QJM vol. 97 no. 12 © Association of Physicians 2004; all rights reserved.
Commentary |
The obesity epidemic: prospects for prevention
From the 1Department of Health and Human Sciences, University of Essex, Wivenhoe, and 2Department of Epidemiology and Public Health, Queen's University of Belfast, Belfast, UK
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Summary |
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Some 20–25% of UK adults are obese according to the WHO criterion (BMI
30 kg/m2). Type 2 diabetes, increasingly recognized as a major complication of overweight and obesity, is beginning to appear in UK adolescents, following the trends in the US. Epidemiological data indicate that the prevalence of overweight and obesity has doubled or tripled in the past few decades in the US, in Europe, and even in many developing countries. Thus obesity is increasingly seen as a public health problem requiring concerted action by both governmental and non-governmental organizations. A sound understanding of the root causes is crucial, if strategies for the prevention and treatment of this epidemic are to be developed. Many epidemiological studies suggest that physical activity at work, school or at leisure has declined to minimal levels, and that sedentary behaviours such as television viewing and computer games have become major pastimes. Thus energy requirements are substantially less than those for recent generations. Further, the food industry produces high-calorie foods which children and adults consume as snack meals, giving a substantial surfeit to their daily energy requirement. In children, a few school-based, preventive intervention trials have shown some promising results. Many negative trials have also been reported, and practical difficulties remain in the widespread implementation of appropriate protocols. Initiatives have been introduced by the government to increase the physical education syllabus in school to a minimum of 2 h/week, and the promotion of fruit and vegetables. Further research is required on the physiological and psychological causes of overweight and obesity in children and adults, and randomized, controlled, school and community-based trials are required to pilot preventative initiatives. Monitoring of the progress in prevention at both organizational and outcome level is required, and also of adverse outcomes such as a rise in the prevalence of eating disorders. |
Introduction |
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We have always had fat patients, whose personalities are as varied as the next person's (see Figure 1), but increasingly excess weight is becoming seen as a public health problem rather than one just for the individual patient and their physician. Recognition of the complications of excess weight and of the scale of the problem has led to the forecast that obesity will soon overtake the smoking habit as the predominant ‘preventable’ cause of death in Western countries.1,2
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Definition and complications |
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Garrow has proposed a simple definition of overweight and obesity as a condition in which body fat stores are charged to an extent which impairs health.3 In the 1940s and 1950s, insurance companies published tables of ‘desirable weights’, which were based on their own mortality data. More recently epidemiological studies have used Quetelet's index of body weight standardized for height (kg/m2) and, on the basis of data from these studies, the normal (desirable) range lies between 18.5 and 24.9 kg/m2, with overweight defined as 25.0 to 29.9 kg/m2 and obesity as 30.0+ kg/m2. The World Health Organization (WHO) classification4 is shown in Table 1.
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The subcategories of obesity (I–III) are more likely to be of clinical use, rather than for epidemiological monitoring, as the more severe grades of obesity are unlikely to be well represented in epidemiological surveys. There has been recent interest in central obesity as measured by the waist-hip ratio. But the evidence that central obesity provides a better risk marker than BMI for future cardiovascular disease and insulin resistance remains controversial,5,6 and secular trend data are lacking. Globally, 58% of cases of type 2 diabetes, 21% of coronary heart disease, and between 8% and 42% of some cancers are attributable to overweight and obesity.7
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Prevalence and trends: adults and children |
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The International Obesity Task Force2 indicates that the prevalence of obesity in European adults is between 10–25% in women and 10–20% in men and that the prevalence has increased by 10–40% in Europe in the past 10 years. The scale of the epidemic in England in adults and in children is shown in Box 1.
| Box 1 Obesity in England, 1980–2002: some summary facts
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Whilst the BMI cut-points for adults of 25 kg/m2 and 30 kg/m2 for overweight and obesity have been established worldwide, there was no commonly accepted standard of determining overweight and obesity in children until Cole et al.11 published reference curves for children aged 2–18 years based on pooled data from several countries; cut-points for BMI at 6-monthly increments in age, which correspond to the percentiles of children with a BMI of 25 kg/m2 (overweight) or 30 kg/m2 (obese) at 18 years of age are used. Previously childhood obesity had been defined in various ways: weight for height percentiles, percent of ideal body weight and absolute weight,12 etc., and therefore it was difficult to make comparisons of prevalence and trends between and within populations. In the US, the Surgeon General has reported that 15.3% of 6–11-year-olds and 15.5% of 12–19-year-olds are considered obese,13 and the heaviest children are getting heavier.14 In Australia, 23% children are overweight, with a further 6% being obese.15 The prevalence of overweight is also increasing in many countries with transitional economies such as Brazil and China;16 although in Russia, overweight decreased from 15.6% to 9.0% in children and adolescents, but this is more likely to be due to problems with food distribution and availability, than to good dietary practice.16 Trends in central obesity, measured by waist circumference, have greatly exceeded the trends in BMI in children aged 11–16 years between 1977 and 1997 in Britain,17 but waist circumference has not been adopted as an international standard. A recent report to the World Health Organization by the International Obesity Taskforce recommends that BMI cut-points such as those proposed by Cole et al.11 be used as a standard definition for surveys, but that further research was required; also on waist circumference, and on the shorter-term health consequences of childhood obesity.18 While childhood obesity does not necessarily indicate adult obesity, epidemiological studies have found that about a third of obese preschool children and a half of obese school-age children become obese adults.19 The scale of the problem suggests that only a concerted sustained effort by health professionals in collaboration with many other disciplines and organizations will reverse this trend for future generations.
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Causes |
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Overweight and obesity occurs when there is an imbalance between energy intake and expenditure. This imbalance can result in overweight and obesity in children in a matter of years or, in adults, often in the longer term, in decades. The regulation of energy balance is highly complex, involving societal, behavioural, genetic, hormonal, and neural influences.20 Results from twin studies suggest that the tendency to obesity is inherited,21,22 at least in part, and that responsiveness to dietary intervention is genetically determined.23 Children with overweight or obese parents have a greater risk of obesity,24,25 which may have both genetic and environmental components. The ability to taste seems to be fully functional before birth, and infants and children appear to develop a preference for salt and a dislike for bitter and strong tastes.26,27 The discovery of mutations in the gene encoding leptin in two obese Pakistani children,28 and the striking effects of leptin administration to those children with a deficiency,29 have stimulated interest in the role of genetics in obesity. Single genes are thought to produce a small effect at population level, and it is the interaction between the genetic defects associated with obesity and other factors, such as prenatal factors which may account for high levels of obesity.22 However the two main preventable causes of obesity are lack of physical activity and chronic consumption of excess calories.
The majority of obese adults were of normal weight as children, and it is a result of excess energy intake over a period of many years which leads to obesity. Therefore learning to eat healthily and to exercise is vital. However, being a thin child doesn't necessarily offer protection against later obesity. Indeed, it has been found that children who were the thinnest in childhood and became fat adults had the highest levels of adult ill health.30 This suggests that it is the change in relative size between birth and adulthood that is associated with increased risk of adult disease, rather than size itself.31 Therefore, getting the energy balance right is important, not only from birth, but from before conception. Both maternal undernutrition and overnutrition may affect later levels of obesity in offspring. Maternal overnutrition, leading to an increase of transfer of nutrients to the foetus, may change energy metabolism in the foetus, leading to obesity.32 However, there is a body of evidence linking low birth weight and other markers of foetal size to cardiovascular risk factors,33 including increased central fat distribution and waist-hip ratio.34,35
There seem to be critical periods even within gestation, as data from the Dutch famine cohort show that, in 19-year-old men, exposure to famine in the first two trimesters of pregnancy was associated with an almost two-fold increase in obesity, while those exposed to famine in the third trimester had reduced rates of obesity at 19 years.36 Also maternal smoking during pregnancy has been associated with later obesity in children.37–39 Infant feeding is also important in the development of obesity, with children who were bottle-fed having higher rates of obesity than those who were breast-fed,40,41 but this relationship has been inconsistent in adults.42,43
Dietary intake
Growth rate in children and adolescents is a sensitive indicator of dietary adequacy, even though the energy required for growth is a minor component of total energy.44 Children are expected to gain around 12 kg in weight and 30 cm in height between the ages of 5 and 10 years, with the percentage of body fat remaining constant for boys and increasing for girls. Boys are heavier than girls until about 8 years, when girls become heavier with the onset of puberty and the associated deposition of fat.45 In order for the child's full potential for height and healthy weight to be achieved, adequate nutrition is essential. The National Diet and Nutrition Survey (NDNS) of young people in Britain aged 4–18 years46 found that mean energy intakes were lower than the estimated average requirement, but that children were taller and heavier than similar populations in previous studies.47 The most commonly eaten foods were white bread, savoury snacks, chips, biscuits, other forms of potatoes, and chocolate. These foods all contain a high proportion of refined carbohydrates which cause a large post-prandial insulin response; foods like these with a high glycaemic index (based on the physiological response to glucose) have been linked with the development of type 2 diabetes.48
The NDNS46 also found that children ate less than half the recommended five portions of fruit and vegetables per day. This is comparable to figures from the US, which show that only one in five children and one in four adults consume five portions daily.49 Sugared fizzy drinks were consumed by three-quarters of the NDNS children, compared to less than half who usually drank the sugar-free versions.46 Ebbeling et al.50 estimate that an excess calorie consumption of only 120 kcal per day (the equivalent of one sugared soft drink) would produce a 50 kg increase in body weight over 10 years. Consumption of fast food is also increasing worldwide. A large fast food meal such as a double cheeseburger, French fries, soft drink and dessert would contain 2200 kcal,50 which is equivalent to the recommended daily intake of calories for adults, and is unlikely to be the only food consumed in a day.
In NDNS, carbohydrate provided 51% of food energy intake in boys and girls, and fat provided about 35% of food energy, with saturated fat comprising 14% of energy which is above the adult recommendations of 11%. The relationship between fat intake and obesity is unclear,44 and studies in the US show that despite a decrease in the proportion of dietary fat, obesity is increasing.51,52 Since reported intakes in the NDNS were lower than the estimated average requirements, and under-reporting was considered unlikely, diet is clearly not the only factor implicated in the increase in obesity.46
Physical activity
Rates of physical activity have been declining and it has been estimated that between 40–69% of British children over the age of 6 are spending <1 h/day being moderately active;45 in a recent study in 3–5-year-olds in Glasgow, total energy expenditure was significantly lower than the UK estimated average requirement for energy (by 837 kJ/day).53 In the US, >60% of adults do not meet the recommended targets for physical activity.13 More people are engaged in sedentary work than before, spend more time in traffic queues and in front of televisions and computer monitors in their spare time, and the amount of time spent walking and cycling has decreased.54 A large cohort study in American nurses found that in 50 000 non-obese and non-diabetic nurses, those who watched more television and had a sedentary lifestyle were at greater risk of developing obesity or diabetes during 6 years of follow-up. The authors estimated that 30% (95%CI 24–36%) of new cases of obesity and 43% (95%CI 32–52%) cases of type 2 diabetes could be prevented by adopting a relatively active lifestyle (<10 h/week TV viewing and 30 min of brisk walking per day).55 In American schools, many schoolyards are closed out of school hours because of the risk of litigation54 and the situation is similar in Britain. Here, in the 1980s, about 67% of children aged 5–10 years walked to school, just over 20% travelled by car and <10% by bus. By 2000, only 55% walked, 35% went by car and 10% by bus.45 In the US, one third of adolescents watched 5 h of television per day, and there was less obesity in those children who watched <1 h of television per day.56 Advertisements for food are scheduled most frequently when children are likely to be watching.57 A recent report from New Zealand which followed 1000 children until the age of 26 years found that television viewing in childhood and adolescence was associated with higher BMI, lower cardiorespiratory fitness, increased cigarette smoking and raised serum cholesterol.58 Schools play a vital role in providing physical activity for children and adolescents, but budget cuts have resulted in resulted in less, and sometimes no physical activity, on school timetables,54,59 despite the knowledge that children and adolescents who participate in team sports or exercise programmes are less likely to be overweight.60
Family environment also plays an important role in the development of obesity. Lengthy periods of television viewing, lack of family meals at home, eating out at restaurants, childhood neglect and depression have all been associated with increased childhood risk of obesity.50 Environmental risk factors for childhood obesity have been recently reviewed elsewhere.18
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Prevention and treatment |
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The promotion of healthy eating and regular physical activity is essential both for the prevention of future obesity61 and also for treating those who are already overweight and obese.62 Recent large multi-centre trials have shown that significant weight loss is achievable using a contribution of diet and physical activity, and can prevent the onset of type 2 diabetes in subjects at risk.63 It remains to be seen whether similar protocols can be effectively introduced into public health and primary care practice. Guidelines for prevention and treatment have been introduced both in the US13 and in Britain,7 but implementation plans for these guidelines may take some years to be accepted and established given the number of organizations potentially involved, and the complex task of promoting collaboration between them. Given the scale of the epidemic the burden of treatment services will lie with primary care although a recent report from the Royal College of Physicians, Royal College of Paediatrics and Child Health and the Faculty of Public Health emphasizes the role of nutritional support services.64 The National Audit Office Report identified only 12 separate obesity clinics in England,65 and a Cochrane Review of the management and organization of care for overweight and obese people found that there were ‘few’ solid leads about improving obesity management, and that further research was required to identify cost-effective strategies for improving the management of obesity.66 In the UK, guidelines for the use of Orlistat and Sibutramine as anti-obesity agents were issued by the National Institute for Clinical Excellence,67 and the Chief Medical Officer's Report7 noted that their use had trebled since these guidelines were issued. Surgical treatment for type II and III grades of obesity appears to be more cost-effective than medical therapy, and may be underutilized.68 As weight loss is more difficult to achieve and maintain in adulthood,69 prevention and treatment in childhood and adolescence is essential. However as children are less mature intellectually and psychologically than adults, treatment plans have been mainly based on the family or school in order to provide the extra support that children will need. In surveys in Northern Ireland it was found that the level of dieting among obese 13- and 14-year-olds was low (38% in girls and 15% in boys)70 but in 5–8-year-olds, overweight and obese children were reported by parents to be taking healthier food choices.71 However, as has been shown elsewhere46 there was a marked trend for children from poorer families to have unhealthy food choices, which may indicate that national health promotion should be particularly directed at less well-off parents.
Education on good nutrition is vital, as it may influence eating habits. This is a useful form of prevention when focused on families as, not only could it change adult dietary preferences, but those of children also; food preferences are formed in childhood, mainly through parental food choice, and affect childhood dietary intake. These preferences are carried through to adult life and can cause adult health effects.72 Educating parents of obese children has been shown to produce positive changes in the children's dietary intake.73
Current recommendations from the American Heart Association74 are: (i) All children age 2 and older should participate in at least 30 min of enjoyable, moderate-intensity activities every day. (ii) They should also perform at least 30 min of vigorous physical activities at least 3–4 days each week to achieve and maintain a good level of cardiorespiratory (heart and lung) fitness.
School-based initiatives to improve nutrition are being developed. In Britain, teaching about food and nutrition may occur in Science, Design and Technology or in Personal, Social and Health Education.45 The Healthy Schools Programme encourages awareness of local health issues and schools attempt to present consistent informed messages about healthy eating.75 School ‘travel plans’ have also been developed, which encourage the reduction of car use, thus increasing physical activity. The National School Fruit Scheme will ensure that all 4–6-year-olds will receive a free piece of fruit on each school day; there are other initiatives, such as fruit tuck shops and food dudes, which are summarized in a British Nutrition Foundation review.45 Similar campaigns are also occurring in the US, which include programmes to increase the amount of physical activity in schools, sponsored walking to school, ‘turn-off televisions’ campaigns, and also the introduction of salad bars in schools.45 There have also been school-based interventions for children who are overweight or obese. One such intervention showed that a 10-week education programme resulted in a 15.4% decrease in the percentage overweight in obese children. and caused a reversal in the steady weight gain that these children were experiencing.76 In the US, there are a large number of community-based strategies, including walking trails, working with owners of restaurants to increase the amount of healthy items on their menus, cookery classes and community gardens.
Despite all these initiatives in the US, in Britain and elsewhere, the results from education programmes have been disappointing.50 This may be due at least in part to the influence of the multi-national food industry and advertising.77 The food industry spends billions of pounds advertising, and the budget for promoting healthy nutrition is minute in comparison.77 Others have also suggested food marketing controls18 but governments may be reluctant to intervene too radically against a powerful food industry. However in the US, some new initiatives based on long-term changes combining comparatively small increases in physical activity and decreases in calories have been sponsored by the food industry.78 A recent initiative by the World Health Organization includes recommendations restricting the availability of high energy foods,79 but this may be resisted by the food industry. Fiscal policies to promote healthier foods have been proposed for the prevention of coronary disease80 but have not been encouraged by official scientific representatives of the US Department of Agriculture.81
Doctors and other health professionals also have a major role to play in the treatment of obesity. Data from the US show that 58% of obese patients have never had counselling for weight loss.49 Obesity is often not considered a serious medical condition by doctors, and is generally only treated when other disease risk factors are present, rather than before these risk factors develop or are made worse by obesity.2
A recent report by the Select Committee on Health for the UK Parliament, which consulted widely, made a number of major recommendations,82 including the establishment of a strategic framework for preventing and treating obesity within the NHS with a full range of treatment options. It also advised that children should have access to appropriate services and should be screened for overweight and obesity annually at school. The committee also noted the complexity of the task, the need to obtain the collaboration of the food industry and to review progress in 3 years.
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Future research |
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Although increases in physical activity have been shown to reduce weight in children who are obese,73 it is not yet known what level of activity is needed.83 There is little known about the association of behavioural or environmental change with obesity. More evidence is required from longitudinal studies, particularly those based on cohorts of infants or children. A metaanalysis on the evidence for tracking of obesity into adulthood may provide useful information for public health strategies. There is also a need for small-scale trials that can measure the effects of behavioural and environmental change in both children and adults. A summary review of diet, physical activity and behavioural approaches to the management of obesity and overweight has been published recently.84 It will also be important to monitor for undesirable side effects of trials and public health initiatives—for example, a rise in the incidence of eating disorders such as anorexia nervosa and bulimia.85 These studies need to be followed up with large-scale intervention trials, and finally long-term trials that can assess the impact of changes over a period of many years. The role of genetics in overweight and obesity also needs further research and, in particular, the interplay between genes linked with obesity and major factors in the environment.
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Footnotes |
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Address correspondence to Dr J.W.G. Yarnell, Department of Epidemiology and Public Health, Queen's University of Belfast, Belfast, UK. e-mail: h.porter{at}qub.ac.uk
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References |
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