Measures of abdominal obesity within body mass index categories, 1981 and 2007-2009
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by Margot Shields, Mark S. Tremblay, Sarah Connor Gorber and Ian Janssen
Body mass index (BMI), a measure of weight in relation to height, is the most widely used indicator of obesity. A BMI-based system1-3 has long been employed to classify adults in categories based on health risk.
In Canada, the prevalence of overweight and obesity is monitored according to this system (Text Table 1). In recent decades, the percentage of Canadian adults classified as having excess weight based on BMI has increased substantially.4
Text table 1 Body mass index (BMI) health risk classification
However, criticism of this classification is that it provides no information about the distribution of body fat.1 Evidence suggests that within the normal, overweight and obese class I categories, health risk rises with the level of abdominal obesity.5,6 Consequently, several organizations, including Health Canada, have recommended that a combination of BMI and waist circumference be used to classify weight-related health risk.1,2,7
Recent research has found that, on average, the waist circumference of Canadian adults with a given BMI value is now larger than in the past,8 similar to findings for the United States.9 Therefore, it is important to measure and monitor abdominal obesity within BMI categories.
This article documents changes in obesity indicators between 1981 and 2007-2009 in Canadians aged 20 to 69 years, based on measured data collected in two population-based health surveys (see The data). The primary objective was to examine changes in abdominal obesity within BMI categories, based on waist circumference, waist-to-hip ratio, and waist-to-height ratio.
Adiposity measures increase
Among men and women, all four adiposity measures increased significantly between 1981 and 2007-2009 (Table 1). However, the pattern of change differed by BMI category.
Among men in the normal-weight category, the means for the four adiposity measures were not statistically different at the two time points. By contrast, among women in the normal-weight category, mean BMI rose by 0.2 kg/m2, and waist circumference, by 4 cm. Significant increases were also observed for waist-to-hip ratio and waist-to-height ratio.
Among those in the overweight category, a significant increase in mean BMI was observed for both sexes. Increases in means for abdominal obesity were particularly notable among overweight women: 7 cm for waist circumference; 0.05 for waist-to-hip ratio; and 0.03 for waist-to-height ratio. Among overweight men, the increases were 2 cm for waist circumference and 0.01 for waist-to-hip ratio; the increase for waist-to-height ratio did not attain statistical significance.
Increases in abdominal obesity measures tended to be greatest for people in the obese categories. For instance, the increase in waist circumference was 11 cm among class II/III men, and 9 cm among class II/III women.
Health risk categories
Health risk ratings for the obesity indicators are presented in Table 2. Ratings for the abdominal obesity indicators are provided within the BMI categories for normal weight, overweight and obese class I. Rating are not presented for obese class II/III, because in both 1981 and 2007-2009, nearly all adults in these categories were at increased/high risk according to the abdominal obesity indicators (data not shown).
Among normal-weight men, the percentages at increased/high health risk based on their waist circumference, waist-to-hip ratio and waist-to-height ratio were not statistically different in 2007-2009 than in 1981. By contrast, among normal-weight women, the percentage at increased/high risk according to each of these measures was approximately three times higher in 2007-2009 than in 1981. Based on their waist circumference, the percentage of normal-weight women at increased/high risk rose from 8% to 23%. The corresponding increases were from 6% to 16% for waist-to-hip ratio, and from 6% to 18% for waist-to-height ratio.
In 2007-2009, 62% of overweight men were at increased/high health risk based on their waist circumference, up from 49% in 1981; the percentage at high risk rose from 8% to 20%. However, the percentage of overweight men at increased/high risk based on their waist-to-hip ratio and waist-to-height ratio was not different at the two time points. Based on their waist circumference, 54% of overweight women were at high risk in 2007-2009, up from 17% in 1981. The percentage of overweight women at increased/high risk based on their waist-to-hip ratio more than doubled from 22% to 51%, and the percentage at increased/high risk based on their waist-to-height ratio rose from 68% to 87%.
In 1981, 62% of men and 82% of women in obese class I were classified at high health risk based on their waist circumference; by 2007-2009, the percentages had risen to 84% and 94%, respectively. Based on their waist-to-hip ratio, the percentage of obese class I men at increased/high risk rose from 90% to 96%; among women, the figure rose from 48% to 75%. At both time points, virtually all men and women in obese class I were classified at increased/high risk based on their waist-to-height ratio.
Shifting distributions
Figure 1 illustrates shifts in the distribution of waist circumference measurements toward the higher risk ranges among people in the normal-weight, overweight and obese class I categories. While the distributions in 1981 and 2007-2009 among normal-weight men were virtually the same, a shift among normal-weight women meant that a higher percentage of them had waist circumferences that put them in the increased and high-risk ranges. Among overweight people of both sexes, there was a shift to larger waist circumferences, on average, particularly among women. For men and women in obese class I, most of the waist circumference distribution fell beyond the high-risk cut-point in 1981. Nonetheless, there was a pronounced shift to even larger waist circumferences, on average, by 2007-2009.
Trends in the distributions for waist-to-hip ratio and waist-to-height ratio measurements within BMI categories were similar (data not shown). As well, the distribution of BMI values within BMI categories shifted toward higher values. For example, among normal weight women, the percentage whose BMI was at the high end of the category (24 kg/m2 or more) rose from 13% in 1981 to 21% in 2007-2009.
Conclusion
Current surveillance of obesity in Canada relies almost exclusively on BMI. This study reveals that abdominal obesity within BMI categories is increasing. As a result, Canadians with a given BMI are, on average, at higher risk of obesity-related health conditions than they were in 1981. The Canadian body weight classification system and the Canadian clinical practice guidelines recommend that adults' waist circumference also be measured to assess obesity-related health risks .1,2 The results of this study underscore the importance of measuring and monitoring abdominal obesity within BMI categories.
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