The Epidemiology of Obesity

Transcription

1 International Textbook of Obesity. Edited by Per Bjorntorp. Copyright 2001John Wiley & Sons Ltd Print ISBNs: (Hardback); (Electronic) 2 The Epidemiology of Obesity Jacob C. Seidell National Institute of Public Health and the Environment, Bilthoven, The Netherlands CLASSIFICATION OF OBESITY AND FAT DISTRIBUTION The epidemiology of obesity has for many years been difficult to study because many countries had their own specific criteria for the classification of different degrees of overweight. Gradually during the 1990s, however, the body mass index (BMI; weight/height ) became a universally accepted measure of the degree of overweight and now identical cut-points are recommended. This most recent classification of overweight in adults by the World Health Organization is shown in Table 2.1(1). In many community studies in affluent societies this scheme has been simplified and cut-off points of 25 and 30 kg/m are used for descriptive purposes. The prevalence of very low BMI ( 18.5 kg/m ) and very high BMI (40 kg/m or higher) is usually low, in the order of 1 2% or less. Already researchers in Asian countries have criticized these cut-points. The absolute health risks seem to be higher at any level of the BMI in Chinese and South Asian people, which is probably also true for Asians living elsewhere. There are some developments that indicate that the cut-points to designate obesity or overweight may be lowered by several units of BMI. This would of course greatly affect the estimates of the prevalence of obesity in these populations. For instance, the prevalence of overweight measured as BMI 27 kg/m in the 1989 China Health and Nutrition Survey (2) was 6% in the North, 3% in Central China and 1% in the South. If the cut-off point was lowered to 25 kg/m the prevalence would be increased to, respectively, 15%, 9% and 6%. In countries such as China and India, each with over a billion inhabitants, small changes in the criteria for overweight or obesity potentially increase the world estimate of obesity by several hundred million (currently estimates are about 250 million worldwide). Much research over the last decade has suggested that for an accurate classification of overweight and obesity with respect to the health risks one needs to factor in abdominal fat distribution. Traditionally this has been indicated by a relatively high waist-tohip circumference ratio. Recently it has been accepted that the waist circumference alone may be a better and simpler measure of abdominal fatness (3,4). Table 2.2 gives some tentative cut-points for the waist circumference. These are again based on data in white populations. In June 1998 the National Institutes of Health (National Heart, Lung and Blood Institute) adopted the BMI classification and combined this with waist cut-off points (6). In this classification the combination of overweight (BMI between 25 and 30 kg/m ) and moderate obesity (BMI between 30 and 35 kg/m ) with a large waist circumference ( 102 cm in men or 88 cm in women) is proposed to carry additional risk. GLOBAL PREVALENCE OF OBESITY AND TIME TRENDS In many reviews it has been shown that obesity (defined as a BMI of 30 kg/m or higher) is a International Textbook of Obesity. Edited by Per Björntorp. 2001John Wiley & Sons, Ltd.

2 24 INTERNATIONAL TEXTBOOK OF OBESITY Table 2.1 WHO classification of overweight and obesity (1) Classification BMI (kg/m ) Associated health risks Underweight 18.5 Low (but risk of other clinical problems increased) Normal range Average Overweight 25.0 or higher Pre-obese Increased Obese class I Moderately increased Obese class II Severely increased Obese class III 40 or higher Very severely increased prevalent condition in most countries with established market economies (7). There is a wide variation in prevalence of obesity between and within these countries. It is quite easy to find instances of at least a twofold difference in the prevalence of obesity within one country (e.g. Toulouse in France with a prevalence of obesity of 9% in men and 11% in women and Strasbourg in France with 22% of men and 23% of women being obese). Usually, obesity is more frequent among those with relatively low socioeconomic status and the prevalence increases with age until about years of age, after which the prevalence declines (8). In most of these established market economies it has been shown that the prevalence is increasing over time (8). Tables 2.3 and 2.4 show the increases in the prevalence of obesity in men and women aged years in several centres participating in the WHO MONICA project (9). It is clear that there is a rapid increase in the prevalence of obesity in most centres from countries in the European Union, particularly in men. In centres in countries from Central and Eastern Europe the prevalences of obesity in women may have stabil- Table 2.2 Sex-specific cut-off points for waist circumference. Level 1was established to replace the classification of overweight (BMI 25 kg/m ) but not combined with a high waist-to-hip ratio (WHR 0.95 in men and 0.80 in women). Level 2 was based on classification of obesity BMI 30 kg/m and BMI between 25 and 30 kg/m in combination with high waist-to-hip ratio (5) Level 1( alerting zone ) Prevalence Level 2 ( action level ) Prevalence Men 94 cm ( 37 inches) 24.1% 102 cm ( 40 inches) 18.0% Women 80 cm ( 32 inches) 24.4% 88 cm ( 35 inches) 23.9% Table 2.3 Prevalence of obesity (age standardized % with BMI 30 kg/m ) of centres in EU countries participating in the first round of the MONICA study (May 1979 to February 1989) and the third round (June 1989 to November 1996) Men Women EU country (Centre) First round Third round First round Third round Belgium (Ghent) Denmark (Glostrup) Finland (north Karelia) Finland (Kuopio) Finland (Turku/Loimaa) France (Toulouse) France (Lille) Germany (Augsburg, urban) Germany (Augsburg, rural) Italy (area Brianza) Italy (Friuli) Spain (Catalonia) Sweden (North) United Kingdom (Belfast) United Kingdom (Glasgow) Mean

3 THE EPIDEMIOLOGY OF OBESITY Table 2.4 Prevalence of obesity (age standardized % with BMI 30 kg/m ) of centres in countries outside the European Union participating in the first round of the MONICA study (May 1979 to February 1989) and the third round (June 1989 to November 1996) 25 Men Women Country (centre) First round Third round First round Third round Other European countries Iceland (Iceland) Switzerland (Vaud/Fribourg) Switzerland (Ticino) Poland (Warsaw) Poland (Tarnobrzeg) Russia (Moscow) Russia (Novosibirsk) Czech Republic (rural CZE) Yugoslavia (Novi Sad) Mean Outside Europe China (Beijing) USA (Stanford) Figure 2.1 Time-trends of the prevalence of obesity in adults in the Health Survey for England (ages 16 ), and in the Netherlands (MORGEN project, ages years), ized or even slightly decreased but still those prevalences remain among the highest in Europe. The study by Molarius et al. (9) showed that the social class differences in the prevalence of obesity are increasing with time. Obesity is increasingly becoming an almost exclusively lower class problem in Europe. Figure 2.1shows the extraordinary increase in the prevalence of obesity in England. In the mid 1980s the prevalence of obesity in men from the Netherlands and England was about the same but in 1997 it was at least twice as high in england. The most recent ( ) estimates of obesity in adults in the USA are about 20% in men and 25% in women (8). In other parts of the world obesity is also frequent. Martorell et al. recently described the prevalence of obesity in young adult women aged years (10). The estimated prevalence of obesity was on average 10% in Latin American countries and 17% in countries in North Africa and the Middle East. Obesity is uncommon in sub-saharan Africa, China and India, although in all regions the prevalence seems to be increasing, particularly among the affluent parts of the population in the larger cities (11). In these countries we quite often see the para-

4 26 INTERNATIONAL TEXTBOOK OF OBESITY doxical condition of both increasing undernutrition and overnutrition. This is clearly related to growing inequalities in income and access to food in these regions. In addition, it has already been mentioned that classification criteria based on Europid populations (i.e. those of European ancestry) might not be appropriate for Asian populations. There is some uncertainty around most national estimates of obesity prevalence because of the lack of solid data, and the large differences between countries within the same region and secular trends. The numbers corresponding to the midpoint of the estimates add up to about 250 million obese adults, which is about 7% of the total adult world population. It does not seem unreasonable that the true prevalence of obesity is likely to be in the order of 5 10%. In most countries the prevalence of overweight (BMI between 25 and 30 kg/m ) is about two to three times the prevalence of obesity, which would mean that there may be as many as one billion people who are overweight or obese. EXPLANATIONS FOR THE GROWING EPIDEMIC OF OBESITY On an ecological or population level these time trends are not too difficult to explain although exact quantification of different factors is almost impossible. On the one hand, the average energy supply per capita is increasing. The World Health Report (12) has estimated that the average energy supply per capita in the world was 2300 kcal in 1963, 2440 kcal in 1971, and 2720 kcal in 1992; and it is estimated that in 2010 this will be 2900 kcal. These increases are obviously not evenly distributed across the world s population and, sadly, many remain undernourished although in Asia (particularly China and India) and most of Latin America these numbers are declining. The number of people with access to at least 2700 kcal has increased from billion in to 1.8 billion in and is estimated to grow to 2.7 billion in Even when corrected for the increase in the world s population this implies a more than 10-fold increase in the number of people having access to high caloric diets. The globalization of agricultural production and food processing has not only affected the quantity of energy available per capita but also the energy density. At the same time, there are continuing changes in the physical demands of work and leisure time. Increasingly we are at leisure during working hours and we work out during leisure time. Mechanization of many types of work and changes in transportation are causing ever-increasing numbers of people to be sedentary for most of the time. Increasing sedentary behaviour has been proposed as one of the principal reasons for a further increase in the prevalence of obesity in countries with established market economies. Sedentary behaviour is poorly measured by the number of hours engaged in sports only. Large and important differences can be seen in the number of hours spent at sedentary jobs and behind television or computer screens during leisure time. Transportation is almost certainly a factor as well. For example, of short trips in the Netherlands 30% are done by bicycle and 18% by walking. In the UK these percentages are 8% by cycling and 12% by walking and in the USA 1% by bicycle and 9% by walking (13). These daily activities accumulated over a year can easily explain the small but persistent changes in energy balance needed to increase the prevalence of obesity. Given the changes in lifestyles over the last decades in many parts of the world it is not surprising that people gain weight on the average although for many individuals this seems to remain a mystery. With small changes in average body weight the prevalence of obesity increases rapidly. For every unit increase in BMI there is an increase in the prevalence of obesity of around five percentage points (14). PREVALENCE OF A LARGE WAIST CIRCUMFERENCE The data of the WHO MONICA population (second survey carried out between 1987 and 1992) have recently been analysed with respect to waist cut-off points (15). From this analysis it is clear that the use of these single cut-off points of the waist circumference to replace classification by BMI and waist-to-hip ratio varies greatly from country to country. The prevalence of a large waist circumference ( 102 cm in men and 88 cm in women) and of obesity (BMI 30 kg/m ) is shown in Table 2.5.

5 THE EPIDEMIOLOGY OF OBESITY Table 2.5 Prevalence of a large waist circumference (102 cm or more in men or 88 cm or more in women) and of obesity (BMI 30 or more) in 19 centres participating in the WHO MONICA study (second round, ). Adapted from reference Men Women Population Large waist Obesity Large waist Obesity (countries by alphabetical order) (%) (%) (%) (%) Australia (Newcastle) Australia (Perth) China (Beijing) Czech Republic (Rural) Denmark (Glostrup) Finland (Kuopio Province) Finland (North Karelia) Finland (Turku/Loimaa) France (Toulouse) Germany (Augsburg Rural) Germany (Augsburg Urban) Germany (Halle County) Italy (Area Brianza) Italy (Friuli) Spain (Catalonia) Sweden (Göteborg) Sweden (Northern Sweden) United Kingdom (Glasgow) Yugoslavia (Novi Sad) In general, the prevalence of a large waist is higher than the prevalence of obesity and this is because it also includes overweight subjects with abdominal obesity. OBESITY IN CHILDREN AND ADOLESCENTS Comparison of prevalence data of obesity in children and adolescents around the world remains difficult because of the lack of standardization and interpretation of indicators of overweight and obesity in these age groups. Usually local or national percentile distributions for weight-for-age, weightfor-height, or BMI-for-age are used. Not only do these differ between regions and nations but they are also subject to change over time. In addition, different percentile cut-off points are used for the definition of overweight or obesity (e.g. 85th, 90th, 95th and 97th percentiles are used in different countries). Another difficulty with these criteria is that when they are applied to older adolescents they do not correspond to the criteria for classification of overweight based on BMI for adults. Recently Cole et al. (17) used data from six large nationally representative cross-sectional growth studies from various parts of the world. They established centiles of the distribution of BMI by age. Those centile curves that, at age 18 years, passed through the widely used cut-points of 25 and 30 kg/m for adult overweight and obesity were then used to define BMI cutpoints by age. These proposed cut-points are less arbitrary and more internationally based than current alternatives. With respect to the interpretation of criteria of overweight in different age groups it is also important to know whether or not they are predictive of later obesity. It is now generally accepted that body weight before the age of 6 years has very limited predictive power for the chances of becoming an overweight or obese adult irrespective of the family history of obesity (18). Data at this age may, however, be predictive in another way, as has been suggested by Rolland-Cachera and others (19). The BMI-for-age from infancy until adulthood has the form of a J-shape. The nadir of this curve usually is in the age range of 5 to 7 years of age. It has been

6 28 INTERNATIONAL TEXTBOOK OF OBESITY Figure 2.2 Time-trend in the prevalence of obesity (BMI 30 kg/m ) among Danish (open bars) and Swedish (striped bars) male conscripts (adapted from references 2, 28) suggested that when this nadir occurs at a relatively early age ( early-adiposity-rebound ) the chances of adult obesity are higher than when there is a relatively late adiposity rebound (19,20). In addition, time trends in overweight may be sensitive indicators of secular changes in energy balance. The World Health Organization has now tentatively recommended the use of BMI-for-age as an indicator of overweight or obesity (14). In the Netherlands, the French reference curves ( 97th percentile of BMI-for-age) have been used to evaluate some recent trends and a slight increase in the prevalence of obesity during the early 1990s was observed (21). Similar trends have also been observed in other countries, particularly the USA (22 24) and the United Kingdom (25). Military conscript data have been shown to be particularly useful in giving an unbiased view of long-term national time trends. Such data have been reported from Denmark (26) and in Sweden (27). Figure 2.2 shows these time trends in overweight and obesity among young Danish and Swedish men and they illustrate a persistent increase in both countries. Currently, a subgroup of the WHO International Obesity Task Force (IOTF) is trying to develop international BMI-by-age standards that can be used universally and which are preferably based on longitudinal tracking data of BMI for children and adolescents and which match around age 20 with the adult classification of BMI. Body mass index may not be a very precise indicator of body fatness on an individual level but there are many studies that support the use of BMI as an indicator of fatness on a population level (5,28). The interpretation of these increases in childhood and adolescent obesity rates is difficult. Explanations require unbiased and precise estimates of energy intake and energy expenditure and these are often unavailable. Small secular changes in obesity may be the result of minute shifts in energy balance which are all well within the margin of error of all available methods. This is further complicated by the likelihood that reported energy intake in children is considerably underestimated (29). However, the USA is among those countries in which, despite a dramatic recent increase in the prevalence of obesity, there is no good evidence for any appreciable change in energy intake over the last decades and there may even have been some improvement (30). Some crude evidence suggests that the reduction in energy expenditure in children and adults is the most important determinant and it is not difficult to see that quite major changes in lifestyle have occurred in youngsters over the last few decades (16). Several studies report low physical activity in obese children compared to their lean counterparts (31,32). This may be the cause or the consequence of their obesity. Prospective studies, however, have also linked sedentary behavior such as television viewing to the development of obesity (33,34). CONCLUSIONS The increase in the prevalence of obesity among children, adolescents and adults in many countries around the world is alarming. Prevention of obesity should be among the high priorities in public health. This should be particularly aimed at encouraging healthy lifestyles in all age groups including children and adolescents. This cannot be achieved by efforts aimed at the individual level. Communities, governments, the media and the food industry need to work together to modify the environment so that it is less conducive to weight gain (1). REFERENCES 1. WHO. Obesity: Preventing and Managing the Global Epidemic. Geneva: WHO, WHO/NUT/NCD/ Popkin BM, Leyou G, Fengying Z, Guo X, Haijiang M, Zohoori N. The nutrition transition in China: a cross-

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