TNFRSF11B and obesity due to melanocortin 4 receptor deficiency: Most studies recruited apparently healthy obese children [22–24], adolescents [24], or young adults [25–27] without other metabolic comorbidities (i.e., metabolically healthy obesity) to show that circulating OPG was lower in obese compared to lean individuals, although some studies showed comparable levels [28, 29] or even increased serum OPG levels in obese than normal-weighted participants [30].