutrition 66.10 (2012): 1153-1159.
12. Steptoe, Andrew, et al. “The effects of chronic tea intake on platelet activation and inflammation: a double-blind placebo controlled trial.” Atherosclerosis 193.2 (2007): 277-282.
13. Samman, Samir, et al. “Green tea or rosemary extract added to foods reduces non- heme iron absorption.” The American Journal of Clinical Nutrition 73.3 (2001): 607-612.