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Epidemiological, in vitro and animal studies suggest that grape polyphenols, such as those present in wine, have favorable effects on the metabolic syndrome. However, controversy remains whether treatment with grape polyphenols is effective in humans. Here, we aimed to systemically review the effects of grape polyphenols on metabolic syndrome components in humans.We systematically searched Medline, EMBASE and the Cochrane database for all clinical trials assessing the effects of grape polyphenols on insulin sensitivity, glycemia, blood pressure or lipid levels. We screened all titles and reviewed abstracts of potentially relevant studies. Full papers were assessed for eligibility and quality-rated according to the Jadad scale by two independent assessors.Thirty-nine studies met the eligibility criteria. In individuals without component criteria of the metabolic syndrome, only low- and medium-quality studies were found with primarily neutral results. In individuals with the metabolic syndrome or related conditions, one of two high-quality studies suggested improvement in insulin sensitivity. Glycemia was improved in 2 of 11 lower-quality studies and 2 of 4 high-quality studies. Seven of 22 studies demonstrated a significant decrease in blood pressure, but only one was of high quality. Two of four high-quality studies pointed towards effects on total cholesterol while other lipidemic parameters were not affected.No compelling data exist that grape polyphenols can positively influence glycemia, blood pressure or lipid levels in individuals with or without the metabolic syndrome. Limited evidence suggests that grape polyphenols may improve insulin sensitivity.