Effects of High-Monounsaturated-Fatty-Acid (MUFA) Diet and Melatonin Supplementation on Lipid Metabolism in Female Rats

Background: This study investigated the effects of melatonin (50 mg/kg/day) and a high-monounsaturated-fatty-acid (MUFA) diet (60% MUFAs; PUFA/SFA ratio = 5), alone or combined, on lipid metabolism-related gene expression in female Sprague–Dawley rats. Methods: Thirty-two eight-week-old rats were randomized into four groups: control (C), melatonin (M), experimental oil diet (E), and combined melatonin plus experimental oil diet (ME). Treatments lasted eight weeks, with melatonin given orally at 17:00–19:00 daily. Results: In the ME group, hepatic expression of peroxisome proliferator-activated receptor α (PPARα) and acyl-CoA oxidase (ACO) mRNA was markedly upregulated, while the E group showed significantly higher expression of hormone-sensitive lipase (HSL) in white adipose tissue. Additionally, ME rats exhibited increased fibronectin type III domain-containing protein 5 (FNDC5) mRNA with reduced peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α), but serum irisin remained unchanged. Conclusions: Melatonin and a high-MUFA diet induced modest and non-uniform changes in lipid metabolism-related gene expression. However, these transcriptional responses were not accompanied by measurable physiological or metabolic outcomes, indicating that the observed effects represent early molecular adaptations rather than functional metabolic modulation.