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Type 2 Diabetes Systematic Literature Review

Type 2 diabetes (prevention and management)

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Reference: Schwingshackl, L., et al., Olive oil in the prevention and management of type 2 diabetes mellitus: a systematic review and meta-analysis of cohort studies and intervention trials. Nutr Diabetes, 2017. 7(4): p. e262.

One-sentence summary: High olive oil intake was associated with a decreased risk of T2D and improved glucose metabolism in persons with T2D.

Study type: A systematic review and meta-analysis of 4 cohort studies and 29 RCTs on olive oil, T2D and glycemic control.

Diet: Diets containing olive oil (any type).

Outcomes measured: T2D risk and glycemic control (HbA1c and fasting plasma glucose).

Population: 22 studies in Europe, 8 studies in North America, 2 studies in Australia/New Zealand and 1 study in Asia. In the RCTs, only persons with T2D were included.

Key results:

 T2D risk:

  • Highest vs. lowest category of olive oil use reduced the risk of T2D by 16% (RR = 0.84; 95% CI: -0.77 to -0.92; P<0.01) (5 studies).
  • The dose-response meta-analysis found that each 10-gram increase in olive oil reduced the risk of T2D by 9% (RR = 0.91; 95% CI: -0.87 to -0.95; P<0.01) (4 studies).
    • There was evidence of a non-linear relationship (P<0.01), where the risk of T2D decreased by 13% with increasing intake of olive oil up to ~15-20 grams/day, and no additional benefit above this.

2. Glycemic control

a. HbA1c

  • Olive oil resulted in a reduction in HbA1c vs. the control group (Mean difference = -0.27%; 95% CI -0.03 to -0.17; P<0.01) (22 studies)
    • Subgroup analysis by the type of control showed a significant effect vs. a low-fat diet only (Mean difference = -0.35%; 95% CI -0.48 to -0.23; P<0.01) (8 studies)
    • There was no significant difference vs. fish oil (13 studies) or vs. PUFA-rich diets (2 studies).
  • The reduction of HbA1C by 0.35% vs. a low-fat diet is clinically significant: a decrease in HbA1c of just 0.1% has previously been estimated to reduce cardiovascular disease by about 7% in persons with T2D.

 b. Fasting plasma glucose

  • Olive oil resulted in a reduction in fasting plasma glucose vs. control group (Mean difference = -0.44mmol/L; 95% CI -0.66 to -0.22; P<0.01) (25 studies)
    • Subgroup analysis by the type of control found a reduction in fasting plasma glucose vs. fish oil (Mean difference = -0.29mmol/L; 95% CI -0.54 to -0.04; P=0.02) (14 studies) and vs. PUFA-rich oils (Mean difference = -0.85mmol/L; 95% CI -1.35 to -0.35; P<0.01) (4 studies)
    • There was no-significant difference vs. low-fat diets (8 studies).

Quality assessment: The quality of the meta-analysis was rated using the Nutri-Grade system and was low for T2D risk and moderate for glycemic control (HbA1c and fasting glucose).


  • There was considerable heterogeneity in the trial design, with the number of participants ranging from n=6 to n=215, and the trial length ranging from 2 weeks to 4.1 years.
  • The type of olive oil was not always specified.
  • There were only a small number of cohort studies available to determine the association with T2D risk (4 studies).

The bottom line: Olive oil reduced T2D risk and improved measures of glycemic control in people with T2D, at clinically significant levels. Since improvements in these outcomes have previously been described in individuals adhering to a Mediterranean diet, olive oil may be an important mediating factor in this dietary pattern.

Other reviews:

Dinu, M., et al., Mediterranean diet and multiple health outcomes: an umbrella review of meta-analyses of observational studies and randomised trials. Eur J Clin Nutr, 2018. 72(1): p. 30-43.

Dow, C., et al., Diet and risk of diabetic retinopathy: a systematic review. Eur J Epidemiol, 2018. 33(2): p. 141-156.

Jannasch, F., J. Kroger, and M.B. Schulze, Dietary Patterns and Type 2 Diabetes: A Systematic Literature Review and Meta-Analysis of Prospective Studies. J Nutr, 2017. 147(6): p. 1174-1182.

Emadian, A., et al., The effect of macronutrients on glycaemic control: a systematic review of dietary randomised controlled trials in overweight and obese adults with type 2 diabetes in which there was no difference in weight loss between treatment groups. Br J Nutr, 2015. 114(10): p. 1656-66.

Esposito, K., et al., A journey into a Mediterranean diet and type 2 diabetes: a systematic review with meta-analyses. BMJ Open, 2015. 5(8): p. e008222.

Huo, R., et al., Effects of Mediterranean-style diet on glycemic control, weight loss and cardiovascular risk factors among type 2 diabetes individuals: a meta-analysis. Eur J Clin Nutr, 2015. 69(11): p. 1200-8.

Sayon-Orea, C., S. Carlos, and M.A. Martinez-Gonzalez, Does cooking with vegetable oils increase the risk of chronic diseases?: a systematic review. Br J Nutr, 2015. 113 Suppl 2: p. S36-48.

Schwingshackl, L., et al., Adherence to a Mediterranean diet and risk of diabetes: a systematic review and meta-analysis. Public Health Nutr, 2015. 18(7): p. 1292-9.

Sleiman, D., M.R. Al-Badri, and S.T. Azar, Effect of mediterranean diet in diabetes control and cardiovascular risk modification: a systematic review. Front Public Health, 2015. 3: p. 69.

Carter, P., et al., A Mediterranean diet improves HbA1c but not fasting blood glucose compared to alternative dietary strategies: a network meta-analysis. J Hum Nutr Diet, 2014. 27(3): p. 280-97.

Koloverou, E., et al., The effect of Mediterranean diet on the development of type 2 diabetes mellitus: a meta-analysis of 10 prospective studies and 136,846 participants. Metabolism, 2014. 63(7): p. 903-11.

Ajala, O., P. English, and J. Pinkney, Systematic review and meta-analysis of different dietary approaches to the management of type 2 diabetes. Am J Clin Nutr, 2013. 97(3): p. 505-16.

Maghsoudi, Z. and L. Azadbakht, How dietary patterns could have a role in prevention, progression, or management of diabetes mellitus? Review on the current evidence. J Res Med Sci, 2012. 17(7): p. 694-709.

Esposito, K., et al., Prevention and control of type 2 diabetes by Mediterranean diet: a systematic review. Diabetes Res Clin Pract, 2010. 89(2): p. 97-102.

Walker, K.Z., et al., Diet and exercise in the prevention of diabetes. J Hum Nutr Diet, 2010. 23(4): p. 344-52.

Kastorini, C.M. and D.B. Panagiotakos, Dietary patterns and prevention of type 2 diabetes: from research to clinical practice; a systematic review. Curr Diabetes Rev, 2009. 5(4): p. 221-7.