Mediterranean Diet Adherence and One-Year Metabolic Changes in Patients with Papillary Thyroid Cancer: An Observational Study

Background/Objectives: The Mediterranean diet (MD) has been associated with favorable metabolic outcomes in the general population. However, evidence of these associations among thyroid cancer survivors remains limited. This study examined whether higher MD adherence at diagnosis is associated with longitudinal changes in insulin resistance and lipid profiles in patients with papillary thyroid cancer (PTC). Methods: We analyzed 345 Korean patients aged ≥20 years with histologically confirmed PTC at a tertiary hospital between April 2023 and March 2024. MD adherence at baseline (diagnosis) was assessed using the Korean Mediterranean Diet Adherence Screener and categorized into tertiles. Changes in body mass index (BMI), hemoglobin A1c (HbA1c), homeostasis model assessment of insulin resistance (HOMA-IR), metabolic score for insulin resistance (METS-IR), homeostasis model assessment of β-cell function (HOMA-β), and triglyceride-glucose (TyG) index were evaluated between baseline and one year post-diagnosis. A stratified analysis was conducted according to BMI (<25 vs. ≥25 kg/m²). Results: During the one-year follow-up, patients with PTC experienced significant reductions in BMI, HbA1c, METS-IR, HOMA-β, triglycerides, LDL-cholesterol, and the TyG index, whereas HDL-cholesterol levels increased. Patients in the high MD adherence group showed decreased HOMA-IR and increased HDL-cholesterol levels compared to those in the low adherence groups. In BMI-stratified analyses, reductions in insulin and HOMA-IR were observed only among patients with obesity in the high MD adherence group. Conclusions: Higher adherence to the MD at diagnosis was associated with decreases in insulin resistance markers and an increase in HDL-cholesterol levels among patients with PTC during the first year after diagnosis.