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G C Patton a Centre for Adolescent Health, Department of
Paediatrics, University of Melbourne, Parkville Victoria 3052, Australia, b Clinical Epidemiology and Biostatistics, Royal
Children's Hospital Research Institute, Parkville Victoria 3052, Australia
Correspondence to: Professor Patton
patton{at}cryptic.rch.unimelb.edu.au
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Abstract |
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Objective:
To study the predictors of new eating
disorders in an adolescent cohort.
Design:
Cohort study over 3 years with six waves.
Subjects:
Students, initially aged 14-15 years, from 44 secondary schools in the state of Victoria, Australia.
Outcome measures:
Weight (kg), height (cm), dieting
(adolescent dieting scale), psychiatric morbidity (revised clinical
interview schedule), and eating disorder (branched eating disorders
test). Eating disorder (partial syndrome) was defined when a subject met two criteria for either anorexia nervosa or bulimia nervosa according to the Diagnostic and Statistical Manual of Mental
Disorders, fourth edition (DSM-IV).
Results:
At the start of the study, 3.3% (29/888) of female subjects and 0.3% (2/811) of male subjects had partial syndromes of eating disorders. The rate of development of new eating
disorder per 1000 person years of observation was 21.8 in female
subjects and 6.0 in male subjects. Female subjects who dieted at a
severe level were 18 times more likely to develop an eating disorder
than those who did not diet, and female subjects who dieted at a
moderate level were five times more likely to develop an eating
disorder than those who did not diet. Psychiatric morbidity predicted
the onset of eating disorder independently of dieting status so that
those subjects in the highest morbidity category had an almost
sevenfold increased risk of developing an eating disorder. After
adjustment for earlier dieting and psychiatric morbidity, body mass
index, extent of exercise, and sex were not predictive of new eating disorders.
Conclusions:
Dieting is the most important predictor
of new eating disorders. Differences in the incidence of eating
disorders between sexes were largely accounted for by the high rates of earlier dieting and psychiatric morbidity in the female subjects. In
adolescents, controlling weight by exercise rather than diet restriction seems to carry less risk of development of eating disorders.
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Key messages
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Introduction |
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Eating disorders in young women are common and associated with significant mortality and morbidity. 1 2 Lifetime risks in women have been estimated at 8% for bulimic syndromes and around 3% for anorexic syndromes. 3 4 The features of eating disorders most commonly emerge in mid-adolescence, before the development of full syndromes.5 Cross sectional surveys have confirmed that eating disorders are common in adolescent women; around 0.5% have anorexia nervosa, 1% have bulimia nervosa, and 3% to 5% have subclinical syndromes. 6 7
Rigorous population based studies on risks for eating disorders have
been few. A recent case-control study of young adults linked dieting,
psychiatric morbidity, and obesity to eating disorders.5 But because these subjects were studied on average eight years after
the first appearance of symptoms, it was difficult to distinguish influences on the course of the eating disorder from factors that caused it. Case-control studies are also restricted in the evaluation of factors that vary with time, for example, dieting and psychiatric morbidity, when an eating disorder can influence both the presence of a
risk factor and its recall. The interpretation of available cohort
studies has been marred by incompleteness of follow up, restriction to
only two measurement points, and uncertain reliability in the
measurement of both eating disorders and putative risk factors.
8 9
Many of these earlier limitations have been
addressed in our prospective study of eating disorders in almost 2000 Australian secondary school students.
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Subjects and methods |
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Sample population and methodology
Between August 1992 and July 1995, we collected data on subjects
in a cohort study of adolescent health in the state of Victoria,
Australia. The cohort was defined using a two stage procedure. At stage
1, we randomly selected 45 schools from a stratified frame of
government, catholic, and independent schools (total number 60 905).
At stage 2, a single intact class from each participating school was
selected at random to constitute the wave 1 sample. At the second wave
of data collection 6 months later, when the wave 1 sample had moved
into year 10, a second intact class from each participating school was
selected at random (fig 1). One school from the initial cross sectional
survey was unavailable for study leaving a total of 44 schools.
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Measures
Eating disorders
The branched eating disorders test was
used to assess criteria for eating disorders according to the
Diagnostic and Statistical Manual of Mental Disorders,
fourth edition (DSM-IV).11 The test was designed for use
in adolescent community samples and covers symptoms of eating disorders
over the previous 3 months. A partial syndrome of eating disorder was
defined when a subject met two DSM-IV criteria for either anorexia
nervosa or bulimia nervosa. The definitions of partial syndrome using
the test had high agreement with the eating disorders examination in an
earlier community based validation study (sensitivity 1.0, specificity 0.99, and positive predictive value 0.7).12
Weight was measured to the nearest
0.1 kg with subjects in minimal school uniform, and 1 kg was deducted to account for clothing. Height was measured with a stadiometer to the
nearest centimetre, with shoes removed. Self reported weights were used
for those subjects who had left school.
Dieting and exercise
Dieting level was categorised using
the adolescent dieting scale, which comprises nine items and measures
three dieting strategies: calorie counting, reducing food quantities at
meals, and skipping meals.13 Two types of exercise
frequency were measured: daily participation in sport or a formal
exercise routine. Dieting levels in schools were estimated from the
rates of moderate dieting at inception of the cohort, and the schools
were then divided into thirds (on the basis of dieting rates within the
school) to give a measure of exposure to dieting in school peers.
Psychiatric morbidity
Mental health status was evaluated
with a computerised form of the revised clinical interview schedule, which is a branched questionnaire for assessing common symptoms of
psychiatric disorder in non-clinical populations.
14 15
Total scores were used to categorise subjects on four levels of
psychiatric morbidity: 0-5, 6-11, 12-17, and 18 or higher. A threshold
of 12 or higher indicated when a doctor might be expected to have concern about a subject's mental health.
14 15
Analysis
We analysed the data with STATA software. Estimation
procedures were used with logistic regression analyses to allow for
probability weighting and complex survey design. Where appropriate,
confidence intervals for rates and proportions were based on methods
that use Poisson or binomial distributions to allow for small numbers.
We calculated incidence rates for eating disorder by dividing the
numbers of incident cases by total person time at risk. Person time at
risk was calculated using midpoints between relevant data waves for new
cases. Multivariable analysis of prediction of a new disorder was
performed with a discrete time version of the Cox proportional hazards
model, which may be fitted using standard generalised linear modelling
methods.16 A binary outcome was defined
that is, first
onset of eating disorder
for each at risk time interval, and this was
analysed using the "glm" command in STATA with the
complementary log-log link function, including an offset term equal to
the natural logarithm of the length of the time
interval.17
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Results |
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Overall, 1947 of 2032 students (95.8%) completed study measures at least once during the study. Figure 1 shows the response rates of the total sample across the six waves. The sex ratio of the cohort (males 47.0%) was similar to that in schools in Victoria at the time of sampling.18 The mean age (SD) at wave 1 was 14.5 (0.5) years and at completion of wave 6 it was 17.4 (0.4) years.
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Cross sectional profile of eating disorders
At the completion of sampling in wave 2, 3.3% (95% confidence
interval 2.1% to 4.5%) of female subjects and 0.3% (0.1% to 1.1%)
of male subjects had partial syndromes of eating disorders. Of the 30 female subjects with an eating disorder, two (7%) had bulimia nervosa
and 24 (80%) had partial syndrome of bulimia nervosa giving a
prevalence rate of 2.7% (1.6% to 3.8%) at baseline in female
subjects. Four of the 30 female subjects (13%) had partial syndrome of
anorexia nervosa giving a prevalence rate of 0.5% (0% to 1.0%) in
female subjects at baseline. Three male subjects had partial syndrome
of bulimia nervosa. Overall, 8% (69/892) of females and 0.5% (4/793)
of males dieted at a severe level, and 60% (536) of females and 29%
(226) of males dieted at a moderate level.
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Discussion |
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Our study showed that 8% of 15 year old girls dieted at a severe level and a further 60% dieted at a moderate level. Female subjects who dieted at a severe level were 18 times more likely to develop a new eating disorder within 6 months than those who did not diet, and over 12 months had an almost 1 in 5 chance of developing a new eating disorder. Female subjects who dieted at a moderate level were five times more likely to develop a new eating disorder than those who did not diet, and over 12 months had a 1 in 40 chance of developing a new eating disorder. In contrast, fewer than 1 in 500 subjects in the non-dieting group developed an eating disorder in the next year. Even ostensibly moderate dieting increased the risk for eating disorders, and around two thirds of new cases were in female subjects who had been in the moderate rather than severe dieting group. Although these findings suggest that dieting is a very important risk factor for adolescent eating disorder, it is arguable that for those who already diet severely a process leading to eating disorder has begun.13
Psychiatric morbidity carried independent risks for developing an eating disorder. After adjusting for previous dieting, those subjects with high psychiatric morbidity had a more than six times increased risk of developing a new eating disorder. In the high morbidity group, 6% of female subjects developed an eating disorder within 12 months compared with <1% of those in the low morbidity group. Female subjects in the severe dieting group with high levels of psychiatric morbidity had a greater than 1 in 4 chance of developing an eating disorder within 12 months.
One notable strength of our study was the multiwave design, which permitted evaluation of risk factors that vary with time (dieting, psychiatric morbidity, exercise). Other strengths derived from using a representative cohort, having high initial participation rates and low attrition rates. As school retention rates were 98% in the year of initial sampling, our cohort closely represented the broader adolescent population.18 The problem of adequately defining dieting and subclinical eating disorder was addressed through the use of standardised and reproducible measures linked to DSM-IV criteria.11 A prevalence estimate of just over 3% for partial syndromes at the start of the study is similar to reports of earlier school based samples.6 Rates of anorexia nervosa were, however, low compared with one study that used weight records in case identification but similar to those found in earlier surveys on the basis of self reported questionnaires.7 Sensitivity to questions about weight and eating may lead to non-participation in surveys by subjects with anorexia nervosa, so that despite high overall study participation, anorexic syndromes may be under-represented.14
A sex ratio of around 4:1 for the development of eating disorder was lower than the initial prevalence ratio of around 10:1, raising a possibility that differences in both inception rates and early course of the eating disorder contribute to the difference in prevalence between the sexes. More importantly, the hazard ratio for sex in relation to onset of eating disorder was small after adjustment for dieting and psychiatric morbidity; both strongly associated with being a female subject. This suggests that a predominance of bulimia nervosa in female subjects arises largely from differential rates of adolescent dieting and psychiatric morbidity.
Dieting by children and adolescents, rather than other means of
weight control, is likely to remain a much debated issue as obesity in
young people increases.19 Previous reports have suggested that a risk factor for eating disorders is participation in sports, particularly those that require thinness, such as
gymnastics.
20 21
In our study, however, daily
participation in sport did not raise risks above those already
associated with dieting, suggesting that food restriction associated
with specific sports underlayed the reported association. In adolescent
weight control, promotion of exercise rather than restriction of
dietary intake may prove less of a risk in the development of eating disorders.
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Acknowledgments |
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Contributors: GCP, the principal investigator on the Victorian adolescent health cohort study, initiated the study, took part in the development and selection of study measures, and undertook data analysis and writing of the paper; he will act as guarantor for the paper. RS took part in the development, validation, and selection of study measures, and contributed to data analysis and editing of the paper. CC supervised data collection and data cleaning and analysis and contributed to both the writ-ing and editing of the paper. JBC oversaw the data analysis and contributed to the writing and editing of the paper. RW contributed to the data analysis as well as the writing and editing of the paper.
Funding: Victorian Health Promotion Foundation and the National Health and Medical Research Council. Competing interests:
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References |
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Psychol Med
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Psychol Med
1990;
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382-394.(Accepted 29 January 1999)
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