Intestinal-level anti-inflammatory bioactivities of catechin-rich green tea: Rationale, design, and methods of a double-blind, randomized, placebo-controlled crossover trial in metabolic syndrome and healthy adults

Abstract

Metabolic endotoxemia initiates low-grade chronic inflammation in metabolic syndrome (MetS) and provokes the progression towards more advanced cardiometabolic disorders. Our recent works in obese rodent models demonstrate that catechin-rich green tea extract (GTE) improves gut barrier integrity to alleviate the translocation of gut-derived endotoxin and its consequent pro-inflammatory responses mediated through Toll-like receptor-4/nuclear factor κB (TLR4/NFκB) signaling. The objective of this clinical trial is to establish the efficacy of GTE to alleviate metabolic endotoxemia-associated inflammation in persons with MetS by improving gut barrier function. We plan a double-blind, placebo-controlled cross-over trial in persons with MetS and age- and gender-matched healthy persons (18-65 y; n = 20/group) who will receive a low-energy GTE-rich (1 g/day; 890 mg total catechins) confection snack food while following a low-polyphenol diet for 28 days. Assessments will include measures of circulating endotoxin (primary outcome) and secondary outcomes including biomarkers of endotoxin exposure, region-specific measures of intestinal permeability, gut microbiota composition, diversity, and functions, intestinal and systemic inflammatory responses, and catechins and microbiota-derived catechin metabolites. Study outcomes will provide the first report of the GTE-mediated benefits that alleviate gut barrier dysfunction in relation to endotoxemia-associated inflammation in MetS persons. This is expected to help establish an effective dietary strategy to mitigate the growing burden of MetS that currently affects ~35% of Americans.

Keywords: BMI, body mass index; Catechin; Endotoxemia; GTE, green tea extract; Gut barrier function; Gut dysbiosis; Gut microbiota; Inflammation; LBP, LPS binding protein; LPS, lipopolysaccharides; MetS, metabolic syndrome; Metabolic syndrome; NFκB, nuclear factor κB; PCoA, principal coordinates analysis; SCFA, short chain fatty acid; TLR4, Toll-like receptor-4; TNF- α, tumor necrosis factor-α; Tea.

Fig. 1

Chemical structures of major catechins in green tea. Adapted from Masterjohn and Bruno [16].

Fig. 2

GTE potentially alleviates systemic metabolic syndrome complications attributed to metabolic endotoxemia-associated inflammation. GTE is hypothesized to favorably alter gut microbiota composition to increase commensal populations that biosynthesize short chain fatty acids while decreasing pathogenic bacteria populations that provoke intestinal inflammation. GTE is also likely to limit the absorption of Gram-negative bacteria-derived endotoxins that otherwise activate TLR4/NFκB signaling. Abbreviations: GTE, green tea extract; LPS, lipopolysaccharide (i.e. endotoxin); NFκB, nuclear factor κB; TLR4, Toll-like receptor-4. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

Study design of the planned double-blind, placebo-controlled, randomized cross-over trial. Persons with metabolic syndrome (MetS) and age- and gender-matched healthy adults will be recruited from the Columbus, Ohio area. Fasted blood samples will be collected on days 0, 14, and 28 for metabolic assessments (e.g. endotoxemia, catechins, systemic inflammatory responses, clinical chemistries). Gut microbiota composition and functions will be assessed from pooled fecal samples collected on any three of four terminal days of the intervention. On day 28, participants will ingest a 5-sugar probe solution and collect urine for 24-h for the assessment of region-specific gut permeability.

Fig. 4

Persons with metabolic syndrome (MetS) have metabolic endotoxemia. Serum endotoxin was evaluated at 2-wk intervals in persons with MetS and age-/gender-matched healthy adults who completed an earlier study [51]. Measures were performed using a high-sensitivity fluorescence-based endotoxin kit according to the manufacturer's instructions (PyroGene recombinant Factor C Assay; Lonza Int). Data (means ± SE; n = 10/group) were analyzed by 2-way RM ANOVA to assess effects due to time and MetS status. Endotoxin was unaffected by time, but was >2-times higher among MetS persons at each time point (*, P < 0.05).