activity, calibrated dietary energy intake, BMI category, and incident stone formation, adjusting for nephrolithiasis risk factors. A likeli- hood ratio test was used to evaluate the importance of individual variables in the multivariate models. The potential interactions be- tween calibrated dietary energy intake, weekly physical activity, and BMI were tested by including the product of the variables in a discrete regression model. A correlation matrix evaluated the potential co- linearity of these three variables.
Exploratory analyses evaluated the effect of exercise intensity on incident stones accounting for total physical activity. The proportion of total METs per week spent engaged in mild, moderate, and strenuous exercise was determined. The categorization of a particular activity as mild, moderate, or strenuous was predetermined by the WHI. Each participant was then categorized into a primary exercise type (mild, moderate, or strenuous exerciser) on the basis of the greatest contributor to their total METs per week. The association between the participant’s primary exercise type and incident kidney stones was evaluated in separate multivariate analyses stratified by total METs per week category adjusting for nephrolithiasis risk fac- tors. This directly tested the significance of exercise intensity in women with similar total expended METs per week. Second, the association between total METs per week category and incident kid- ney stones was assessed in separate adjusted multivariate analyses stratified by primary exercise type. This evaluated the independent effect of total METs per week of physical activity among women primarily performing a similar intensity of activity.
HRs, aHRs, and 95% CIs were determined. All P values were two tailed, and statistical significance was set at P,0.05. Analyses were performed using Stata IC, version 10 (Stata Corp., College Station, TX), and SAS software, version 9.1 (SAS Institute, Cary, NC).
This study received institutional review board exemption. For the original WHI Observational Study, the appropriate institutional review board approvals were obtained at all participating institutions and written informed consent was obtained from all participants.