People consistently drinking green tea may not be getting all the benefits as hoped.
Based on studies in 2015, green tea (unfermented) has been the main focus internationally amongst other teas, including half-fermented (oolong tea), fully fermented (black tea), and post-fermented (pu-erh tea) for health and nutrition. Tea is one of the most popular drinks on a global scale, and green tea, specifically, is predominantly utilized in China and Japan.
Green tea is considered to have benefits for cancer, type 2 diabetes (T2D), obesity, atherosclerosis, and cardiovascular disease (CVD). These benefits include protective effects from polyphenols and catechins, hepatoprotective, antibacterial, antiviral, and neuroprotective effects. But are people really protected from the onset of T2D when they drink green tea? Despite these benefits, evidence of the correlation between drinking green tea and T2D continues to fluctuate. “A published cohort study (Published online doi: 10.1371/journal.pone.0036910) in Europe reported that people who drank at least four cups of tea per day had a 16% lower risk of developing T2D than non-tea drinkers. Yang et al. (BMJ Open. 2014; 4(7): e005632) found that daily tea consumption of more than three cups was associated with a decreased risk of T2D. A meta-analysis also reported that tea consumption was inversely associated with T2D risk. On the other hand, Odegaard et al. found that only black tea, but not green tea, was associated with a lower incidence of T2D in Chinese Singaporeans; (BMJ Open. 2014; 4(7): e005632) and Hayashino et al. (Diabetes Care 26(6):1714-8 ) observed a positive association between oolong tea intake and risk of diabetes among the Japanese. A community-based cohort study conducted in Japan did not find green tea consumption to be related to the risk of diabetes, but another study conducted in Japan reported a positive association between green tea consumption and risk of T2D in women.” There is still conflict between these various studies on the association between drinking green tea and developing T2D. As a result, this topic was examined once again, this time with Chinese adults.
A cohort study conducted by International Journal of Epidemiology gathered 119,373 participants from both the Shanghai Women’s and Men’s Health Study in China who did not have diabetes. Surveys were given to collect tea-drinking information, both at baseline and follow-up. Caffeine blood levels and T2D incidence were analyzed in the follow-up surveys. Follow-up occurred every 2-4 years. The hazard ratio and 95% confidence interval were found via cox regression analysis. Caffeine metabolites and T2D risk correlation was examined with logistic regression analysis.
It was found that individuals who drank more green tea had a greater onset risk of T2D versus people who weren’t drinking green tea [HR = 1.20 (95% CI = 1.14–1.27)]. Both men and women in the study had this risk during the entire follow-up period. Body mass/size and smoking behavior did not affect the association between green tea drinking and diabetes incidence. Diabetes incidence was associated with caffeine plasma levels as well, based on the tea drinking (P = 0.03).
One possible reason may have been the levels of pesticides within the green tea leaves in China. “We found that the median level of serum organochlorine pesticides (OCPs) was much higher in Chinese women living in urban Shanghai than levels reported in other populations.” (Environment International 33(2):157-63) It was also speculated that tea drinkers had a lifestyle that was not as healthy as non-tea drinkers. “On the other hand, it is also possible that the less healthy lifestyle (e.g. cigarette smoking, alcohol drinking, and lack of leisure time exercise) and poorer health status of tea drinkers compared with non-drinkers may partly explain the positive associations.” A third consideration was that diabetes was less likely to be diagnosed in individuals of low income, who composed more of the non-current tea drinkers. So the low-income residents may have also had diabetes without being affected by drinking green tea, but the tea and T2D correlation still maintained its status after lifestyle and the participants’ economic status were adjusted for, even though they could not be ruled out. Further studies could examine these confounding variables, including the effect of plasma caffeine metabolites from tea on T2D onset risk.
- Green tea drinking’s association with the risk of developing type 2 diabetes has varied in multiple studies. Some demonstrate positive associations, some confirm negative ones.
- This study on the Chinese population demonstrated a positive association, implying that drinking green tea does put an individual at risk of type 2 diabetes. Plasma levels of caffeine also showed a positive association with the risk of type 2 diabetes.
- Some hypotheses for this association include pesticide residues being in the green tea leaves, poor lifestyles of green tea drinkers, and diabetes being underdiagnosed in low-income non-current tea drinkers. These were adjusted for but could not be ruled out.
Cai, Hui; Gao, Yu-Tang; Gerszten, Robert E; Ji, Bu-Tian; Li, Honglan; Liu, Xiaona; Shu, Xiang; Shu, Xiao-Ou; Xiang, Yong-Bing; Xu, Wanghong; Wang, Thomas; Zheng, Wei. “Green tea consumption and risk of type 2 diabetes in Chinese adults: the Shanghai Women’s Health Study and the Shanghai Men’s Health Study.” International Journal of Epidemiology. 2018. Oxford Academic. https://academic.oup.com/ije/article-abstract/47/6/1887/5086723?redirectedFrom=fulltext. 10 January 2019.
Wilson T, Temple NJ. “Beverage Impacts on Health and Nutrition.” New York, NY: Humana Press, 2016. https://www.springer.com/us/book/9783319236711. 10 January 2019. Published online 2012 May 30. doi: 10.1371/journal.pone.0036910
Annahita Forghan, Pharm.D. Candidate 2019, LECOM College of Pharmacy