Epidemiologic studies have revealed that consuming green tea or coffee reduces diabetes risk. We evaluated the effects of the combined consumption of green tea catechins and coffee chlorogenic acids (GTC+CCA) on postprandial glucose, the insulin incretin response, and insulin sensitivity. Eleven healthy men were recruited for this randomized, double-blinded, placebo-controlled crossover trial. The participants consumed a GTC+CCA-enriched beverage (620 mg GTC, 373 mg CCA, and 119 mg caffeine/day) for three weeks; the placebo beverages (PLA) contained no GTC or CCA (PLA: 0 mg GTC, 0 mg CCA, and 119 mg caffeine/day). Postprandial glucose, insulin, glucagon-like peptide-1 (GLP-1), and glucose-dependent insulinotropic polypeptide (GIP) responses were measured at baseline and after treatments. GTC+CCA consumption for three weeks showed a significant treatment-by-time interaction on glucose changes after the ingestion of high-fat and high-carbohydrate meals, however, it did not affect fasting glucose levels. Insulin sensitivity was enhanced by GCT+CCA compared with PLA. GTC+CCA consumption resulted in a significant increase in postprandial GLP-1 and a decrease in GIP compared to PLA. Consuming a combination of GTC and CCA for three weeks significantly improved postprandial glycemic control, GLP-1 response, and postprandial insulin sensitivity in healthy individuals and may be effective in preventing diabetes.

Catechins, phytochemicals contained mainly in green tea, exhibit antiviral activity against various acute infectious diseases experimentally. Clinical evidence supporting these effects, however, is not conclusive. We performed a placebo-controlled, single-blind, randomized control trial to evaluate the clinical effectiveness of consumption of catechins-containing beverage for preventing acute upper respiratory tract infections (URTIs). Two hundred and seventy healthcare workers were randomly allocated to high-catechin (three daily doses of 57 mg catechins and 100 mg xanthan gum), low-catechin (one daily dose of 57 mg catechins and 100 mg xanthan gum), or placebo (0 mg catechins and 100 mg xanthan gum) group. Subjects consumed a beverage with or without catechins for 12 weeks from December 2017 through February 2018. The primary endpoint was incidence of URTIs compared among groups using a time-to-event analysis. A total of 255 subjects were analyzed (placebo group n = 86, low-catechin group n = 85, high catechin group n = 84). The URTI incidence rate was 26.7% in the placebo group, 28.2% in the low-catechin group, and 13.1% in the high-catechin group (log rank test, p = 0.042). The hazard ratio (95% confidence interval (CI)) with reference to the placebo group was 1.09 (0.61–1.92) in the low-catechin group and 0.46 (0.23–0.95) in the high-catechin group. These findings suggest that catechins combined with xanthan gum protect against URTIs.

We examined the effects of the timing of acute and consecutive epigallocatechin gallate (EGCG) and catechin-rich green tea ingestion on postprandial glucose in mice and human adults. In mouse experiments, we compared the effects of EGCG administration early (morning) and late (evening) in the active period on postprandial glucose. In human experiments, participants were randomly assigned to the morning-placebo (MP, n = 10), morning-green tea (MGT, n = 10), evening-placebo (EP, n = 9), and evening-green tea (EGT, n = 9) groups, and consumed either catechin-rich green tea or a placebo beverage for 1 week. At baseline and after 1 week, participants consumed their designated beverages with breakfast (MP and MGT) or supper (EP and EGT). Venous blood samples were collected in the fasted state and 30, 60, 120, and 180 min after each meal. Consecutive administration of EGCG in the evening, but not in the morning, reduced postprandial glucose at 30 (p = 0.006) and 60 (p = 0.037) min in the evening trials in mice. In humans, ingestion of catechin-rich green tea in the evening decreased postprandial glucose (three-factor analysis of variance, p < 0.05). Thus, catechin intake in the evening more effectively suppressed elevation of postprandial glucose.

Epidemiologic studies show that the risk of diabetes can be reduced by ingesting green tea or coffee. Previous studies have shown that simultaneously taking green tea catechins (GTC) and coffee chlorogenic acid (CCA) alters postprandial gastrointestinal hormones secretion and improves insulin sensitivity. However, there is no evidence on the acute effects of GTC and CCA on incretin and blood glucose, and on the respective dose of polyphenols. In this randomized, double-blind, placebo-controlled crossover study, we examined the effective dose of GTC and CCA on postprandial glucose, insulin, and incretin responses to a high-fat and high-carbohydrate cookie meal containing 75 g of glucose in healthy men. Study 1 (n = 18) evaluated two doses of GTC (270 or 540 mg) containing a fixed dose of CCA (270 mg) with 113 mg of caffeine and a placebo (0 mg GTC and 0 mg CCA) with 112 mg of caffeine. Study 2 (n = 18) evaluated two doses of CCA (150 or 300 mg) containing a fixed dose of GTC (540 mg) and a placebo with 99 mg of caffeine. The single combined ingestion of GTC and CCA significantly altered the incretin response and suppressed glucose and insulin levels. These findings suggest that the effective minimum dose is 540 mg of GTC and 150 mg of CCA.

Background: Green tea catechins and inulin may improve metabolic health and modulate the gut microbiota; however, double-blind trials evaluating visceral fat, insulin resistance, and microbiota concurrently are limited. Methods: We conducted a double-blind, parallel-group, randomized, placebo-controlled trial in Japanese adults aged 20–75 years with visceral fat area (VFA) ≥ 80 cm² and BMI ≥ 23 kg/m² (trial registration: Japan Registry of Clinical Trials (jRCT), jRCTs021230004 (registered 16 May 2023)). Participants were randomized to a catechin + inulin beverage (catechins 400 mg/day; inulin 2.3 g/day) or placebo for 12 weeks. The primary outcome was the change in VFA (bioelectrical impedance). Secondary outcomes included HOMA-IR, metabolic markers, liver fat assessed by the controlled attenuation parameter (CAP), and genus-level gut microbiota. Results: Ninety-six participants were randomized (catechin + inulin, n = 49; placebo, n = 47); 47 and 44, respectively, were included in the full analysis set for the primary outcome. The 12-week change in VFA did not differ between groups (mean change: +0.91 vs. +4.61 cm²; p = 0.243). HOMA-IR decreased in the catechin + inulin group and increased in the placebo group, yielding a significant between-group difference (mean change: −0.32 vs. +0.18; p = 0.020). No other secondary outcomes showed significant between-group differences. In exploratory microbiota analyses, no genus remained significant after false discovery rate correction; however, Coprococcus and Bifidobacterium showed nominal between-group differences (unadjusted p < 0.05). Changes in Coprococcus were inversely correlated with changes in HOMA-IR (r = −0.28; p = 0.010). Conclusions: Catechin plus inulin for 12 weeks did not reduce visceral fat but was associated with improved insulin resistance. Exploratory analyses suggest a potential association between increased Coprococcus and improved HOMA-IR, warranting further investigation.

Purpose: This study examined the effects of acute and chronic oral intake of tea catechins and ornithine supplementation on exercise-induced ammonia accumulation and cycling performance. Methods: Sixteen healthy young men participated in a randomized, double-blind, cross-over, placebo-controlled study. For the acute trials, the participants consumed either tea catechins and ornithine (CO) or placebo (P) and performed cycling exercises at an intensity corresponding to 75% of the maximum heart rate for 60 min, followed by a 15-min cycling performance test. The participants continued to consume each designated supplement for 13 days. For the chronic trials, the participants repeated the same protocol as the acute trials on day 14. After a washout period, the participants changed the supplement and repeated the same protocol as above. Results: Plasma catechins (acute ES = 3.61; chronic ES = 2.64, p < 0.001) and ornithine (acute ES = 4.28; chronic ES = 2.25, p < 0.001) concentrations were higher in both acute and chronic CO trials than those in P trials. No differences were found in plasma ammonia concentration measured during the whole experimental period and in mean power output during the performance test among trials. Subjective fatigue during 60-min cycling was lower in both acute and chronic CO trials than those in P trials (acute ES = 0.32, chronic ES = 0.60, p < 0.001). Conclusions: A single dose and 14-day oral intake of tea catechins and ornithine supplementation did not suppress exercise-induced ammonia accumulation or enhance cycling performance. Clinical trial: Clinical trial registration ID: UMIN000035267.