BDNF and nutritional deficiency disease: Studies had shown that some factors could increase the risk of bad mood in adulthood, such as small natural hippocampus (51), BDNF gene polymorphism (52), abnormal expression of serotonin 2A receptor gene (HRT2A) (53) and tryptophan hydroxylase 2 gene (TPH2) (54), small weight at birth (55), abuse in childhood, loss of parents, malnutrition and discordant family environment (56, 57).