Mediterranean Diet Patterns Are Positively Associated with Maximal Fat Oxidation and VO2max in Young Adults: The Mediating Role of Leptin

<b>Background:</b> Adherence to the Mediterranean diet (MedDiet) offers multiple metabolic benefits. However, its relationship with maximal fat oxidation (MFO) and cardiorespiratory fitness (VO₂max), alongside the potential mediating role of leptin, remains underexplored in young adults. <b>Objective:</b> The objective was to investigate the associations between MedDiet adherence and the body mass index (BMI), MFO, and VO₂max and to evaluate whether leptin mediates these relationships. <b>Methods:</b> Sixty-five young adults (<i>n</i> = 23 women), aged 18-38, were assessed for body composition, MedDiet adherence (14-Item Mediterranean Diet Adherence Screener), MFO, and VO₂max through indirect calorimetry. Plasma leptin concentrations were measured in fasting conditions. Multiple linear regression models were performed, adjusting for sex, age, and both. Mediation analyses were conducted. <b>Results:</b> Higher MedDiet adherence was significantly associated with lower BMI (β = -0.339, <i>p</i> = 0.006) and leptin values (β = 0.284, <i>p</i> = 0.022) and higher absolute MFO (β = 0.338, <i>p</i> = 0.006) and VO₂max values (β = 0.462, <i>p</i> < 0.001). These associations remained significant in all models except BMI and leptin when adjusted for sex and sex and age. Leptin was positively associated with the BMI (β = 0.550, <i>p</i> < 0.001) and inversely associated with absolute MFO (β = -0.650, <i>p</i> < 0.001) in all models. There was a trend in the association between leptin and VO₂max (β = -0.233, <i>p</i> = 0.061) only in the unadjusted model. Mediation analysis revealed that the leptin levels significantly mediated the associations between MedDiet adherence and BMI (β = -0.358, 95% CI [-0.677, -0.077]) and VO₂max (β = 1.043, 95% CI [0.280, 1.833]). <b>Conclusions:</b> MedDiet adherence is associated with a lower BMI and higher MFO and VO₂max in young adults. Our findings further suggest that leptin plays a mediating role in how MedDiet adherence influences the BMI and VO₂max.