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Overweight Canadian children and adolescents
by Margot Shields
Notable rise among adolescents
Over the past 25 years, the prevalence of overweight and obesity in children and adolescents has risen, with the most substantial increases observed in economically developed countries.1 According to the results of the 2004 Canadian Community Health Survey: Nutrition (CCHS), a substantial share of Canadian youth are part of this trend.
The 2004 CCHS was the first time in many years that interviewers directly measured the height and weight of a nationally representative sample of Canadians (see Data sources and analytical techniques). In the past, most health surveys relied on respondents to report their height and weight, a practice that tended to underestimate the prevalence of overweight and obesity among adolescents and adults2, 3, 4, 5 (see Methodology makes a difference).
The last time that the height and weight of a nationally representative sample of Canadian children and adolescents (aged 2 to 17) were directly measured was in 1978/79 as part of the Canada Health Survey. Results from that survey and the 2004 CCHS can be compared to get a better picture of the increase of overweight and obesity among young Canadians during the past quarter century.
In 1978/79, 12% of 2- to 17-year-olds were overweight, and 3% were obese—a combined overweight/obesity rate of 15%. By 2004, the overweight rate for this age group was 18% (an estimated 1.1 million), and 8% were obese (about half a million)—a combined rate of 26%.
Increases in overweight and obesity were similar among boys and girls (Chart 1). In 2004, the combined overweight/obesity rate for each sex was about 70% higher than it had been in 1978/79, and the obesity rate was 2.5 times higher. However, trends differed for various age groups.
For example, the percentage of children aged 2 to 5 who were overweight/obese remained virtually unchanged. By contrast, the overweight/obesity rate of adolescents aged 12 to 17 more than doubled from 14% to 29%, and their obesity rate tripled from 3% to 9% (Chart 2).
Overweight and obesity are based on body mass index (BMI), a measure that takes weight and height into account (BMI = weight in kilograms divided by height in metres squared). For adults aged 18 or older, BMI cut-offs of 25 and 30 are used to classify individuals as overweight and obese, respectively, based on health risks associated with being in these weight classes.6 For children and adolescents, the cut-offs are lower, and they also account for age (see Calculating overweight and obesity in children and adolescents).
The average BMI of adolescents aged 12 to 17 rose from 20.8 in 1978/79 to 22.1 in 2004. This produced a shift in the BMI distribution of the age group toward the heavy end of the continuum. The most pronounced increases were in the percentages of adolescents whose BMI exceeded 25 or 30, the overweight and obese thresholds for adults (Chart 3). This is of particular importance, given that adolescence is a critical period for the development of adult obesity.1, 7, 8, 9, 10
Youth overweight and obesity rates varied across the country, with the highest rates tending to be in the Atlantic provinces. In 2004, the combined overweight/obesity rate of 2- to 17-year-olds was significantly above the national level (26%) in Newfoundland and Labrador (36%), New Brunswick (34%), Nova Scotia (32%), and also, Manitoba (31%) (Chart 4). The prevalence of obesity was significantly higher than the national figure (8%) in Newfoundland and Labrador (17%) and New Brunswick (13%).
The combined overweight/obesity rate was significantly below the national level in Québec (23%) and Alberta (22%), but the obesity rate in these provinces was similar to the national rate.
Since the early 1960s, the height and weight of a nationally representative sample of Americans have been directly measured as part of the National Health and Nutrition Examination Survey (NHANES). Based on the most recent NHANES data (1999-2002), the combined overweight/obesity rate of 2- to 17-year-olds was similar in the United States and Canada (Chart 5), but the American obesity rate was slightly higher (10% versus 8%).
Overweight/obesity and obesity rates for boys in the two countries were similar (Chart 6). However, for girls, the overweight/obesity rate in Canada was higher at ages 2 to 5, but lower at ages 12 to 17. At ages 12 to 17, American girls were almost twice as likely as Canadian girls to be obese: 13% versus 7%.
For young people of both sexes in the United States, the prevalence of overweight and obesity increased with age. The overweight/obesity rate of American boys rose from 14% at ages 2 to 5 to 33% at ages 12 to 17; for American girls, the increase was from 17% to 31%. In Canada, too, boys’ overweight/obesity rate rose from 19% at ages 2 to 5 to 32% at ages 12 to 17. By contrast, Canadian girls’ rate was relatively stable at around 25% regardless of age.
If the prevalence of overweight and obesity among youth is still increasing, differences between Canada and United States may be greater, because the American rates are based on earlier data (collected from 1999 to 2002).
Another factor in comparisons between the two countries is the ethnic composition of the population. In the United States, Black, Hispanic and Mexican-American children and adolescents had relatively high overweight/obesity rates (more than 30%) (Chart 7). These groups represent about one-third of American youth, but constitute a very small share of the population in Canada. When comparisons were made between white Canadian and American youth, the overweight/obesity and obesity rates did not differ significantly.
In Canada, young people of Aboriginal origin (off-reserve) had a significantly high combined overweight/obesity rate (41%). Their obesity rate was 20%, two and a half times the national average. By contrast, youth of Southeast Asian or East Asian origin had a low overweight/obesity rate (18%) (Chart 8). These differences between ethnic groups persisted when age and socio-economic factors were taken into account. However, because of the relatively small numbers in these ethnic groups, the national overweight/obesity and obesity rates were not strongly influenced by these differences.
American data show that children’s consumption of fast food has increased dramatically over the past two decades, and that a large majority of children and adolescents do not have an adequate number of daily servings of fruit and vegetables.1, 10 Based on data from the 2004 CCHS, 59% of Canadian children and adolescents were reported to consume fruit and vegetables less than five times a day (see Definitions). These young people were significantly more likely to be overweight/obese or obese than were those who ate fruit and vegetables more frequently (Chart 9).
Some studies have found that physical activity is protective against childhood obesity,11 while others have not found such a relationship1 (see Limitations). Analysis of CCHS data shows that physical activity levels were not associated with overweight and obesity at ages 6 to 11 (Chart 10), but by ages 12 to 17, associations were significant, though only for boys (Chart 11). Sedentary boys were more likely than active boys to be obese (16% versus 9%). Unexpectedly, a higher proportion of active and moderately active boys were overweight (but not obese), compared with boys who were sedentary.
Watching television, playing video games and using the computer are common pastimes for many Canadian children. Time spent on these activities is referred to as “screen time.” In 2004, over a third (36%) of children aged 6 to 11 logged more than 2 hours of screen time each day (Chart 12). These children were twice as likely to be overweight/obese as were those whose daily viewing totalled an hour or less (35% versus 18%), and about twice as likely to be obese (11% versus 5%).
For adolescents aged 12 to 17, screen time was measured on a weekly basis. Their overweight/obesity rates ranged from 23% of those whose viewing amounted to fewer than 10 hours a week to 35% of those who spent 30 or more hours a week in front of a screen (Chart 13).
The relatively recent introduction and rapid proliferation of video games and home computers make it difficult to track trends in screen time. In 1988, the Campbell’s Survey on Health and Well-being asked 12- to 17-year-olds how many hours they watched television—the weekly average was 9. In 2004, average weekly television hours were almost the same, at 10. However, when time spent on the computer and playing video games is included, adolescents’ total average screen time doubles to 20 hours a week.
All associations between these lifestyle factors (fruit and vegetable consumption, physical activity and screen time) persisted when the effects of age and socio-economic status were controlled.
For adults, lower socio-economic status tends to be associated with obesity. While the same relationship has been observed for children, the association is usually not as strong, and results have been inconsistent.12, 13, 14
According to the 2004 CCHS, children and adolescents in middle-income households were more likely to be overweight/obese or obese than were those in high-income households (Chart 14). Overweight/obesity rates and obesity rates for youth in low-income and high-income households were similar.
The pattern was clearer for education. Young people in households where no members had more than a high school diploma were more likely to be overweight/obese than were those in households where the highest level of education was postsecondary graduation (Chart 15).
In 2004, 18% of adolescents aged 12 to 17 reported that they had at least one diagnosed chronic condition. This rate did not vary significantly whether adolescents were in the normal weight range, overweight (not obese) or obese. Nevertheless, young people’s perceptions of their health did differ, depending on their weight (Chart 16). Boys who were obese were much less likely than those whose weight was in the normal range to report their health as excellent or very good. For girls, diminished health perceptions were evident not only among those who were obese, but also among those who were overweight. These associations between weight and health perceptions persisted for both sexes when socio-economic status and the presence of a chronic condition were taken into account.
The past 25 years have seen a considerable increase in the percentage of Canadian children and adolescents who are overweight or obese. The increase was particularly notable among 12- to 17-year-olds, whose overweight/obesity rate more than doubled, and whose obesity rate tripled.
The burden to the health care system of childhood obesity is difficult to quantify because the related physical health problems are usually not evident until later in life. Nonetheless, the upturn in the prevalence of overweight/obesity among young people is important because overweight/obesity in adolescence often persists into adulthood.1, 7, 8, 9, 10 A recent Canadian study based on longitudinal data found that once an adult is overweight, further weight gain is likely; very few return to the normal weight range.15
However, some of the factors associated with overweight and obesity among young people are modifiable. Increased consumption of fruit and vegetables, more physical activity and less time devoted to sedentary activities such as watching television and playing video games may help reverse the upward trend.
The author thanks Larry MacNabb and Dr. Mark Tremblay for their assistance and helpful suggestions during the analysis, and Wayne Millar for his help in producing the variances estimates based on SUDAAN.
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