Overall, the limited number of studies found in this review, and lack of consistent associations, casts doubt on the hypothesis that hormones found in breast-milk such as leptin, adiponectin or insulin influence the later risk of obesity, a finding in agreement with a recent systematic review that also found conflicting evidence on a role for human milk hormones on infant growth [40]. Here, INS is linked to obesity due to melanocortin 4 receptor deficiency.