How does the Mediterranean diet impact insulin resistance and sensitivity in obese individuals?

In a recent study published in Nutrients, researchers evaluate the associations between adherence to the Mediterranean diet (MD), insulin secretion (IS), resistance (IR), and sensitivity in obese or overweight individuals.

Study: The Mediterranean Diet: Effects on Insulin Resistance and Secretion in Individuals with Overweight or Obesity. Image Credit: monticello / Shutterstock.com

MD and IR

Obesity and being overweight are chronic conditions associated with various comorbidities, such as cardiovascular disease, type 2 diabetes (T2D), cancers, dyslipidemia, and hypertension. IR is one of the major drivers of complications related to obesity and is a cardiovascular risk factor among people without T2D.

Reports suggest that over one-third of adults may be prediabetic; however, most people remain unaware of this predisposition to diabetes. Thus, the diagnosis and treatment of IR and prediabetes are critical to mitigating cardiometabolic conditions.

Several factors may trigger IR or prediabetes, such as unhealthy diets, family history of T2D, excess body fat, and low physical activity. Therefore, lifestyle changes are the first-line defense to prevent prediabetes and IR.

MD adherence is associated with lower T2D risk, with one meta-analysis reporting a 14% reduced risk of T2D for each two-point increase in MD adherence score. MD is characterized by the consistent consumption of plant-based foods with extra-virgin olive oil, moderate animal fat and protein intake, and restricted consumption of processed foods and sweets.

Notably, it remains unclear whether the benefits of MD are due to the diet as a whole or specific components of the diet.

About the study

The present study evaluated the associations between MD adherence, IR, IS, and insulin sensitivity indices in obese or overweight individuals. Patients attending an obesity center at a university hospital in Italy were screened for inclusion between January 2022 and January 2023. Eligible participants were adults aged 18-65 with a body mass index (BMI) exceeding 27 kg/m² and stable body weight in the past three months.

Subjects were excluded from the study if they were pregnant/lactating or had T2D, type 1 diabetes, cerebrovascular or cardiovascular diseases, cancers, chronic inflammatory conditions, and hepatic/kidney failure. Individuals on glucose-lowering therapy, antipsychotic drugs, glucocorticoids, glucagon-like peptide 1 (GLP1) receptor agonists, and medications that induce weight gain were also excluded from the study.

Study participants provided information on demographics, lifestyle habits, and medical history. MD adherence was examined using the 14-item PREDIMED questionnaire.

An oral glucose tolerance test was performed on each participant to estimate plasma insulin and glucose levels. Homeostatic model assessment (HOMA) for IR (HOMA-IR) was used to evaluate fasting indices.

HOMA for β-cell function was used to assess IS capacity. Insulin activity after glucose load was determined using the insulin sensitivity index (ISI) and oral glucose insulin sensitivity (OGIS) method. Pearson’s or Spearman rank correlation was performed to explore correlations between study variables.

Study findings

The study recruited 62 participants, with a mean age of 49 years, including 55 females. The average BMI was 35.8 kg/m².

Twelve participants had low MD adherence, 35 had intermediate adherence to MD, and 15 reported high MD adherence. Individuals with high MD adherence were older than those with low adherence.

Lifestyle habits, disease prevalence, BMI, and sex distribution were not different across MD adherence groups. Likewise, glycated hemoglobin (HbA1c) concentrations, as well as the prevalence of impaired fasting glucose and impaired glucose tolerance, were similar across groups.

Fasting plasma glucose levels were not different across groups; however, fasting insulin levels were significantly lower in participants with high MD adherence. HOMA-IR was also lower in individuals with high MD adherence. The high MD adherence group also exhibited significantly elevated insulin sensitivity than other groups.

β-cell function was lower in participants with high MD adherence than those with intermediate adherence; however, there were no differences between the low and high adherence groups. The PREDIMED score correlated with ISI and, inversely, with HOMA-IR. Among MD items, fish intake correlated with ISI and, inversely, with HOMA-IR and β-cell function, which remained significant after adjusting for BMI.

Conclusions

High MD adherence was associated with reduced IR at fasting and post-glucose load in overweight or obese individuals, with this effect primarily associated with fish intake. These findings have clinical implications, as existing guidelines emphasize lifestyle changes as preventive strategies to reduce the risk of prediabetes and IR complications.

One notable limitation of the current study is its cross-sectional design, which does not allow for establishing a cause-effect relationship.