Joint and nonlinear associations of serum folate and physical activity with depressive symptoms

To examine the joint association of serum folate and physical activity on depressive symptoms, and to investigate the nonlinear relationship between serum folate and depressive symptoms.

Data were analyzed from 5031 participants aged ≥20 years in the 2015-2018 NHANES, with complete records on depressive symptoms, physical activity, and serum folate levels. Weighted multivariable logistic regression models were employed to assess the associations between these factors. Restricted cubic splines were used to explore the nonlinear relationship between serum folate and depressive symptoms. Additionally, the population attributable fraction (PAF) was calculated to estimate the proportion of depressive symptoms that could be avoided.

After full adjustment, both high serum folate level and physical activity were significantly negatively associated with depressive symptoms. The restricted cubic spline regression revealed an L-shaped relationship between serum folate and depressive symptoms, with the threshold around 35 nmol/L. Notably, the joint association of serum folate and physical activity was significant, especially in individuals aged 60 years or younger, where the synergistic effect confers particularly notable protective benefits, surpassing the sum of their separate effects. PAF analysis indicated that high serum folate level and physical activity reduced depressive symptoms by 7.1 % and 12.8 %, respectively, and their concurrent presence led to a 25.5 % reduction.

This study suggests that serum folate and physical activity have a synergistic effect on depressive symptoms, and serum folate may exhibit a threshold effect. Comprehensive intervention strategies may be of significant importance in preventing depressive symptoms.