Entry #: 45
Date: 27 March 2018
Section: Phenolic compounds
Topic: Olives and diabetes risk
Type: Human volunteer trial
OliveNetTM Journal Club
Expert review of literature related to olives and olive oil
D. Elizabeth McCord, Nancy B. Ray and Tom C. Karagiannis
Nutritional implications of olives and sugar: attenuation of post-prandial glucose spikes in healthy volunteers by inhibition of sucrose hydrolysis and glucose transport by oleuropein
Kerimi et al
Citation / Year
(1) / 2018
Oleuropein, sugar, diabetes risk, post-prandial glucose spike, maltase, sucrose, α-amylase
The beneficial health effects of the Mediterranean with the relatively high consumptions of olives and olive oil have been highlighted by a number of large-scale epidemiological and intervention studies, with the PREDIMED study being a recent prime example (2). This study focussed the key antioxidant secoiridoid, oleuropein, which is a major component of olive leaves and found in olives (up to 70mg/kg and higher in unprocessed olives) and olive oil, and has been shown to possess potent biological activity in numerous models of disease (3-5). In the context of cardiovascular disease and diabetes, the beneficial effects of oleuropein have been highlighted in numerous animal models (6, 7). Further, in humans oleuropein has been shown to improve vascular function and inflammation, and to lower fasting insulin levels and Hb1Ac (a key marker of diabetes), in patients with type 2 diabetes (8, 9). However, the effect is not clear and there have contrary studies. For example in one study six-week supplementation with 500 mg oleuropein with 100 mg green coffee been extract, and 150 mg beetroot powder did not have an effect on glucose or insulin levels (10). The aim of this study was to add some clarity to the literature by exploring the potential of oleuropein in the context of diabetes risk, by investigating in detail the effects of the compound on sugar digestion and absorption. As an extension a series of healthy human volunteer trials were performed to examine the effects of oleuropein on post-prandial glucose levels.
Key points and implications
In the first part of this work, a detailed analysis of the effects of oleuropein on sugar digestion and absorption was performed, using digestive enzyme extracts and in vitro cell culture models, respectively. In summary, using a water soluble olive leaf preparation, the findings indicated that oleuropein inhibits: 1) intestinal maltase, 2) human sucrase, 3) transport of glucose across Caco-2 cells, 4) GLUT2-mediated uptake of glucose in Xenopus oocytes, and 5) only modest inhibition of α-amylase. These findings highlight that oleuropein has the potential to directly influence sugar digestion and absorption, at least in vitro. Following these findings are series of relatively small-scale (minimum n=10 and with adequate statistical power), healthy volunteer trials (eight studies in total), were performed to assess the effects of oleuropein on post-prandial glucose levels. Oleuropein was administered in the form of capsules (Bonolive® with 41.8±0.9% w/w/ oleuropein or Olecol® 19.8±0.5% w/w oleuropein), or olives (Kalamata containing 34.8±0.4 mg oleuropein per 100 g), and carbohydrate was administered in the form of sucrose, glucose, or bread, allowing for interesting permutations. In summary the findings highlighted that while oleuropein did not have an effect on post-prandial glucose levels when consumed with relatively high quantities of sucrose (50g) or bread, it did attenuate post-prandial glucose levels when consumed with 25 g sucrose, suggesting modification of digestion in healthy volunteers. Overall, these findings are very encouraging, indicating that oleuropein can influence sugar digestion and absorption, and highlight the importance of manipulating carbohydrate intake and modifying dosages to achieve maximal results in prevention of diabetes risk.
- A. Kerimi et al., Nutritional implications of olives and sugar: attenuation of post-prandial glucose spikes in healthy volunteers by inhibition of sucrose hydrolysis and glucose transport by oleuropein. European journal of nutrition, (2018).
- R. Estruch et al., Primary prevention of cardiovascular disease with a Mediterranean diet. The New England journal of medicine 368, 1279-1290 (2013).
- V. Neveu et al., Phenol-Explorer: an online comprehensive database on polyphenol contents in foods. Database : the journal of biological databases and curation 2010, bap024 (2010).
- Z. Janahmadi, A. A. Nekooeian, A. R. Moaref, M. Emamghoreishi, Oleuropein offers cardioprotection in rats with acute myocardial infarction. Cardiovascular toxicology 15, 61-68 (2015).
- M. Romero et al., Antihypertensive effects of oleuropein-enriched olive leaf extract in spontaneously hypertensive rats. Food & function 7, 584-593 (2016).
- K. Murotomi et al., Oleuropein-Rich Diet Attenuates Hyperglycemia and Impaired Glucose Tolerance in Type 2 Diabetes Model Mouse. Journal of agricultural and food chemistry 63, 6715-6722 (2015).
- S. W. Kim et al., Oleuropein prevents the progression of steatohepatitis to hepatic fibrosis induced by a high-fat diet in mice. Experimental & molecular medicine 46, e92 (2014).
- S. Lockyer, G. Corona, P. Yaqoob, J. P. Spencer, I. Rowland, Secoiridoids delivered as olive leaf extract induce acute improvements in human vascular function and reduction of an inflammatory cytokine: a randomised, double-blind, placebo-controlled, cross-over trial. The British journal of nutrition 114, 75-83 (2015).
- J. Wainstein et al., Olive leaf extract as a hypoglycemic agent in both human diabetic subjects and in rats. Journal of medicinal food 15, 605-610 (2012).
- R. H. Wong, M. L. Garg, L. G. Wood, P. R. Howe, Antihypertensive potential of combined extracts of olive leaf, green coffee bean and beetroot: a randomized, double-blind, placebo-controlled crossover trial. Nutrients 6, 4881-4894 (2014).