**5.2 The relationship between melatonin and lipid profile**

Melatonin positively affects the lipid profile regarding CVD [54]. The intestinal system and liver play a significant active role in the metabolism and production of lipoproteins. Lipids are digested in the intestine system and then transported from the intestine to the liver as chylomicrons. In the liver, it is biochemically transformed, and free fatty acids are converted to triglycerides (TGs) and phospholipids. They are transported into the blood system by lipoproteins. LDL is the form of lipoprotein that carries cholesterol into cells, and it also tends to be oxidized by free oxygen radicals, damaging cells and promoting inflammation [55]. Some studies have focused on the damage caused by highly reactive oxygen species (ROS), which leads to atherosclerotic progression. An experimental study reported that melatonin supplementation with an atherogenic diet increased atherosclerotic lesions in the proximal aorta in hypercholesterolemic mice, in contrast to the majority of other studies, by increasing the susceptibility of atherogenic lipoproteins to γ-radiolysis and Cu2+ oxidation [55]. In another study, the administration of melatonin for 14 months in individuals with fatty liver disease (not alcoholic) showed that LDL and triglyceride levels decreased compared to the health group [56]. Melatonin administration reduced blood pressure as well as LDL-cholesterol levels [54]. Melatonin protects macromolecules from oxidation damage with its direct effect and free radical scavenging effect by stimulating antioxidant enzymes. The positive effects of melatonin on the lipid profile may be related to its anti-inflammatory and antioxidative effects. In addition, melatonin decreases lipid levels by increasing the conversion of endogenous cholesterol to bile acids and suppresses cholesterol synthesis and accumulation.
