Can the Mediterranean diet lower my blood sugar?

The Mediterranean diet is an eating model based on the dietary patterns of countries surrounding the Mediterranean Sea.¹ Its key components are its high monounsaturated/saturated fat ratio and high intake of plant-based foods. There are limited studies directly comparing the Mediterranean diet with other diets, but there is some indirect evidence that suggests it may be more effective at improving glycemic control for those with type 2 diabetes mellitus due to its ability to reduce fasting glucose, HbA1c, fasting insulin, and HOMA-IR (Homeostatic Model Assessment for Insulin Resistance) levels.^2-10

A parallel-arm randomized controlled trial  by Doménech et al. studied the one-year effect of the Mediterranean diet supplemented with either extra virgin olive oil (MedDiet + EVOO) or nuts (MedDiet + Nuts).² The authors enrolled 235 subjects with high cardiovascular risk who were randomized from the PREDIMED study to either the MedDiet + EVOO group, MedDiet + Nuts group, or a low-fat control group. Compared to baseline, the mean changes in fasting glucose were −6.13 mg/dL (95% confidence interval [CI] [-11.62­ – -0.64]), −4.61 mg/dL (95% CI [-9.82 – 0.60]), and 3.5 mg/dL (95% CI [-0.51 – 7.54]), respectively. Compared to the control group, fasting glucose was significantly reduced in the MedDiet + EVOO group.

A 2018 umbrella review by Dinu et al. expanded their analysis beyond fasting glucose. They assessed 16 medium-to-high quality meta-analyses of randomized controlled trials (RCTs) that investigated the Mediterranean diet’s effect on multiple health outcomes, including HbA1c, fasting glucose, fasting insulin, and HOMA-IR.³ Studies that compared a Mediterranean diet intervention to a control diet (typically a low-fat diet, low-carb diet, or usual American dietary habits) were included. Nine studies (n=1089) found that those following a Mediterranean diet had a significant reduction in HbA1c of 0.30% (95% CI [0.46 – 0.14], p=0.0003). Twenty-three studies found a significant reduction in fasting glucose of 6.7 mg/dL (95% CI [7.4 – 5.9], p<0.0001). With regards to insulin, five studies (n=557) showed a significant reduction in fasting insulin of 0.55 µU/mL (95% CI [0.81 – 0.29], p=0.0001). Lower fasting insulin may be associated with improved glucose tolerance.¹¹ Lastly, ten studies (n=1742) found a significant reduction in HOMA-IR of 0.45 (95% CI [0.74 – 0.16], p<0.0001), indicating a decrease in insulin resistance.

A more recent network meta-analysis by Pan et al. in 2019 supported these results.⁴ The authors analyzed ten RCTs that involved five dietary patterns: the Mediterranean diet, low carb diet, high carb diet, low fat diet, and regular diet. Directly compared to the low fat diet, the Mediterranean diet led to significant reductions in HbA1c of 0.45% (95% CI [0.55 – 0.34]) and in fasting plasma glucose of 22.3 mg/dL (95% CI [28.3 – 16.4]). The authors ranked the dietary patterns with the P-score, which measures the mean extent of certainty that one treatment is better than its competitors.¹² They found that the Mediterranean diet had the highest P-scores for HbA1c (P-score=0.8174) and fasting plasma glucose (P-score=0.7662). However, because there was a lack of direct comparisons between diets it is not possible to conclude that one is superior over the other.

^References

1. Rees K, Takeda A, Martin N, et al. Mediterranean-style diet for the primary and secondary prevention of cardiovascular disease. Cochrane Database Syst Rev. 2019;3:Cd009825.
2. Domenech M, Roman P, Lapetra J, et al. Mediterranean diet reduces 24-hour ambulatory blood pressure, blood glucose, and lipids: one-year randomized, clinical trial. Hypertension. 2014;64(1):69-76.
3. Dinu M, Pagliai G, Casini A, Sofi F. 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):30-43.
4. Pan B, Wu Y, Yang Q, et al. The impact of major dietary patterns on glycemic control, cardiovascular risk factors, and weight loss in patients with type 2 diabetes: A network meta-analysis. J Evid Based Med. 2019;12(1):29-39.
5. Carter P, Achana F, Troughton J, Gray LJ, Khunti K, Davies MJ. 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):280-297.
6. Nordmann AJ, Suter-Zimmermann K, Bucher HC, et al. Meta-analysis comparing Mediterranean to low-fat diets for modification of cardiovascular risk factors. Am J Med. 2011;124(9):841-851.e842.
7. Schwingshackl L, Chaimani A, Hoffmann G, Schwedhelm C, Boeing H. A network meta-analysis on the comparative efficacy of different dietary approaches on glycaemic control in patients with type 2 diabetes mellitus. Eur J Epidemiol. 2018;33(2):157-170.
8. Esposito K, Maiorino MI, Bellastella G, Chiodini P, Panagiotakos D, Giugliano D. A journey into a Mediterranean diet and type 2 diabetes: a systematic review with meta-analyses. BMJ Open. 2015;5(8):e008222.
9. Ajala O, English P, Pinkney J. Systematic review and meta-analysis of different dietary approaches to the management of type 2 diabetes. Am J Clin Nutr. 2013;97(3):505-516.
10. Viscogliosi G, Cipriani E, Liguori ML, et al. Mediterranean dietary pattern adherence: associations with prediabetes, metabolic syndrome, and related microinflammation. Metab Syndr Relat Disord. 2013;11(3):210-216.
11. Weyer C, Hanson R, Tataranni P, Bogardus C, Pratley R. A high fasting plasma insulin concentration predicts type 2 diabetes independent of insulin resistance: evidence for a pathogenic role of relative hyperinsulinemia. Diabetes. 2000;49(12):2094-2101.
12. Rucker G, Schwarzer G. Ranking treatments in frequentist network meta-analysis works without resampling methods. BMC Med Res Methodol. 2015;15:58.