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Figure 2.2 Time-trend in the prevalence of obesity (BMI >30kg/m2) 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).

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