The Association of Restrained Eating With Weight Change OverTime in a Community-Based Sample of Twins

Ellen A. Schur1, Susan R. Heckbert2, and Jack H. Goldberg2,31Department of Medicine, University of Washington, Seattle, Washington, USA2Department of Epidemiology, University of Washington, Seattle, Washington, USA3Vietnam Era Twin Registry, VA Epidemiologic Research and Information Center, Seattle,Washington, USA

AbstractWe investigated the association of restrained eating with BMI and weight gain while controllingfor the influence of genes and shared environment. Participants were 1,587 twins enrolled in theUniversity of Washington Twin Registry (UWTR). Restrained eating was assessed by the Hermanand Polivy Restraint Scale. Height and weight were self-reported on two occasions. Analyses usedgeneralized estimating equations or multiple linear regression techniques. Restraint Scale scoreswere positively associated with both BMI (adjusted = 0.39 kg/m2; 95% confidence interval (CI)= 0.340.44; P < 0.001) and weight gain (adjusted = 0.33 pounds; 95% CI = 0.170.49; P 100 cigarettes in your lifetime? and encompasses both past and present smokers.Current smoking was assessed with the question, Do you now smoke cigarettes every day,some days, or not at all? Twins were asked, in the past 12 months, how many times theyhad attempted to lose weight, if they were trying to lose weight continuously, and how theyhad tried to lose weight (reduced the number of calories or the amount of food you ate;switched to food with lower calories or less fat, but ate a normal amount; followed a specialdiet such as Atkins or Zone; increased exercise; skipped meals/fasted; took diet pillsprescribed by a doctor; took laxatives or vomited) (25). Finally, twins reported how manytimes during the past 12 months they had lost 5 pounds on purpose (none, 12, 34, 5).

DemographicsSeveral demographic factors were considered potential confoundingvariables. Age and gender were determined from UWTR records. Race was determined byself-report from the initial enrollment surveys (Do you consider yourself to be Hispanic orLatino? and What race do you consider yourself to be?), as was education (What is thehighest level of school you completed?).

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Statistical analysisDescriptive statistics were calculated as means and s.d. for continuous variables andproportions for categorical variables. Response bias to the 2006 Health Survey was assessedby comparing baseline BMI among health survey responders to nonresponders. Cronbachs was calculated for the Restraint Scale score. Total Restraint Scale scores were imputed fortwins missing 1 (n = 51) or 2 (n = 12) responses for the scale by calculating the percentageof total possible points for the nonmissing responses and multiplying this percentage by 32(the total possible points for the complete 10-item scale). We divided the sample into tertilesby Restraint Scale score (high 16, intermediate = 1216, and low 11). Group differencesby gender and by Restraint Scale score category were tested by using generalized estimatingequations regression models with robust s.e. to account for the clustered structure of the twinpairs. Linear, logistic, and multiple logistic regression models were used as indicated toevaluate continuous and categorical variables, respectively. Significance was determined byWald tests.

For initial analyses, all individual twins were included in a linear regression to assess thecross-sectional relationship between the Restraint Scale score and BMI as assessed in the2006 Health Survey. Again, generalized estimating equations with robust s.e. were used toaccount for the clustered structure of the twin pairs. The covariates of age, gender,education, race, smoking, and physical activity were chosen a priori and included inadjusted regression models. We present predicted adjusted least squared mean BMIs fromregression results.

The initial cross-sectional analysis of individual twins, described above, does not directlycompare BMI within twin pairs. For twin pairs in which both members completed the 2006Health Survey, we calculated the within- pair differences in Restraint Scale, BMI, andcovariates by randomly identifying one index twin in the pair and comparing the co-twin tothe index twin across all measures. Thus, within-pair differences have both positive andnegative values. These within-pair difference measures were used in a multiple linearregression model. Within-pair analyses account for potential genetic and familyenvironmental confounding factors, as well as age and gender. Further analysis stratified bytwin zygosity examined these within-pair effects in separate regression models for MZ andDZ pairs. Because MZ pairs share 100% of their DNA, these within-pair comparisons arefully controlled for genetic factors.

We continued our analysis by examining the relationship of the Restraint Scale score torecent weight change. In this analysis, we used the difference in weight between individualtwins initial enrollment survey and the 2006 Health Survey as our primary outcome. Aswith the cross-sectional analysis of BMI, we performed adjusted analyses of the relationshipbetween the Restraint Scale score obtained on the 2006 Health Survey and weight changeover time. A priori covariates included baseline age, smoking status, and education. We alsoadjusted for BMI at baseline and included a term for the length of time between the baselineweight and the 2006 assessment. Physical activity data were not available at baseline andwere not included in the model. As described above for the BMI outcome, we again used awithin-pair analysis of the Restraint Scale score and weight change to account for geneticand familial factors. Finally, because natural growth might have occurred among theyounger twins in the sample, we also conducted a sensitivity analysis restricted to twins age25 and older.

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ResultsTwin participants

Sixty-eight percent of the participants were women. Women had higher average (s.d.)Restraint Scale scores (14.4 5.4 vs. 11.0 5.7; P < 0.001), but men and women weresimilar on other characteristics (data not shown). Among all twins, the mean BMI was 25.3 5.3 kg/m2 with a range of 15.355.4 kg/m2. Fifty-three percent of the sample were normalweight, 24% overweight, and 16% obese. Twins gained a mean of 4.7 14.0 pounds overthe average of 3.1 1.0 years between their initial survey and the 2006 Health Survey.There was no indication of response bias by BMI among twins responding to the 2006Health Survey; responders mean BMI at the time of their initial enrollment survey was 24.7kg/m2 (95% confidence interval (CI) 24.524.9) and nonresponders mean BMI was 24.6kg/m2 (95% CI 24.424.8). Seventy-three percent of twin pairs were MZ and 27% were DZ.The majority of twins were white (89%), and 45% had completed an undergraduate degreeor higher. Obesity rates were consistent with available data on white, well-educated personsin Washington State and King County (26,27). Sixty-eight percent of twins had neversmoked cigarettes. The Restraint Scale was internally reliable, with a Cronbachs of 0.76in our twin sample.

Characteristics of the twin participants categorized by their Restraint Scale scores arepresented in Table 1. The mean (s.d.) Restraint Scale scores for the low, intermediate, andhigh tertiles were 7.6 2.8, 14.0 1.4, and 20.2 3.1, respectively. Physical activity, race,and zygosity did not differ across Restraint Scale categories, but gender, age, education,lifetime history of smoking, time between surveys, BMI category, lifetime maximumweight, and number of weight-loss attempts in the past year did. The high Restraint Scalescore group (n = 459) tended to be female, be older, have smoked at some time in their lives,have fewer persons with a high-school education or less, have a higher proportion of obeseindividuals, and report both higher lifetime maximum weights and a greater number ofrecent weight-loss attempts. In this group, 81% reported reducing calories or amount of foodeaten in order to lose weight in the past year, 56% changed the type of food eaten, 30% useda special diet (i.e., Atkins, Zone), 73% increased exercise, 35% skipped meals or fasted, 4%took diet pills prescribed by a doctor, and 7% took laxatives or vomited. In addition, 47%reported losing 5 pounds on purpose one to two times in the past year, whereas 19%reported 34 weight-loss episodes and 11% reported losing 5 pounds more than five timesin the past year.

The association of restrained eating with BMI and weight change over timeWe found a significant overall association between Restraint Scale score and BMI in ourinitial cross-sectional analysis of individual twins (Table 2), controlling for the effects ofage, gender, physical activity, education, race, and smoking history (adjusted = 0.39 kg/m2; 95% CI = 0.340.44; P < 0.001). The value gives the adjusted average difference inBMI per 1-point difference on the Restraint Scale score. In the analysis of weight changeover time, restrained eating at the time of the 2006 Health Survey was significantlyassociated with an increase in weight since the time of the initial enrollment survey(adjusted = 0.33 pounds; 95% CI = 0.170.49; P < 0.001), controlling for the effects ofage, gender, education, race, smoking history, initial BMI, and time elapsed between self-reported weights. The average recent weight change was almost 7 pounds among the highestRestraint Scale scorers (Table 2), although the study design cannot distinguish betweentwins whose heightened attempts at cognitive restraint followed the weight gain and thosewhose heightened attempts preceded it.

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In the sensitivity analysis restricted to twins aged 25 or older (n = 1,141), the resultsremained highly significant for the association of Restraint Scale scores with both BMI (P 16) P valuea

Number, N (%) 610 (38) 518 (33) 459 (29)Gender

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Tabl

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BMIa

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