Olive oil in the prevention and management of type 2 diabetes mellitus: a systematic review and meta-analysis of cohort studies and intervention trials

Two analyses were performed: (1) risk of type 2 diabetes (olive oil intake) using prospective cohort studies and randomized trials in adults ≥18 years; baseline diabetes-free for incidence analysis; 4 cohorts totaling about 183,370 participants with 15,784 T2D cases; follow-up 5.7–22 years; (2) effects of olive oil on glycemic control in adults with established T2D; randomized controlled trials (parallel or crossover), 29 trials, 3,698 participants; interventions delivered as olive oil (main added fat) or capsules; durations 2 weeks–4.1 years.

Olive oil (either as main added fat in the diet or as olive oil capsules); dosage and duration varied by study; taken orally.

Prevention: highest olive oil intake reduced T2D risk by 16% (RR 0.84; 95% CI 0.77-0.92); each 10 g/day increase associated with ~9% lower risk (RR 0.91; 95% CI 0.87-0.95); nonlinear relation with benefit up to ~15-20 g/day, plateau thereafter. Management: olive oil interventions lowered HbA1c by 0.27 percentage points (95% CI -0.37 to -0.17) and fasting plasma glucose by 0.44 mmol/L (95% CI -0.66 to -0.22) versus controls; stronger HbA1c reductions observed with olive oil supplied in pure form vs capsules (not consistently significant) and when compared with low-fat diets. Overall, olive oil could be beneficial for T2D prevention and management, with effects influenced by oil type and comparator; evidence quality varies.

Heterogeneity in trial designs and comparators; variations in olive oil type (EVOO vs refined) and dose; many studies did not specify oil type; limited high-quality, long-term data; dietary assessment error and potential publication bias; meta-evidence not consistently high.