Association between salivary flow rate and depressive symptoms with adjustment for genetic and family environmental factors in Japanese twin study

Objectives The association between salivary flow rate (SFR) and depressive symptoms have been inconclusive. The present study aimed to investigate the association between SFR and depressive symptoms with and without adjustment for genetic and family environmental factors. Materials and methods We conducted a cross-sectional study using twins and measured SFR and depressive symptoms as the outcome and explanatory variables, respectively. We also performed three-step regression analyses by first analysing the association between SFR and depressive symptoms without adjustment for genetic and family environmental factors (individual-level analyses). We then performed between–within analyses using monozygotic (MZ) and same-sex dizygotic (DZ) twin pairs, and finally using only MZ twin pairs. These between–within analyses estimated the coefficients adjusted for genetic and family environmental factors. Furthermore, differences in the associations between individual-level and between–within suggest confounding by genetic and family environmental factors. Results We conducted 448 twins aged ≥20 years. In individual-level analyses in males and between–within analyses using MZ and same-sex DZ male twin pairs, SFR associated with depressive symptoms. In between–within analyses using only MZ male twin pairs, SFR did not associate with depressive symptoms. In females, SFR did not associate with depressive symptoms in both individual-level and between–within analyses. Conclusions The present study revealed that the association between SFR and depressive symptoms was affected by common genetic factors in males. Clinical relevance Understanding this association between SFR and depressive symptoms with adjustment for genetic and family environmental factors could lead to an important consideration for the prevention and treatment of hyposalivation.