**3. Ghrelin**

#### **3.1. Generalities**

Ghrelin is a peptide hormone produced in the gastrointestinal tract, and it has an important role in regulating the use of energy in human organism. Ghrelin undergoes posttranslational changes resulting in two circulating forms: unacylated ghrelin (UAG) and acylated ghrelin (AG) [51]. This hormone acts directly on hypothalamus and indirectly by increasing the expression of orexigenic peptides such as neuropeptide Y, Agouti-related protein, proopiomelanocortin, and corticotropin-releasing hormone [52].

In addition to its effect on hunger, ghrelin has important effects on glucose homeostasis, energy homeostasis, heart, muscular atrophy, bone metabolism, and tumors [53]. Recent studies emphasize that AG excess is correlated with insulin resistance and metabolic alterations; thereby, the AG/UAG ratio could play a role in the development of MetS [54].

#### **3.2. Ghrelin and metabolic syndrome**

Ghrelin is inversely associated with MetS components, and progressively lower ghrelin levels are being correlated with its severity. Ukkola O et al. emphasized the correlation of low ghrelin levels in obese patients with metabolic syndrome [55]. Also, the positive correlation of ghrelin levels with hypertension, insulin resistance, and obesity has been confirmed by numerous studies. McLaughlin et al. have concluded that ghrelin correlates with MetS mainly based on obesity as well as they identified lower ghrelin levels in patients with MetS and obesity than in non-obese MetS patients [10]. Likewise, many studies confirm the relation between MetS and ghrelin [11–16].
