Rethinking Mediterranean Diet Adherence and Body Composition: A 3-Year Prospective Observational Study in Patients with Type 2 Diabetes and Arterial Hypertension

Background/Objectives: Type 2 diabetes mellitus (T2DM) and arterial hypertension (AH) frequently coexist and are major risk factors for cardiovascular disease (CVD) and chronic kidney disease. Although the Mediterranean diet (MeDi) is widely recommended in cardiometabolic management, longitudinal real-world evidence regarding dietary adherence, body composition, and the role of continuous dietitian-led follow-up remains limited. Methods: This three-year prospective observational study included 158 adults with T2DM and AH treated at the Outpatient Clinic for Clinical Nutrition, University Hospital Centre Split, Croatia. Body composition was assessed using multi-frequency bioelectrical impedance analysis, dietary adherence using the Mediterranean Diet Serving Score (MDSS), and renal and metabolic parameters using standard laboratory methods. Participants were stratified according to attendance at scheduled dietitian-led follow-up visits. Results: Significant reductions were observed in body weight, body mass index (BMI), waist circumference, muscle mass, skeletal muscle index, and phase angle (PhA) during follow-up (all p < 0.001). Lipid profile improved, with reductions in total cholesterol and low-density lipoprotein levels and an increase in high-density lipoprotein concentrations. In contrast, renal function declined, as evidenced by increased creatinine and urea levels and decreased estimated glomerular filtration rate (eGFR). CVD incidence increased significantly during follow-up (p < 0.001). Although overall adherence to the MeDi improved, only 17.8% of participants achieved high adherence. Participants attending regular dietitian-led follow-up demonstrated significantly higher MDSS scores, greater adherence to the MeDi, and higher phase angle values compared with participants without regular follow-up. Conclusions: Adults with T2DM and AH demonstrated partial enhancement of MeDi adherence over three years; although the lipid profile also improved, this could not be attributed to diet and may largely reflect pharmacological treatment. These changes were accompanied by progressive muscle mass loss, declining renal function, and increased cardiovascular burden. Continuous dietitian-led follow-up was associated with superior dietary quality and better nutritional status, supporting the importance of individualized, multidisciplinary approaches integrating sustained nutritional counselling, lean mass preservation, and structured physical activity in long-term cardiometabolic care.