To our knowledge, this is the first study investigating the effects of decaffeinated green tea supplementation on biomarkers of inflammation and features of metabolic syndrome in obese population in the US. Our study results show that green tea beverage or extract supplementation selectively lower plasma SAA versus the no treatment group. However, green tea intervention did not affect inflammatory markers including CRP, IL-6, IL-1β, sVCAM-1, sICAM-1, adiponectin and leptin or features of metabolic syndrome.
SAA, a family of apolipoproteins expressed in hepatocytes and adipocytes has been shown to increase oxidative stress, decrease endothelial nitric oxide synthase (eNOS) activity, and impair reverse cholesterol transport by HDL particles [40
]. A cross-sectional study among overweight or obese postmenopausal women has shown lower SAA levels among women with higher quality dietary patterns versus those with lower scores [44
]. A 6-week green tea polyphenol supplementation significantly decreased SAA and severity of colitis in an animal model of chronic inflammation at dietary achievable doses [45
]. However, catechin supplementation for six weeks in apo E-deficient mice had no effects on plasma SAA [46
]. Clinical studies investigating the effects of dietary factors including polyphenols on SAA are limited. Previously, Nantz et al [47
]. have reported that green tea polyphenols significantly reduced SAA in healthy adult volunteers. The subjects took 200 mg of decaffeinated green tea extracts twice a day for 3 weeks. In contrast, polyphenols from soy, instead of green tea, showed no effect on plasma SAA. In a 3-month study involving a one month control phase, followed by one month each of high- and low-soy isoflavone intervention, no treatment differences were observed on plasma SAA among hypercholesterolemic men and postmenopausal women [48
]. In our study, administration of green tea beverage or extract, equivalent to approximately 440mg or 460mg EGCG, respectively, was effective in reducing SAA in obese subjects with metabolic syndrome. In comparison to our study, the null effects reported by Jenkins et al. [48
] may be explained by the short duration of each intervention, differences in study sample, and possible differential effects of soy flavonoids versus green tea flavonoids in lowering SAA. Cardiovascular epidemiology has shown SAA to be an independent predictor for cardiovascular disease in women with suspected myocardial ischemia and for early mortality in patients with acute coronary syndromes [49
]. Furthermore, baseline serum SAA in our subjects with metabolic syndrome (mean±SE, 53.0±1.0 µg/mL) was higher than the levels reported by Nantz et al. [47
] or Jenkins et al. [48
] in their healthy or hypercholesterolemic subjects, respectively, and was comparable to SAA reported in women with advanced CVD [50
]. Thus, our data suggest the possible anti-inflammatory role of green tea flavonoids in reducing elevated SAA in obese subjects with metabolic syndrome. However, this observation needs to be confirmed in larger clinical trials.
CRP, an acute phase protein synthesized by the liver has shown to be a powerful predictor of cardiovascular risk compared to other inflammatory markers [51
]. In our study subjects with metabolic syndrome, all participants had baseline CRP level >3mg/L (mean±SE, 6.2±1.2 mg/L), a high-risk category for CVD [53
]. Serum CRP has been inversely associated with dietary flavonoid intake in US adults, especially with the intakes of foods and beverages high in flavonols (onions, apples, tea), anthocyanidins (berries), procyanidins (dark chocolate) and isoflavones (soy) [54
]. However, limited clinical data exist on the effects of tea flavonoids on CRP. Both acute and chronic (4 weeks) consumption of 450ml and 900ml of black tea, respectively, showed no effects on CRP in patients with coronary artery disease [56
]. Acute consumption of green tea (6g) by smokers failed to show any effects on CRP compared to the matched caffeine or water group [25
]. In another 4-week study among smokers, de Maat et al. [24
] reported no effects of green tea intervention (3.6g polyphenols) on plasma CRP levels. We report similar findings in our 8-week study which showed no effects of green tea (960ml) or green tea extract (870mg green tea catechins) supplementation on plasma CRP levels. It is possible that a higher dose of green tea flavonoids or a combination of different flavonoids might be effective in lowering CRP and this remains an area of further investigation.
Circulating levels of adipocytokines, such as IL-6, IL-1β and adhesion molecules like sVCAM-1 and sICAM-1 expressed in vascular endothelium, have been associated with increased risks of cardiovascular events in apparently healthy subjects or in patients with existing CAD [57
]. While limited in vitro data show the inhibitory effects of green tea polyphenols or EGCG on IL-1β and/or IL-6 synthesis [61
] and/or expression of VCAM-1 [63
], human studies are inadequate and remain inconclusive. Lee et al. [23
] showed a selective effect of green tea beverage in lowering P-selectin, an adhesion molecule in smokers in an uncontrolled 4-week study. However, no effects were seen in IL-6 in type 2 diabetic subjects following green tea intervention for 4 weeks [26
]. We found no significant change in circulating levels of cytokines or adhesion molecules in our study subjects with metabolic risk factors. Thus, flavonoid modulation of cytokines and adhesion molecules in a clinical setting appears to be selective and warrants further investigation. Also, baseline mean concentrations of IL-6 (23.5±12.0 µg/L), IL-1β (0.22±0.02 pg/mL), sVCAM-1 (250.1±24.2 ng/mL) and sICAM-1 (107.3±9.7 ng/mL) were lower in our subjects with metabolic syndrome in comparison to previously reported levels in subjects with metabolic syndrome or advanced CVD [59
], and this may account for the null effects of green tea intervention. Furthermore, certain classes of anti-hypertensive medications, such as angiotensin II antagonists and angiotensin converting enzyme inhibitors, have been shown to exert anti-inflammatory effects [66
]. Since a significant number of our study subjects were on stable medications for hypertension, this could possibly contribute to the observed null effects of green tea intervention on inflammatory markers including CRP, IL-6, IL-1β or adhesion molecules.
Adiponectin, an anti-inflammatory adipocytokine has shown to be reduced in subjects with metabolic syndrome versus subjects with null or fewer features of metabolic syndrome [68
]. Subjects in our study had mean baseline adiponectin concentrations of 2.0±0.4 mg/L, which are comparable to previously reported studies in which subjects in the lowest quartile of adiponectin (< 5.0 mg/L) had highest incidence or risks of metabolic syndrome [70
]. Green tea catechins have been shown to up regulate adiponectin expression in mouse preadipocyte cells [72
]. However, in our study, green tea supplementation had no significant effects on adiponectin levels. Our results are comparable to previously reported dietary intervention studies in which adiponectin levels were unaltered: following green tea supplementation in type 2 diabetic subjects [26
], after significant diet-induced weight loss [73
], or in obese subjects on a very low-calorie diet [74
]. Thus, the role of dietary factors in modulating adiponectin levels needs further investigation. We also measured leptin: adiponectin ratios in our study, which however showed no significant effects. Leptin, a hormone secreted by adipose tissue has been positively correlated with obesity and metabolic syndrome [75
]. In our study, the average baseline leptin concentrations of 39.0±7.2 ng/mL are higher than previously reported studies in subjects with metabolic syndrome [77
]. A combination of soy, black and green tea polyphenols was shown to significantly reduce serum leptin concentrations in both male and female mice, though the effects cannot be attributed to green tea per se [79
]. While green tea polyphenols and leptin levels have not been previously correlated in humans, our data suggest that the cardio protective effects of green tea are not mediated via circulating leptin levels in obese subjects.
Furthermore, the baseline values for individual components of metabolic syndrome, including glucose and lipid profiles, were mildly elevated in our study subjects. The mean baseline values for blood pressure, glucose, triglycerides and total cholesterol were either normal or slightly elevated in the green tea, green tea extract or no treatment group. Also, the recently updated definition of metabolic syndrome has unifying criteria, and requires the cut points for waist circumference to be specific to the population being studied, and the country of study setting [80
]. In light of this new definition, our sample size of primarily US adult women has only two significant abnormal features out of five: elevated waist circumference and low HDL-cholesterol levels, with blood pressure slightly above the cut points and normal glucose and slightly elevated triglycerides. Thus, mild prevalence of metabolic syndrome in conjunction with low concentrations of interleukins and adhesion molecules in our subjects may contribute to the overall lack of positive effects of an eight-week green tea intervention on these variables. On the other hand, in future studies, a higher dose of green tea or longer study duration may be effective in decreasing elevated CRP or increasing low adiponectin levels in subjects with metabolic syndrome. Other limitations of our study involve a high female: male ratio, and a small study sample which limits generalizability to a larger population. Also, genetic variations in phase-II metabolizing enzymes [81
] may affect the overall metabolism, clearance and thus physiological effects of green tea polyphenols and were not accounted for in this study.