Meta-analyses of randomised controlled trials (RCTs) report that polyphenol-rich diets can modulate a range of cardiometabolic biomarkers, with increasing evidence that inter-individual factors (e.g. age, BMI, or ethnicity) contribute toward the variability in the response to the bioactive(Reference Pinto and Santos1,Reference González-Sarrías, Combet and Pinto2) . This systematic review and meta-analysis assessed the effect of flavanols from cocoa, apple and tea on fasting insulin and HOMA-IR and explored the role of inter-individual variability.
PubMed and Web of Science databases were searched from inception to October 2017 (PROSPERO reg. CRD42016033878). The effect of flavanols supplementation on insulin and HOMA-IR was estimated using a random effects meta-analysis model and reported as standardised mean difference (SMD) and 95%CI. Subgroup analyses (Q tests; multivariate meta-regression) focused on baseline BMI, gender, age, and geographical location to explore the role of inter-individual variability.
Out of 1409 studies identified, 31 RCTs were included for insulin (n = 1792) and 21 RCTs for HOMA-IR (n = 1152). Low heterogeneity was found between studies (insulin I2 = 0%, p = 0.98; HOMA-IR I2 = 5.9%, p = 0.38) with evidence of low publication bias. Flavanol-rich interventions (2–26 weeks; 88 to 1344 mg flavanols/day) decreased both insulin (SMD −0.25, 95% CI −0.33; −0.16) and HOMA-IR (SMD −0.26; 95% CI −0.36, −0.16). Results were consistent across subgroups (Q tests) with lack of effect in subgroups with BMI<25 or male subjects only; multivariate meta-regression showed that baseline BMI (overweight versus lean, coef. −1.07; 95% CI −2.03, −0.08; p = 0.03) and study location (Asia versus other sites, coef. 0.94; 95% CI 0.03,1.84; p = 0.04) impacted on the effect on HOMA-IR significantly. There was no impact of age, gender, baseline BMI or geographical location on the effect on insulin.
Values are showed in SMD (95%CI). n, number of trials; N, number of participants.
Flavanols from tea, apple and cocoa were effective in modulating insulin and HOMA-IR. Inter-individual variability in the response was limited in contrast to previous studies(Reference Pinto and Santos1,Reference González-Sarrías, Combet and Pinto2) . This could be partly explained by the small number of trials reporting data for specific subgroups, and the broad range of doses and duration tested among the studies.