# Nonlinear Association Between BMI and Depression Among Nonalcoholic Fatty Liver Disease: NHANES (2017–2018)

## TL;DR

This study finds a nonlinear link between BMI and depression in people with nonalcoholic fatty liver disease, with higher BMI increasing depression risk up to a certain point.

## Contribution

The study identifies a specific BMI threshold (29.8 kg/m²) where the relationship with depression changes among NAFLD patients.

## Key findings

- BMI was positively associated with depression below 29.8 kg/m².
- Above 29.8 kg/m², BMI no longer significantly predicted depression.
- The nonlinear relationship was confirmed using advanced statistical models.

## Abstract

Background: Depression is one of the most common diseases in the world. Earlier research on the link between body mass index (BMI) and depression has been contentious. This study seeks to investigate the connection between BMI and depression among individuals with nonalcoholic fatty liver disease (NAFLD).

Methods: All data were extracted from the National Health and Nutrition Examination Survey (NHANES) database 2017–2018. The Cox regression technique was employed to analyze the link between BMI and depression. To analyze the potential nonlinear connection between BMI and depression, Cox proportional hazards regression incorporating cubic spline functions and smooth curve fitting was utilized. In addition, a two-segment Cox proportional hazards regression model was used to pinpoint the inflection point at which BMI impacts the likelihood of depression. The Patient Health Questionnaire (PHQ-9) was the primary measure of depressive symptoms.

Results: The mean age of the 1426 participants was 56.05 years with a standard deviation of 15.63 years, and approximately 49.30% of the sample were male. After controlling for confounding variables, BMI demonstrated a positive association with depression (OR:1.05, 95% CI:1.02–1.09, p < 0.004). The two-piecewise Cox proportional hazards regression model identified an inflection point for BMI at 29.8 kg/m2. Below this inflection point (BMI ≤ 29.8 kg/m2), BMI was positively correlated with an increased risk of depression (OR:1.23, 95% CI:1.04–1.45, p < 0.014). Conversely, when BMI exceeded 29.8 kg/m2, the association was not statistically significant (OR: 1.02, 95% CI: 0.98–1.07, p = 0.305).

Conclusion: There is a nonlinear relationship between BMI and depression among patients with NAFLD. BMI was positively related to depression when BMI is less than 29.8 kg/m2.