Entry #: 26
Date: 11 December 2017
Section: Insulin resistance
Topic: Olive oil and insulin resistance and liver inflammation
Type: In vivo model

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OliveNetTM Journal Club

Expert review of literature related to olives and olive oil

D. Elizabeth McCord, Nancy B. Ray and Tom C. Karagiannis

Title

Polyphenol-rich virgin olive oil reduces insulin resistance and liver inflammation and improves mitochondrial dysfunction in high-fat diet fed rats

Author(s)

Lama et al

Citation / Year

(1) / 2016

Keywords

Polyphenol-rich olive oil, olive oil phenolics, inflammation, insulin resistance, mitochondrial dysfunction

Summary

Whereas early research focussed on the beneficial health effects of the major mono- and poly-unsaturated fatty acid component of extra-virgin olive oil, it is now evident the minor phenolic content is important for the potent anti-inflammatory and antioxidant effects of extra-virgin olive oil (2-5). In this study, the authors have utilised a conventional in vivo model to highlight the importance of the phenolic fraction of virgin olive oil in producing anti-inflammatory and antioxidant effects. A well-controlled experiment using four groups of animals (n = 6 per group) was performed. Group 1 received a standard pellet diet (control group; 17% fat, 23% proteins, 60% carbohydrates; 16.1 kJ/g), group 2 received a high fat diet (58.1% fat, 16% proteins, 25% carbohydrates), group 3 received the high fat diet with 1.3% virgin olive oil rich in polyphenols and tocopherols, and group 4 received the high fat diet with a virgin olive oil with tocopherols but not polyphenols. The polyphenol-rich olive oil contained 404.1 mg/kg of polyphenols whereas the oil for group was completely devoid of polyphenols. This experimental design allowed the authors to at least in part, de-lineate the effects attributable to the polyphenolic content. The experiment was performed over a six week period with an oral glucose tolerance test conducted at five weeks.

Key points and implications

The authors have methodically evaluated the anti-inflammatory and antioxidant effects of polyphenol-rich diet compared to the control, high fat diet and high fat diet without polyphenols groups. Firstly, the high fat diet increased serum alanine transaminase – a marker of liver damage – and triglycerides and these were reduced by the polyphenol-rich diet. Similarly, the oral glucose tolerance test, fasting serum glucose levels, and homeostatic model assessment for assessing β-cell function and insulin resistance, highlighted improvements with the polyphenol-rich diet which were more pronounced than the virgin olive oil diet

without polyphenols. This was confirmed by Western blot analysis of: 1) phosphorylated AKT, 2) GLUT2, 3) AMPK, and 4) PPAR-α in the liver. In addition, the polyphenol-rich diet positively modulated the serum levels of pro-inflammatory (TNF-α and IL-1), and anti-inflammatory (IL-10) mediators. Effects on liver inflammation and oxidative stress were also measured by examining mRNA levels of TNF-a and the cyclooxygenase-2 levels, investigating reactive oxygen species and reactive nitrogen species-induced liver damage, by measuring malondialdehyde levels as a marker of lipid peroxidation. The findings indicated that polyphenol-rich diet improved all of the above parameters more effectively than the virgin olive oil diet without polyphenols diet. Finally, improved fatty acid oxidation with the polyphenol-rich diet was observed in isolated hepatic mitochondria. Overall, using a well-controlled experiment and wide array of conventional biochemical and molecular methodologies this study re-iterates the importance of the polyphenolic content of virgin olive oil in producing potential health benefits.

Related publications

  1. A. Lama et al., Polyphenol-rich virgin olive oil reduces insulin resistance and liver inflammation and improves mitochondrial dysfunction in high-fat diet fed rats. Molecular nutrition & food research 61, (2017).
  2. L. Parkinson, S. Cicerale, The Health Benefiting Mechanisms of Virgin Olive Oil Phenolic Compounds. Molecules 21, (2016).
  3. S. Cicerale, X. A. Conlan, A. J. Sinclair, R. S. Keast, Chemistry and health of olive oil phenolics. Critical reviews in food science and nutrition 49, 218-236 (2009).
  4. P. Reboredo-Rodriguez et al., State of the Art on Functional Virgin Olive Oils Enriched with Bioactive Compounds and Their Properties. International journal of molecular sciences 18, (2017).
  5. F. Visioli, A. Poli, C. Gall, Antioxidant and other biological activities of phenols from olives and olive oil. Medicinal research reviews 22, 65-75 (2002).