Multivariate generalized estimation equation analysis demonstrated an inverse association between interaction and perceived stress ( logB = -0.01 ; 95 % confidence interval : -0.02 to -0.01 ; P < 0.00 ) and body mass index ( logB = -0.02 ; 95 % confidence interval : -0.03 to -0.002 ; P = 0.02 ) at group level .