English and Black Walnut Phenolic Antioxidant Activity <i>in Vitro</i> and Following Human Nut Consumption

Abstract

Background: Walnut consumption may reduce the risk of cardiovascular disease by providing antioxidant protection to low density lipoproteins (LDL). Aim: This study compared the phenolic profile and antioxidant activity of English ver- sus black walnuts. Methods: Nuts were extracted in methanol or acetone prior to analysis with HPLC/LC-MS-MS for phenolic identification and quantitation. The ability to prevent oxidation of LDL was examined in vitro using walnut extracts and ex vivo after walnut consumption for 28 days. Results: Flavonoids identified/quantified with HPLC/LC- MS-MS included the phenolic acids 5-caffeoylquinic acid, 3-caffeoylquinic acid (black walnut only), 4-caffeoylquinic acid, and the flavonol glycosides quercetin-3-rutinoside, quercetin-3-galactoside, quercetin-3-pentoside, quercetin-3- arabinoside, quercetin-3-rhamnoside, and the aglycone quercetin (English walnut only). Total phenolic yield of acetone extracts were 166.1 and 24.2 µg/g for English and black walnut respectively, and yield for methanol extracts were 147.6 and 4.1 µg/g for English and black walnut respectively. In vitro LDL oxidation by Cu++ with English walnut ex- tracts significantly extended oxidation lag-time (A234) in a dose dependent manner at 1.0 and 0.1 µg/ml and reduced TBARS formation (1.0 µg/ml). Black walnut extracts reduced TBARS significantly but had no effect on A234. Human consumption of English or black walnuts (30 g nuts/day) for 28 days resulted in no differences in LDL antioxidant ca- pacity (A234) between groups or within groups. Conclusion: This study suggests that the English walnuts have a pheno- lic profile and in vitro antioxidant capacity that is better than black walnuts, but that walnut consumption for 28 days does not improve LDL resistance to oxidation.