Chronic and acute effects of walnuts on antioxidant capacity and nutritional status in humans: a randomized, cross-over pilot study

Abstract

Background: Compared with other common plant foods, walnuts (Juglans regia) are consistently ranked among the highest in antioxidant capacity. In vitro, walnut polyphenols inhibit plasma and LDL oxidation, while in animal models they lower biomarkers of oxidative stress and raise antioxidant capacity. A limited number of human feeding trials indicate that walnuts improve some measures of antioxidant status, but not others.

Methods: A 19 wk, randomized crossover trial was conducted in 21 generally healthy men and postmenopausal women > or = 50 y to study the dose-response effects of walnut intake on biomarkers of antioxidant activity, oxidative stress, and nutrient status. Subjects were randomized to receive either 21 or 42 g raw walnuts/d during each 6 wk intervention phase with a 6 wk washout between phases. Subjects were instructed to consume their usual diet, but refrain from eating any other tree nuts, seeds, peanuts, or ellagitannin-rich foods during the entire study, and other polyphenol-rich foods for 2 d prior to each study visit.

Results: Compared to baseline levels, red blood cell (RBC) linoleic acid and plasma pyridoxal phosphate (PLP) were significantly higher after 6 wk with 42 g/d walnuts (P < 0.05 for both). Overall, changes in plasma total thiols, and other antioxidant biomarkers, were not significant with either walnut dose. However, when compared to fasting levels, plasma total thiols were elevated within 1 h of walnut consumption with both doses during the baseline and end visits for each intervention phase (P < 0.05 for all). Despite the observed increase in RBC linoleic and linolenic acids associated with walnut consumption, this substrate for lipid peroxidation only minimally affected malondialdehyde (MDA) and antioxidant capacity. The proportional changes in MDA and Oxygen Radical Absorbance Capacity (ORAC) were consistent with a dose-response effect, although no significant within- or between-group differences were observed for these measures.

Conclusions: Walnut consumption did not significantly change the plasma antioxidant capacity of healthy, well-nourished older adults in this pilot study. However, improvements in linoleic acid and pyridoxal phosphate were observed with chronic consumption, while total plasma thiols were enhanced acutely. Future studies investigating the antioxidant effects of walnuts in humans are warranted, but should include either a larger sample size or a controlled feeding intervention.

Trial registration: ClinicalTrials.gov: NCT00626691.

Figures

Figure 1

Acute changes in plasma total thiols within 1 h of walnut consumption. Mean total plasma thiols just prior to consuming a single 21 g dose were 2.90 ± 0.52 and 2.81 ± 0.41 mM, respectively, during the start and end of intervention visits for this phase. Mean total plasma thiols just prior to a single 42 g dose were 2.65 ± 0.63 and 2.55 ± 0.38 mM during the start and end visits for this phase. Changes in plasma total thiols were measured 1 h following walnut consumption, and compared with the respective baseline values at each visit using paired t-tests. P > 0.05 for between group differences, * P < 0.05 for change from baseline values, **P = 0.058 for change from baseline.

Figure 2

Changes in RBC linoleic acid and linolenic acid following 6 wk walnut consumption. Mean baseline RBC 18:n2 was 12.3 ± 1.4 and 11.4 ± 2.0 mol % total fatty acids, and 18:n3 was 0.65 ± 0.64 and 0.52 ± 0.28 mol % total fatty acids for 21 and 42 g/d, respectively. Using paired t-tests, P > 0.05 for between group differences; change from baseline: *P = 0.066, **P = 0.022, ***P = 0.068.