- Susan Chinn, reader in medical statistics (, )
- Roberto J Rona, professor
- Correspondence to: S Chinn
- Accepted 29 September 2000
Objectives: To report trends in overweight and obesity, defined by new internationally agreed cut-off points, in children in the United Kingdom.
Design: Three independent cross sectional surveys.
Setting: Primary schools in England and Scotland.
Participants: 10 414 boys and 9737 girls in England and 5385 boys and 5219 girls in Scotland aged 4 to 11 years.
Main outcome measures: Prevalence and change in prevalence of overweight and obesity, as defined by the international obesity task force, in 1974, 1984, and 1994, for each sex and country.
Results: Little change was found in the prevalence of overweight or obesity from 1974 to 1984. From 1984 to 1994 overweight increased from 5.4% to 9.0% in English boys (increase 3.6%, 95% confidence interval 2.3% to 5.0%) and from 6.4% to 10.0% in Scottish boys (3.6%, 1.9% to 5.4%). Values for girls were 9.3% to 13.5% (4.1%, 2.4% to 5.9%) and 10.4% to 15.8% (5.4%, 3.2% to 7.6%), respectively. The prevalence of obesity increased correspondingly, reaching 1.7% (English boys), 2.1% (Scottish boys), 2.6% (English girls), and 3.2% (Scottish girls).
Conclusion: These results form a base from which trends can be monitored. The rising trends are likely to be reflected in increases in adult obesity and associated morbidity.
Recently the need for estimates of overweight and obesity in children to assess preventive measures, monitor secular trends, and identify high risk groups has been emphasised. 1 2 There has been a lack of consensus over the definitions, but internationally based cut-off points have now been published.3 On the basis of these cut-off points we report prevalence and secular trends in overweight and obesity from 1974 to 1994 in white children in the United Kingdom.
Participants and methods
The national study of health and growth, which started in 1972, included 22 English areas in 1974, 1984, and 1994, six Scottish areas in 1974, and 14 Scottish areas in 1983-4 and 1993-4. All white children from the national study of health and growth were eligible for our study. The samples included too few non-white children for useful analysis; ethnic minority groups were included in a separate inner city sample not reported here. We chose to study (a) 1994 because this was the final year of the national study, (b) trends from 1984 to 1994 in weight for height because these data have been reported,4 and (c) 1974 because this was representative of the earlier years. Areas were chosen by stratified random sampling for England and Scotland separately, with weighting towards poorer areas. If a school needed to be replaced a comparable school was chosen in the same area when possible or from another area in the same stratum.5
When the national study of health and growth began in 1972 a system of ethical committees for community based studies had not been established, but the coordinators at the time obtained ethical approval from St Thomas's Hospital medical ethics committee. Participation of schools was agreed with the health and education authorities and head teacher in each area. Parents were notified of the study in advance and were able to withdraw their child.
Height was measured on a Holtain stadiometer to the last 0.5 cm in 1974 and to the last 0.1 cm in 1984 and 1994; 0.25 cm or 0.05 cm was added as appropriate. Weight was recorded to the last 100 g with a mechanical balance in 1974 and 1984 and electronic digital scales in 1994. Details have been given elsewhere.4
Body mass index was calculated as weight (kg)/(height (m)2). Using linear interpolation between the cut-off points for each six months of age we calculated the percentage of children who were overweight or obese for each country, sex, and year. Children were divided into three age groups, 4 to 6, 7 to 8, and 9 to 11 years. The definitions of overweight and obesity were based on average centiles estimated to pass through body mass index 25 and 30, respectively, at age 18.3 Table 1 shows the cut-off points for ages 4 to 12 years.
Over 97% of children were measured in 1974 and 1984 and over 94% in 1994. Table 2 shows the prevalence and changes in prevalence in overweight. From 1974 to 1984 there was little overall change. From 1984 to 1994 there was an overall increase in all four groups of children: 3.6% in boys and 4.1% and 5.4% in English and Scottish girls, respectively. The increase was greatest in the oldest age group, the differences between age groups being significant in English boys (logistic regression, test of interaction P=0.009). The prevalence reached nearly 20% in the oldest Scottish girls in 1994. Table 3 shows the prevalence of obesity, but this is relatively small and is not shown subdivided as there is low power to detect differences between age groups. A decrease in obesity in boys occurred from 1974 to 1984 and an increase in all groups from 1984 to 1994, with Scottish girls having a final prevalence of 3.2%.
What is already known on this topic
Mean weight for height increased in children in the United Kingdom from 1984 to 1994
Previously there were no agreed definitions of overweight and obesity that could be used to quantify the increase
Internationally agreed definitions have recently been published
What this study adds
Prevalence of overweight was 5-6% in both 1974 and 1984 in white boys and 9-10% in white girls, and it rose to 9-10% in boys in 1994, to over 13% in English girls, and to nearly 16% in Scottish girls
The prevalence of obesity in children is low, but it has increased substantially since 1984
Overweight in children is a serious public health problem in Britain
Although the prevalence of overweight and obesity in children has been reported previously in the United Kingdom and elsewhere, the results have always been difficult to interpret as they have relied on the 85th and 95th centiles of reference or study based values or a preset excess level of relative weight. 2 6 7 We have on several occasions reported increases in weight for height in children,4–8 but until now we have not reported prevalence of obesity. Previously this information would have shed little light on the magnitude of the problem as different studies have used different definitions. The recently agreed cut-off points for overweight and obesity in children have given us the opportunity to provide baseline information and, uniquely, trends in overweight and obesity over a 20 year period.3 The data from the national study of health and growth for 1990 formed the greater part of the data for children aged 5 to 11 years in the United Kingdom dataset, which contributed to the international standards.9 Our study gives an appropriate base against which estimates from other studies in the United Kingdom and elsewhere can be compared with the same methodology.
Because of the reduced power of an analysis of a dichotomous variable compared with an analysis of the underlying continuous measure, the results do not show a clear picture of the age group differences in the increase in prevalence. An analysis of a weight for height index, however, showed a clear trend of a greater increase in older age groups from 1972 to 1994, which was particularly noticeable in Scottish children.8
Although debate over the cut-off points will continue, this should not detract from the urgency of tackling the problem of obesity. Our data indicate that overweight and obesity on the basis of body mass index have increased noticeably since 1984. Most studies have shown poor prediction of adult obesity from child assessments but a consistent positive correlation between child and adult overweight and obesity.10 Rising trends in children will almost certainly be represented in later trends in adult overweight and obesity and probably in an increase in associated adult morbidity.
We thank our colleagues in the study team, all parents, children, and helpers in the study areas, and Professor Tim Cole for supplying the international cut-off points in advance of their publication.
Contributors: SC carried out all the analyses, wrote the first draft of the paper, and is guarantor. RJR was the project leader for the national study of health and growth and initiated and participated in the writing of the paper.
Funding The national study of health and growth was funded by the Department of Health.
Competing interests None declared.