English and Black Walnut Phenolic Antioxidant Activity in Vitro and Following Human Nut Consumption

Jacki M Rorabaugh, Ajay P Singh, Isabel M Sherrell, Michelle R Freeman, Nicholi Vorsa, Peter Fitschen, Christopher Malone, Margaret A Maher, Ted Wilson

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DOI: 10.4236/fns.2011.23026                

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.

Keywords

Walnut, Lipoprotein, Oxidation, Antioxidant, Phenolic, Flavonol

Conflicts of Interest

The authors declare no conflicts of interest.

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