There is an association between the high consumption of olive oil, the principle fat in the Mediterranean diet, and a reduction in chronic disease risk( 1 ). Data has highlighted the health-promoting effects of phenolic compounds present within the water-soluble fraction of the oil( 2 ). However, olive leaves are the richest source of phenolics within the olive plant (Olea europaea), with the secoiridoid, oleuropein, being the most abundant compound.
In a randomised, controlled crossover clinical trial, 18 subjects (9 male, 9 female) aged 19–40 years consumed either 4 phenolic-rich olive leaf extract (OLE) capsules, (each containing 400 mg olive leaf extract, equivalent to 2.4 g fresh olive leaf and 14.53 mg oleuropein, plus 672.5 mg safflower oil) or a control (900 mg safflower oil) in a random order, separated by a 4-week washout. Blood and urine were collected to ascertain the bioavailability of OLE (data not shown). Subjects consumed a low polyphenol diet for 24 hours prior to each study day. Digital volume pulse-derived stiffness index (DVP-SI) was obtained via PCA2 Pulse Trace at baseline and 30, 60, 90, 120, 180, 240, 360 and 480 minutes after capsule ingestion. A low fat, low polyphenol meal was provided after the 240 minute sample. For whole blood cytokine production, blood collected at baseline, 60, 180 and 360 minutes was diluted 1:1 with RPMI 1640 medium (containing 1% antibiotics), stimulated with LPS (1 μg/ml) and incubated for 24 hours (37 °C; 5% CO2). Supernatants were collected and stored at −80 °C until analysis for interleukin(IL)-1β, IL-6, tumour necrosis factor(TNF)-α, IL-8 and IL-10 using Fluorokine MAP ELISA kits and a Luminex reader. Prism (GraphPad software, USA) was used to analyse the data. Differences in DVP-SI, IL-1 β, IL-6, TNF-α, IL-8 and IL-10 by treatment were identified using a two-way ANOVA with repeated measures. Post hoc analysis was subjected to Bonferroni correction. P values less than 0.05 were treated as significant.
DVP-SI and IL-8 were significantly lower over the course of the study day (P<0.01 and P<0.05 respectively) when subjects consumed OLE capsules vs. the control. Post hoc analysis revealed no significant differences at any individual time point over the day. There was no significant treatment effect on IL-1 β, IL-6, TNF-α or IL-10.
The effect of OLE phenolics on vascular function, an important CVD risk marker, has not been studied previously. The current study demonstrated that OLE phenolics have anti-inflammatory properties during the postprandial period, and decrease arterial stiffness. Further work aims to characterize the mechanisms underlying these effects.