Fourteen brands of commercial resveratrol nutraceuticals were evaluated in order to verify their actual resveratrol content and to control if their health-promoting properties are related to manufacturing quality. Products included pure resveratrol capsules or multi-ingredient formulations with standardized amounts of resveratrol and other phytochemicals. Samples were analyzed for total trans-resveratrol, flavonoids, procyanidin, polyphenol content and results were compared with the declared labeled content. Only 5 out 14 brands had near label values, compliant with GMP (Good Manufacturing Practices) requirements (95–105% content of active constituent), 4 products were slightly out of this range (83-111%) and 3 were in the 8-64% range. Two samples were below the limit of detection. The greater the difference between actual and labeled resveratrol content, the lower was the strenght of antioxidant and antiproliferative activities. Dietary supplements containing pure trans-resveratrol exibited a greater induction of differentiation towards human leukemic K562 cells when compared to multicomponent products. Great differences currently exists among resveratrol food supplements commercially available and GMP-grade quality should not be taken for granted. On the other side, dosages suggested by most “pure”, “high-dosage” supplements may allow a supplementation level adequate to obtain some of the purported health benefits.
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