Grape polyphenols prevent fructose-induced oxidative stress and insulin resistance in first-degree relatives of type 2 diabetic patients

Abstract

Objective: To assess the clinical efficacy of nutritional amounts of grape polyphenols (PPs) in counteracting the metabolic alterations of high-fructose diet, including oxidative stress and insulin resistance (IR), in healthy volunteers with high metabolic risk.

Research design and methods: Thirty-eight healthy overweight/obese first-degree relatives of type 2 diabetic patients (18 men and 20 women) were randomized in a double-blind controlled trial between a grape PP (2 g/day) and a placebo (PCB) group. Subjects were investigated at baseline and after 8 and 9 weeks of supplementation, the last 6 days of which they all received 3 g/kg fat-free mass/day of fructose. The primary end point was the protective effect of grape PPs on fructose-induced IR.

Results: In the PCB group, fructose induced 1) a 20% decrease in hepatic insulin sensitivity index (P < 0.05) and an 11% decrease in glucose infusion rate (P < 0.05) as evaluated during a two-step hyperinsulinemic-euglycemic clamp, 2) an increase in systemic (urinary F2-isoprostanes) and muscle (thiobarbituric acid-reactive substances and protein carbonylation) oxidative stress (P < 0.05), and 3) a downregulation of mitochondrial genes and decreased mitochondrial respiration (P < 0.05). All the deleterious effects of fructose were fully blunted by grape PP supplementation. Antioxidative defenses, inflammatory markers, and main adipokines were affected neither by fructose nor by grape PPs.

Conclusions: A natural mixture of grape PPs at nutritional doses efficiently prevents fructose-induced oxidative stress and IR. The current interest in grape PP ingredients and products by the global food and nutrition industries could well make them a stepping-stone of preventive nutrition.

Trial registration: ClinicalTrials.gov NCT01478841.

Figures

Figure 1

Protective effects of grape PP supplementation on HFrD-induced oxidative stress. Linking oxidative stress to IR, from baseline (white bars) to post–8 weeks of PCB or grape PP supplementation (gray bars) and after 6 days of HFrD with PCB/grape PP supplementations (black bars) with urinary F2-isiprostanes (Isop) (picomoles per millimole of creatinine [creat]) (18 PCB and 20 PP) (A), TBARS (nanomoles per gram of tissue) (8 PCB and 8 PP) (B), and muscle protein carbonylation as fold induction relative to baseline (arbitrary units [AU]) (8 PCB and 13 PP) (C). *P < 0.05 for statistical pre- and postintervention comparisons within each group via the Wilcoxon signed rank sum test. #P < 0.05 for intergroup comparisons by Mann-Whitney U tests. Data are means ± SEM.

Figure 2

Differentially regulated biological pathways in response to HFrD and protective effects of grape PP supplementation on fructose-induced alterations of mitochondrial genes and a major transcription factor of mitochondrial biogenesis. A: Analysis of the 277 differentially regulated probes in response to fructose load between both groups was performed with Babelomics (http://babelomics.bioinfo.cipf.es/index.html). Only biological processes with adjusted P value <0.05 and fold enrichment >2 are displayed. Change in mRNA levels of skeletal muscle CKMT2 (B), CPT1B (C), and UCP3 (D) and regulation of PGC-1α mRNA (E), expressed by reference to hypoxanthine guanine phosphoribosyl transferase mRNA abundance, at baseline (white bars), after 8 weeks of PCB or grape PP supplementation (gray bars), and after 6 days of HFrD with PCB/PP supplementations (black bars). Transcript levels were measured by reverse transcriptase–quantitative PCR as described in research design and methods (11 PCB and 12 PP). *P < 0.05 for statistical pre- and postintervention comparisons within each group via the Wilcoxon signed rank sum test; Mann-Whitney U test was used for intergroup comparisons. Data are means ± SEM. AU, arbitrary units.

Figure 3

Protective effects of grape PP supplementation against HFrD-induced mitochondrial dysfunction. Mitochondrial functional adaptations at baseline (white bars), after 8 weeks of PCB or grape PP supplementation (gray bars), and after 6 days of HFrD with PCB or PP supplementation (black bars). A and C: Mitochondrial V0 without ADP in the presence of pyruvate or palmitoyl-l-carnitine (V0) (micromoles O2 per minute per gram of fibers) (seven PCB and nine PP) on the left, and on the right, basal measures of respiration are divided by citrate synthase activity to correct for mitochondrial content. B and D: ADP-stimulated mitochondrial Vmax rate (micromoles O2 per minute per gram of fibers) (seven PCB and nine PP) in the presence of pyruvate or palmitoyl-l-carnitine on the left, and on the right, Vmax is divided by citrate synthase activity to correct for mitochondrial content. *P < 0.05 for statistical pre- and postintervention comparisons within each group via the Wilcoxon signed rank sum test. #P < 0.05 for intergroup comparisons by Mann-Whitney U tests. Correlations were established with Spearman rank correlations tests; data are means ± SEM.