Onion protocatechuic acid and quercetin glucosides attenuate changes induced by high fat diet in rats
Yellow onion waste from industrial peeling was used to obtain three pure preparations: protocatechuic acid (PA), quercetin diglycosides (QD) and quercetin monoglycosides (QM). PA contained 61% of protocatechuic acid, QD contained 35% of quercetin diglucosides, mainly quercetin-3,4'-diglucoside, and QM, contained 41% of monoglucosides, mainly quercetin-4'-glucoside. The highest antioxidant activity was possessed by PA. The effects of preparations on digestive functions of the gastrointestinal tract of rats as well as biochemical parameters and antioxidant capacity of the blood in model research on Wistar rats sustained by a high-fat diet were assessed (5 groups per 8 animals). Results of the present experiment showed that different onion phenolic preparations differently modulate the enzymatic activity of fecal (P<0.001) and cecal (P<0.001) microbiota. For instance, the QD preparation but not QM, efficiently reduced the fecal and cecal bacterial β-glucuronidase activity. The protocatechuic acid and quercetin monoglycosides showed a beneficial effect regulating blood lipids (reduction of TC (P<0.001), TG (P<0.001), non-HDL increase in HDL (P<0.001)), thereby lowering the risk factors for atherosclerotic lesions AI (P=0.038), AII (P=0.013). In addition, onion phenols showed a strong antioxidant effect, however, with a different mechanism. Protocatechuic acid via serum ACL (P=0.033) increase and hepatic GSSG (P=0.070) decrease, QM via ACW (P<0.001) increase and hepatic TBARS (P=0.002) decrease, QD via serum ACW increase and hepatic GSSG decrease. It can be concluded that onion polyphenols with lower molar weight, i.e. QM more preferably affect the blood lipid profile than QD. However QD more efficiently reduced the fecal and cecal bacterial β-glucuronidase activity.