The differences in mean grain intakes from 1986 to 1994 were compared by using Wilcoxon’s signed-rank tests. We determined the age- and energy-adjusted means for lifestyle and dietary characteristics across increasing quintiles of whole-grain intakes. Tests for linear trend were calculated by assigning the median value for each quintile of intake treated as a continuous variable by using linear (for continuous outcome variables) or logistic (for dichotomous outcome variables) regression models.
Using multivariate linear regression, we examined how changes in intakes of whole grain, refined grain, and added bran and germ (1986–1994) were associated with the dependent variable, namely the change in weight (in kg) in the same period. We used the robust variance estimate (19) to obtain valid inferences and avoid the necessary assumptions of normality for linear regression. In all analyses, there was one observation per participant. Whole grains were modeled as continuous variables when tests for nonlinearity using spline regression were not statistically significant; otherwise, the exposures were categorized. We calculated the multivariate adjusted least-squares means for changes in body weight across categories of change in whole grains and fixed the continuous covariates at their respective median values and the categorical values at their modal values.
Multivariable models included age (in 5-y categories), the respective baseline exposure measure (quintiles or continuous), baseline weight (quintiles), and baseline and change in total calories (continuous variables), alcohol (quintiles), and total physical activity (both in quintiles). Change in smoking status between 1986 and 1994 was included as a categorical variable. Men who were nonsmokers at both time periods were classified as “nonsmokers,” and men who were smokers at both periods were categorized as “habitual smokers.” Men who reported smoking in 1986 and nonsmoking in 1994 were classified as “quitters,” and men who reported never smoking in 1986 and smoking in 1994 were classified as “new smokers.”