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Fiber decreases the antioxidant capacity of phenolic acids in an alkaline milieu in vitro

Phenolic acids and dietary fiber contribute to the health benefits from consuming plant foods. However, fiber may interact with phenolic acids via an affinity to their functional antioxidant moieties. We tested whether fiber (soluble pectin or insoluble cellulose) under different pH in vitro affects the Ferric Reducing Antioxidant Power (FRAP) and Total Phenol (TP) content of 3 phenolic acids: gallic (GA), protocatechuic (PA), and p‐coumaric (CA) acids with 3, 2, and 1 hydroxyl moieties, respectively. Phenolic acids at 30 μmol/L were incubated with 1, 2, or 3% (w/v) fiber at pH 1.5, 3, 5, 7 or 9 for 2 h. After centrifugation, supernatants were collected for the determination of FRAP and TP. The effect of fiber on FRAP and TP of the test compounds was dependent on fiber type and concentration, pH, and phenolic acid. Overall, pectin ≥2% and pH 9 had the greatest impact on study outcomes; e.g., at pH 9, 3% pectin decreased the FRAP of GA, PA, and CA by ≥65% and TP by ≥62%. At pH 1.5, 1% cellulose did not significantly affect the TP of GA and PA with reductions of ≤4%. Thus, fiber in an alkaline milieu decreased the antioxidant capacity of phenolic acids in vitro. This relationship may be relevant to the bioactions of these phytochemicals in the ileum in vivo. Further studies using infrared spectrometry and NMR are warranted to elucidate the chemical/physical interactions between dietary fiber and phenolic acids. Supported by the USDA.