BDNF and Onset: As review by Xu and Xie, genetic mutations in human BDNF and its receptor TrkB result in morbid early-onset obesity (Xu and Xie 2016); furthermore, genome wide associated studies (GWAS) have identified single nucleotide polymorphisms (SNPs) in or near BDNF to be associated with increased body mass index (BMI) (Xu and Xie 2016).