If a regression analysis was to be completed on body mass index (BMI), what could be an independent variable in that analysis? Why? If we could, what other independent variables should be included in the analysis? What statistic(s) would show the value of that regression in understanding BMI?

The article Trajectories of the body mas index among active-duty U.S. soldiers, 2011-2014. In that study, what was the dependent variable and what were the independent variable(s)? Further, how would you use this study to highlight the difference between correlations and causation?