Overall, dietary-negative affect subtyping and overvaluation subtyping were each predictors of specific and important, although different, dimensions of BED outcome. Dietary-negative affect subtyping was found to be a predictor of binge frequency such that participants with the dietary-negative affect subtype reported a greater frequency of binge episodes at the end of treatment compared to the pure dietary subtype. Overvaluation subtyping was found to be a significant predictor of eating disorder psychopathology; participants with clinical overvaluation had greater eating pathology at the end of treatment. Collectively, these findings suggest that negative affect has some role in the maintenance of binge eating, whereas overvaluation has a role in the maintenance of eating disorder psychopathology among patients with BED.
To investigate these two predictors, participants were grouped with two sub-categorization methods. In the first method, cluster analysis revealed two groups, that is, a mixed dietary-negative affect subtype (n
= 22; 29.3%) and a pure dietary subtype (n
= 53; 70.7%). Proportions in these groupings were similar to previous studies (Grilo, Masheb, & Berman, 2001
; Grilo et al., 2001c
; Stice & Agras, 1999
; Stice et al., 2001
; Stice & Fairburn, 2003
). Also consistent with previous reports (Grilo et al., 2001c
; Stice et al., 2001
) were findings that the mixed dietary-negative affect subtype was a more pathological variant of BED as evidenced by the higher levels of depressive symptoms and greater psychiatric comorbidity than the pure dietary subtype. In the second sub-categorization method, participants were categorized as experiencing either clinical overvaluation (n
= 38; 50.7%) or subclinical overvaluation (n
= 37; 49.3%) using research conventions of Fairburn and Cooper (1993)
. The overvaluation subtype appeared to be a more pathological variant of BED as evidenced by the higher levels of eating disorder psychopathology as compared to the subclinical overvaluation subtype. Thus, one clinical implication of these sub-categorization methods is that there are identifiable ways to find subtypes of BED patients who are both more pathological and less likely to have positive treatment outcomes.
While both sub-categorization methods predicted specific BED treatment outcomes, neither method moderated the effects of guided self-help CBT and BWL treatments on any of the outcomes tested. This suggests that guided self-help CBT and BWL perform comparably across BED subtypes.
The significant findings for predictors of treatment outcomes in the present study are important given the lack of reliable a priori predictors for BED outcome. Recent advances in identifying predictors of treatment outcome for BN and BED have included investigations of early treatment response as a predictor (Grilo, Masheb, & Wilson, 2006
; Masheb & Grilo, 2007
; Wilson, Fairburn, Agras, Walsh, & Kraemer, 2002
). Rapid response, defined using receiver operating characteristic curves as a 65% or greater reduction in binge eating by the fourth week of treatment, has been shown to predict important treatment outcomes including binge frequency, binge remission, weight loss, and negative affect (Grilo et al., 2006
; Masheb & Grilo, 2007
). In comparison to rapid response, dietary-negative affect subtyping and overvaluation subtyping appear to have limited predictive utility for BED treatment outcomes. One major advantage of the two categorization methods presented in the current study, however, is that they represent patient characteristics known prior to initiating 4 weeks of specialized treatment. Future research should consider alternative categorization methods such as perhaps testing the joint effects of negative affect and overvaluation on the various specific psychological and pharmacological treatments.
Our study findings should be considered within the context of methodological strengths and weaknesses. Strengths include the strong randomized design and assessment methods. There are also several limitations as well as recent findings that restraint scales are not correlated with actual caloric intake (Stice et al., 2004
). Clustering procedures have potential biases. In the present study we initially conducted a two-cluster solution for dietary-negative affect subtyping in an effort to replicate findings from previous studies. To increase confidence in this decision, we re-ran the cluster analysis and found that a two-cluster solution was superior to a three-cluster solution. Another potential bias in the dietary-negative affect subtyping is whether measures of restraint are needed as cluster variables. This is important given that the dietary-negative affect and pure dietary groups did not differ on these measures. Likewise, the use of restraint measures in cluster analytic methods may prove to differ for BN and BED patients as restraint scales have shown not to be correlated with actual intake (Stice et al., 2004
). It may be, for example, that cluster analysis of negative affect and overvaluation yields a better predictor than the cluster analysis of negative affect and restraint.
Similar to cluster analytic strategies, utilizing cut-off scores to dichotomize patient samples also has potential biases. To minimize these biases, we performed analyses for overvaluation subtyping with both cutoff score and cluster analytic methods and found similar results. The prognostic significance of the categorization methods used in the present study may not generalize to other specialist treatments such as individual CBT or group-administered BWL, other forms of psychological or pharmacological intervention, or other treatments delivered by practitioners in diverse community settings. It is also possible that our inability to find moderating effects was partly due to the limited sample size.
In summary, these two sub-categorization methods each appeared to have specific, albeit limited, utility for predicting BED treatment outcomes. Given the importance of each outcome predicted by these two methods (i.e., binge frequency and eating disorder psychopathology) and our limited knowledge of a priori predictors for BED treatment, future research should test the predictive utility of the joint effects of dietary-negative affect and overvaluation.