Abstract/Summary

A polyphenol rich extract derived from the leaves of olive trees (Olea europaea) may be used as a nutritional supplement. This olive leaf extract (OLE) has a high concentration of the secoiridoid, oleuropein and its derivative, hydroxytyrosol (HT). Recent randomised control trials demonstrate hypotensive and anti-hyperglycaemic activity for OLE in human volunteers; the mechanisms for these effects are still uncertain. The high consumption of olive polyphenols may, in part, explain the noted protection against chronic disease associated with the Mediterranean diet. The classical Mediterranean lifestyle as observed by Ancel Keys, involved not only a healthy diet, but also a high level of physically activity. Research investigating the synergistic benefits of including olive polyphenols as part of an active lifestyle is limited. The aims of this thesis were; first, to explore plausible mechanisms that might explain previously observed anti-glycaemic and hypotensive responses to OLE in human volunteers; and second, to investigate the synergistic hypotensive effects of consuming olive leaf extract alongside increasing physical activity in individuals with slightly elevated blood pressure. Our hypotheses were that olive polyphenols would inhibit enzymes involved in glucose and fat digestion in the small intestine, and that these polyphenols inhibit glucose uptake by the gut epithelia resulting in improved blood sugar control. In relation to their anti-hypertensive effects we hypothesised that they would enhance NO production and suppress angiotensin enzymes. Finally, we hypothesised that there might be a synergistic effect of consuming the OLE and increasing physical activity in terms of reducing blood pressure. OLE may influence the digestion of macronutrients. We show it to inhibit pancreatic αamylase and lipase in a dose dependent manner; IC50 OLE; 3.23 ± 0.33 mM and 1.83 ± 0.03 mM, respectively. Further, we suggest that the rate of sugar absorption in the small intestine may be influenced by the presence of OLE, this is based on our observation of a reduction in the mRNA expression for genes encoding intestinal glucose transporters (SGLT1, GLUT2) in Caco-2 cells which was coupled to a reduction in glucose uptake measured in vitro. To explain the anti-hypertensive effects of OLE, we first studied NO production in HUVEC cells. At concentrations of OLE equivalent to 100 µM oleuropein, NO production is enhanced compared to that observed in untreated controls (p < 0.05). However, we deem these concentrations super-physiological, and more work is needed to establish the potency of olive leaf phenolic metabolites on this cell system. The inhibitory effects of OLE and its principal polyphenols were assessed against renin and against the angiotensin converting enzyme (ACE). OLE inhibited renin and ACE activity with an IC50 value of 63.08 ± 2.90 µM and 60.86 ± 5.68 µM, respectively, again the concentrations required to reduce the activity of these enzymes in order to explain the reductions in blood pressure observed in intervention study are unlikely to be achieved in vivo. Following the in vitro studies, we conducted a double-blind, four-arm parallel trial in 63 pre-hypertensive overweight adults aged 25-70y who were assigned to one of four treatment arms for 12 weeks: i) a placebo control group- with a daily capsule; ii) an increasing physical activity and a placebo capsule group; iii) an olive leaf extract arm (equivalent to 132 mg of oleuropein per day; iv) a physical activity and olive leaf extract arm. The primary outcome measure for the study was the change in 24-h ambulatory blood pressure (ABP). Secondary outcome measures were arterial stiffness index, plasma HbA1C levels, fasted glucose and insulin, serum lipid profiles and body composition. 24-h SBP (-5.80 (±SD 7.63), p = 0.011) and day time SBP (-5.07 (±SD 7.16), p = 0.025) were significantly lower following OLE intake. The PA intervention alone resulted in lower 24-h SBP (-3.69(±SD 9.39), p = 0.031) and daytime SBP (-4.19 (±SD 10.27), p = 0.041). Consuming the OLE alongside being increasing physical active reduced 24-h SBP (-3.88 (±SD 6.65), p = 0.027) relative to the baseline. The magnitude of SBP change observed in this thesis would suggest that regular OLE intake as part of healthy lifestyle may be associated with a 9-14 % reduction in CHD risk and a 20-22 % reduction in risk of stroke. There were however, no effects on stiffness index, body composition, lipids, HbA1c, glucose and insulin in all groups after 12 weeks (p > 0.05). In this thesis, we provide mechanistic explanations for the findings of previous intervention studies. Understanding the biological mechanisms is a key component of the Bradford-Hill criteria when it comes to proving efficacy. Publications arising from this thesis may therefore provide further evidence for the assessment of health claims by olive producers in the European market. In addition, we believe that following this thesis, there is now abundance of data proving that, olive phenolics when consumed chronically, at appropriate doses, reduce blood pressure in pre-hypertensive consumers. However, we still do not have a fundamental grasp of the mechanisms involved or an understanding of the relative potency of the individual phenolics in this crude extract. Nevertheless, the findings presented herein have commercial and public health significance. Consuming the OLE delivers the beneficial polyphenols at high doses and minus the need to ingest large amounts of energy dense olive oil. OLE consumption, in the absence of increasing physical activity, led to a more optimal blood pressure profile, however the benefits of increasing physical activity are wide and we would continue to recommend it to OLE consumers.