Tea is one of the most consumed beverages in the world, and a spectrum of health benefits has been associated with tea consumption. Depending on the type of tea and how that tea is processed (degree of oxidation), it may be the tea polyphenols and their profiles that, when regularly consumed in adequate amounts, contribute to positive health outcomes.
There are at least hundreds of cultivars—from white tea (little oxidation) to pu-erh tea (most oxidized)—within the Camellia sinensis species. When the innate enzymes, polyphenol oxidase and peroxidase, are released from the cells, they act upon polyphenols known as catechins to generate oxidized flavonoids called theaflavins and thearubigin (Sharangi 2009). Theaflavins may activate certain antioxidant enzymes like glutathione-S-transferases, glutathione peroxides, and superoxide dismutase, whereas thearubigin may activate phase II enzymes and function to inactivate carcinogen-induced phase I enzymes (Maki, Reeves, Farmer et al. 2009; Patel and Maru 2008).
Some reports suggest tea catechins may assist in maintaining body weight and modulating energy expenditure and may reduce the risk of some cancers, diabetes, and cardiovascular disease, although the data are inconsistent and limited (Heber, Zhang, Yang, Ma, Henning, and Li 2014; Hursel and Westerterp-Plantenga 2013). Several prospective studies suggest long-term tea drinkers typically present lower BMI and a lower percentage of body fat relative to those who consume less tea (Hughes, Arts, Ambergen et al. 2008).
A recent health claim associated with black tea was denied by the European Food Safety Authority (EFSA) (Turck, Bresson, Burlingame et al. 2018). The premise was that 30 mg of catechins per serving could maintain vascular health through endothelium-dependent vasodilation when consumed for at least four weeks. Supporting evidence suggests the consumption of 240–1,500 mg flavonoids/day from 1 week to 6 months may reduce systolic and diastolic blood pressure (Ras, Zock, and Draijer 2011; Greyling, Ras, Zock et al. 2014). However, EFSA concluded that cause and effect relationship could not be established between the consumption of black tea and maintenance of normal endothelium-dependent vasodilation.
A close examination of the possible mechanism of this putative action suggests dietary flavonoids may modulate vascular tone through endothelium function and possibly activating Ca++-dependent potassium channels and decreasing phosphorylation of specific muscle enzymes (Xu, Leung, Leung, and Man et al. 2015). The principal mediator of increased blood flow appears to function through the upregulation of nitric synthase as a feedback mechanism of inactivation of nitric oxide by reducing its bioavailability (Vaziri, Ni, Oveisi, and Trnavsky-Hobbs et al. 2000).
A recent review suggests a relationship between polyphenol consumption and weight maintenance (Rains, Agarwal, and Maki et al. 2011). A mouse model indicated tea polyphenols contributed to reduced food intake and weight loss, possibly mediated through decreased monocyte chemotactic protein-1 gene expression in various tissues, including the liver (Heber, Zhang, Yang, Ma, Henning, and Li 2014). This gene is responsible for recruiting monocytes, neutrophils, and lymphocytes for routine immunological surveillance of tissues, as well as in response to inflammation (Deshmane, Kremlev, Amini, and Sawaya 2009). In the same study by Heber et al., gene expression of angiogenesis markers such as the adipokines Adipoq and Pedf as well as the angiogenesis-related genes Vegfa and Vegfr2 were modulated following consumption of tea-derived polyphenols. The apparent increase in Adipoq and decrease in Pedf gene expression contributed to increased angiogenic factor gene expression. Of equal interest is the observed elevation of direct anti-inflammation indicators, which may explain possible reduced risk of cardiovascular disease following long-term tea consumption.
A cursory assessment of the pharmacokinetics of polyphenols leads to the conclusion that the absorption and excretion of polyphenols are quite variable (Meng, San, Zhu et al. 2002). For example, data suggest 1.5%–16% of ingested catechins are excreted within 24 hours. The metabolic fragments of polyphenol rings are equally variable and mostly uncertain (Chen, Lee, Li, and Yang 1997). Similar studies among humans indicate only 11%–28% of ingested epicatechin and epigallocatechin glucuronide could be quantified in 24-hr urine samples (Clifford, van der Hooft, and Crozier 2013).
Centuries of tea consumption must be taken seriously in any consideration of human nutrition. The associated health claims and beliefs about tea’s promotion of health have crept into mainstream science with increasing rigor, complexity, and resolution. However, in embracing or refuting these claims and beliefs, we must be vigilant about statistical significance masquerading as clinical significance. Tea consumption certainly represents a delicious, sometimes calming, sometimes bracing effect upon our mood. It is far less clear that this versatile beverage significantly promotes health or reduces the risk of pathology.
Roger Clemens, DrPH, CFS,
Univ. of Southern California’s School of Pharmacy,
Los Angeles, Calif.