Analyses were carried out in order to examine whether dieting intensity makes an independent contribution to predicting disordered and healthy eating when the effects of dietary restraint are accounted for. These associations were examined using logistic regression. Separate regressions were run for each o f four dependent variables: anorexic symptoms, bulimic binge eating, healthy dietary habits and dietary fat consumption. For each analysis the covariates o f BMI and pubertal development and the four level restraint variable was included at step one, and the four level dieting intensity variable at step 2. Results o f these analyses can be seen in table 5.16 and 5.17.
There was a significant effect of restraint on anorexic symptoms (Wald's ^
[3]=189.8 p<0.001) at step 1. The association remained highly significant when dieting intensity was added into the equation (Wald's [3]=124.1 p<0.001), but dieting intensity also made an independent contribution (Wald's [3]=38.1 p<0.001). In the case of bulimic binge eating once again a significant effect of restraint was found at step 1 (Wald's [3]=36.1 p<0.001). A significant though reduced effect of restraint was found at step 2 when dieting intensity was entered into the model (Wald's y^ [3]=16.0 p<0.01), but a small independent contribution was also made by dieting intensity (Wald's y^ [3]=8.3 p<0.05).
Chapter 5. Cross-sectional analyses: dieting and its associations
Table 5.16 Logistic regressions: Restraint and dieting intensity as predictors of disordered eating
Wald %2 Sig Anorexic symptoms
Step 1 Restraint 189.75 .00'(
Step 2 Restraint 124.12 .OO'l
Dieting intensity 38.09 .001 Bulimic binge eating
Step 1 Restraint 36.12 .001
Step 2 Restraint 16.02 .001
Dieting intensity 8.28 .041 Other variables in the equation : BMI and pubertal development
Table 5.17 Logistic regression: Restraint and dieting intensity as predictors of healthy eating.
Wald %2 Sig Healthy eating habits
Stepl Restraint 81.25 .00» Step2 Restraint 50.69 .001 Dieting intensity 10.11 .018 Dietary fat Step 1 Restraint 20.66 .001 Step2 Restraint 7.91 .048 Dieting intensity 5.30 .151
Healthy eating habits were significantly predicted by restraint at step 1 (Wald's %2 [3]=81.2 p<0.001).When dieting intensity was added into the model the effects o f restraint on healthy eating remained significant (Wald's %2 [3]=50.7 p<0.01), whilst dieting intensity also made a small contribution (Wald's %2 [3]=10.1 p<0.05). Restraint significantly predicted dietary fat at step 1 (Wald's %2 [3]=20.7 p<0.001). At step 2 the effect o f restraint was attenuated, but remained marginally significant (Wald's %2 [3]=7.9 p<0.05), whilst there was no significant additional effect o f dieting intensity (Wald's %2 [3]=5.3 ns).
Chapter 5. Cross-sectional analyses: dieting and its associations
5.6 Discussion
Around two thirds of dieters reported having tried to control their weight at some point, which confirms in this sample the much quoted observation that concern about body weight is so common as to be ‘normative’ in young women (Rosen et al 1985). Most of those who have controlled their weight claim to have been on at least one diet in the past year, but reported duration and frequency of dieting episodes demonstrates that the category ‘past year dieter’ covers a large range of different levels of involvement in dieting behaviour ranging from brief, occasional dieting episodes to constant dieting. As adolescents increase the level of their involvement in dieting they appear to be both increasing the frequency and the duration of their dieting, as the two measures are significantly correlated. A greater amount of time spent involved in dieting behaviours may have implications both for a greater commitment to healthy eating and activity behaviours and also for an increased risk of developing disordered eating. Analyses presented in this chapter addressed social, psychological and behavioural factors associated with the time spent involved in dieting.
Family affluence was not found to be associated with dieting intensity in this sample. Whilst dieting has consistently been found to be higher in women of high socio-economic status than in low status women (Wardle and Griffith 2000); these differences have been less reliable in children and adolescents (Dombusch et al 1984; Davies and Fumham 1986). Although there were no significant group differences, the lowest level of affluence was found in the non dieters, and it may be that the association between dieting and affluence is in the process of emerging in this sample. Contrary to expectations no age differences were found between the dieting intensity groups. The lack o f age effects might be due to the small age range of the participants involved, since the participants were all taken from two consecutive school year groups. On the other hand it may indicate that dieting practices are already well established and fairly stable by this age. Further elucidation o f this proposition will subsequently be examined using the longitudinal data.
Chapter 5. Cross-sectional analyses: dieting and its associations
As expected both BMI and puberty were linked to dieting. Moderate and frequent dieters were the most pubertally advanced, whilst non dieters were the least advanced. These findings match those from other studies which have found that puberty is linked to weight concerns and weight loss attempts (Duncan 1985, Wichstrom 1995). Unsurprisingly, body fatness was strongly associated with intensity of dieting with the frequent and moderate dieters having a higher BMI than the occasional dieters, who in turn had a higher BMI than the non dieters.
Body dissatisfaction showed particularly strong positive associations with dieting intensity. Body dissatisfaction intensified alongside dieting, and every group differed significantly from every other group. The results also confirmed the higher levels of psychological distress found in dieters which has been reported in other studies (Rosen et al 1987, Neumark-Sztainer et al 2000). These effects were independent of the higher BMI and greater pubertal development of the more intense dieters. Depression and low self esteem rose sharply with dieting intensity, and elevations were found in even the occasional dieters. The association between dieting intensity and stress was less strong, and significant differences emerged only between the non dieters and the frequent dieters.
Some attitudinal characteristics also distinguished between non-dieters and dieters o f varying intensity. Differences in weight attitudes may be important in explaining individual differences in dieting behaviour once body weight has been taken into account. In this sample, when BMI and puberty were controlled for, both awareness and internalisation o f sociocultural attitudes towards body fat were positively associated with dieting intensity. For both subscales mean scores rose between each intensity group (although in the case o f awareness there was no significant difference between moderate and frequent dieters). Ideal body shape also varied by dieting intensity once the effects of BMI and puberty were controlled for such that dieting groups favoured a thinner ideal than the non-dieters. A thin ideal was not significantly associated with the intensity of
Chapter 5. Cross-sectional analyses: dieting and its associations
dieting within the dieting groups, as there were no differences between the occasional, moderate and frequent dieters. Comparison o f the unadjusted mean ideal scores with the estimated marginal means (adjusted for BMI and puberty) shows that the addition of the covariates amplifies the inter group differences. BMI has been found to be inversely associated with ideal body weight in adult women (Wardle and Johnson in press), indicating that the higher mean BMI of dieters may be suppressing the association between dieting and lower weight ideals.
The fat and thinness threshold were introduced in this chapter as a measure of the acceptability of fatter and thinner body shapes. Dieters were found not to differ from non dieters in terms o f their tolerance for a thin body as there were no between group differences in thinness threshold. Frequent and moderate dieters did, however, have less tolerance for fat than non dieters, selecting a thinner fat threshold. It appears likely that differences in the acceptability of body shapes between dieters and non dieters are more characterised by differing attitudes towards fat than differing attitudes towards thinness. The lack of differences in thinness threshold between the dieting groups seems challenge the view that dieters are motivated by a desire for a very thin body size, since dieters and non-dieters had very similar levels of tolerance for extreme thinness.
Dieting intensity was significantly associated with several healthy eating behaviours: low levels of dietary fat, high involvement in healthy eating practices and eating the recommended five portions of fruit and vegetables per day. In each case the frequent dieters had the highest odds o f falling into the healthy behaviour group whilst the non dieters had the lowest odds. There were no differences between dieting intensity groups for high dietary fibre consumption, but differences were found for physical activity. Occasional dieters had the lowest odds o f a high level of activity whilst frequent dieters had the highest odds. These findings support those reported elsewhere that dieting confers some benefits in terms o f leading to a healthier diet (Story et al 1998, Neumark-Sztainer et al 2000). It should be noted, however, that findings
Chapter 5. Cross-sectional analyses: dieting and its associations
presented here are based purely on self reported eating and aetivity patterns, and may be subject to social desirability bias amongst the dieters. Nonetheless the questions on diet were placed at the start of the questionnaire before there had been any mention of dieting, body weight concerns or weight control in order to minimise biased responding.
As expected dieting was strongly associated with both anorexic and bulimic symptoms. This corresponds to finding of other research which has identified dieting as a major risk factor for eating disorders (Hsu 1996, Patton et al 1990). Anorexic symptoms were particularly strongly associated with dieting such that the frequent dieters had twenty five times higher odds o f falling into the high symptoms group than the non dieters. Dieting was also identified with emotional eating which is a measure of eating disinhibition in response to negative emotions. Disinhibition has been both theoretically and empirically linked with dietary restriction (Herman and Mack 1975, Ruderman 1985) and frequent dieters had more than double the odds of falling into the high emotional eating category compared with non dieters.
The analyses presented here show that self-reported, self-defined dieting intensity seems to show a dose-response relationship with psychological distress and behavioural factors linked to health, and that these effects are independent of physical and developmental differences between dieting groups. The analyses which compared dieting intensity with restraint indicate that a measure o f time spent involved in self-reported dieting, (dieting intensity rating), can predict eating disorder symptoms and some aspects of healthy eating independently of the effects of restraint. This provides further evidence o f the validity and value of this purely quantitative measure of dieting behaviour. Dieting intensity greatly increases the risk of eating abnormalities, but the effects of dieting may not be all negative, as it also leads to reported improvements in dietary quality and healthy eating habits.
Chapter 5. Cross-sectional analyses: dieting and its associations
The findings from this chapter are not new, but they confirm the validity of many of these well-established connections within the present sample. To expand on these findings the following chapter will examine the extent to which the effects of dieting are mediated by body dissatisfaction. Body dissatisfaction is closely linked to dieting and may be an underlying source o f some of the associations described here.