A retrospective, cross-sectional cohort study using the publicly available National Health and Nutrition Examination Survey (NHANES) was conducted in patients who completed the PHQ-9 and the physical activity questionnaire15. The acquisition and analysis of the data within this study was approved by the National Center for Health Statistics Ethics Review Board.
Dataset And Cohort Selection:
The National Health and Nutrition Examination Survey (NHANES) is a program designed by the National Center for Health Statistics (NCHS), which has been leveraged to assess the health and nutritional status of the United States population. The NHANES dataset is a series of cross-sectional, complex, multi-stage surveys conducted by the Centers for Disease Control and Prevention (CDC) on a nationally representative cohort of the United States population to provide health, nutritional, and physical activity data. In the present study, we analyzed adult (≥ 18 years old) patients in the NHANES dataset if they completed both the PHQ9 and daily nutritional values questionnaire, leading to the inclusion of 8,091 total patients.
Assessment Of Clinical Depression:
Depressive symptoms were assessed using the 9-item Patient Health Questionnaire (PHQ-9) depression scale, which consists of nine questions based on DSM-IV symptoms of depression15.
These included “Over the last 2 weeks, how often have you been bothered by any of the following problems: 1) Little or no pleasure in doing things? 2) Felling down, depressed, or hopeless? 3) Trouble falling or staying asleep, or sleeping too much? 4) Feeling tired or having little energy? 5) Poor appetite or overeating? 6) Feeling bad about yourself — or that you are a failure or have let yourself or your family down? 7) Trouble concentrating on things, such as reading the newspaper or watching television? 8) Moving or speaking so slowly that other people could have noticed? Or so fidgety or restless that you have been moving a lot more than usual? 9) Thoughts that you would be better off dead, or thoughts of hurting yourself in some way?” and were assessed on a scale from 0 to 3: 0 = Not at all, 1 = Several Days, 2 = more than half the days, and 3 = nearly every day. Per literature, patients with PHQ-9 total scores ≥ 10 were considered as having clinical depression, with a sensitivity of 89% and a specificity of 89%16.
Dependent Variable:
Nutritional values were extracted from the Individual Foods files in the NHANES datasets. These files included a record for each food and beverage consumed by a participant. These foods were then converted to daily nutrient intake totals for each individual patient and the total daily intake of potassium was extracted.
Model Construction And Statistical Analysis:
Descriptive statistics for all patients, patients with depression, and patients without depression were computed for all demographic covariates and daily potassium intake and compared using chi-squared tests for categorical variables and t-tests for continuous variables. Univariable models were used to assess the effect of exercise and sedentary activity on clinical depression risk. Multivariable models, controlling for age, race, education, sex, income, and work schedule were used to assess the effect of potassium on clinical depression risk independent of demographic and socioeconomic covariates.
Choosing How To Model Potassium:
To test whether potassium had a non-linear relationship with the odds of depression, an univariable model was fit for potassium as a linear covariate and potassium as a restricted cubic spline. An Analysis of Variance test (ANOVA) was then used to observe if the cubic spline had a better fit than the linear logistic model or if it was just overfitting. The better model would then be used in continued analysis.
If the univariable restricted cubic spline found to show that there was a nonlinear relationship between potassium and the odds for clinical depression, this relationship was then simplified into a piecewise function, with a cut-point set based on visualization of the cubic spline and an ANOVA was then used to compare whether the two models were significantly different. If the piecewise function and the restricted cubic spline were not significantly different, the piecewise function would be used in univariable and multivariable models for nonlinear logistic regression for the relationship between potassium and the odds for clinical depression. P-value < 0.05 was considered statistically significant.