Polyphenols

Green tea/Nutrient Depletion:

  • GeneralGeneral: This section focuses on potential interactions with green tea. Most nutrient depletion interactions associated with green tea are predominantly theoretical and generally based on the adverse effect profile of caffeine. For more potential interactions based on caffeine itself, the Natural Standard monograph on caffeine is available.
  • Ascorbic acidAscorbic acid: In human research, single doses of tea catechins decreased ascorbate levels briefly (122); however, effects of green tea extract on ascorbic acid levels were lacking in a separate study (123).
  • ElectrolytesElectrolytes: According to secondary sources, green tea containing caffeine may produce a diuretic effect.
  • Folic acidFolic acid: In human research, researchers noted a potential interaction between tea and folic acid, with even low concentrations (0.3g of extract/250mL) of green and black tea extracts yielding decreased bioavailabilities of folic acid (115). In animal research, green tea extracts at very high dietary concentrations lowered serum folate in rats; however, similar effects were not observed in humans (405). According to in vitro evidence, catechins and tea extracts may inhibit folic acid uptake (114).
  • GlucoseGlucose: From available laboratory and human research, the effect of green tea or its constituents on blood glucose or other markers is mixed. Hypoglycemia was reported by one subject in a clinical trial (76). In human research, caffeine or green tea was shown to modulate blood sugar levels and affect levels of insulin and insulin resistance (203; 132), and EGCG decreased blood glucose and/or insulin in animal (204) and human research (205). However, in patients without diabetes, plasma glucose levels were higher two hours following ingestion of a meal, when the meal was consumed with green tea vs. water (99). Also, preliminary research suggests that green tea lacks an effect on blood sugar or insulin levels in many individuals (152; 76; 134; 151; 163; 132; 136; 129); however, the consumption of green tea was inversely associated with risk for diabetes in epidemiological research (206). Green tea extract decreased streptozotocin-induced increases in blood glucose in rats (100).
  • IronIron: In human research, impaired iron metabolism and microcytic anemia occurred in infants (102). In clinical research, in thalassemia major and intermedia patients, tea caused an inhibition of iron absorption (103). Decreased nonheme absorption has been shown in young women consuming foods with green tea added (104). However, it has also been suggested that green tea does not inhibit iron absorption (368; 369). According to a review, it is recommended that vegetarians drink tea between meals, as tea binds to dietary iron found in plant sources and decreases the availability (absorption) of iron (105). In an epidemiological review, tea consumption was associated with a reduced iron status only in groups with a high prevalence of deficiency (106). Also, one study included in the review suggested that in middle-aged men at risk of iron overload, tea consumption may lower serum ferritin concentrations.
  • LipidsLipids: Research available in systematic reviews and meta-analysis suggests that green tea has beneficial effects on blood lipids (158; 157; 66; 131); however, studies in patients with hyperlipidemia are limited. Also, in vitro (17), animal (216; 204), and human (217; 218; 77; 155; 159; 219; 157; 66; 158; 220; 205; 131; 132; 136; 135; 221) studies have demonstrated the antilipemic effects of green tea and green tea catechins. Most commonly, total cholesterol was reduced, although responses of LDL cholesterol and triglycerides (decrease) and HDL cholesterol (increase) varied between studies. A lack of effect on lipid profiles, however, has also been reported in human trials using green tea (222; 123; 129). In humans, theaflavin-enriched green tea decreased levels of total and LDL cholesterol (155).
  • Vitamin CVitamin C: In human research, single doses of tea catechins decreased ascorbate levels briefly (122); however, effects of green tea extract on ascorbic acid levels were lacking in a separate study (123).