Thursday, October 1, 2009

Childhood Overweight and Obesity

Obesity is a serious health concern for children and adolescents. Data from NHANES surveys (1976–1980 and 2003–2006) show that the prevalence of obesity has increased: for children aged 2–5 years, prevalence increased from 5.0% to 12.4%; for those aged 6–11 years, prevalence increased from 6.5% to 17.0%; and for those aged 12–19 years, prevalence increased from 5.0% to 17.6%.1, 46

Obese children and adolescents are at risk for health problems during their youth and as adults. For example, during their youth, obese children and adolescents are more likely to have risk factors associated with cardiovascular disease (such as high blood pressure, high cholesterol, and Type 2 diabetes) than are other children and adolescents.2

Obese children and adolescents are more likely to become obese as adults.3, 4 For example, one study found that approximately 80% of children who were overweight at aged 10–15 years were obese adults at age 25 years.3 Another study found that 25% of obese adults were overweight as children.5 The latter study also found that if overweight begins before 8 years of age, obesity in adulthood is likely to be more severe.

This Web site provides information about childhood overweight and obesity, including how overweight and obesity are defined for children, the prevalence of obesity, the factors associated with obesity, and the related health consequences.

Defining Childhood Overweight and Obesity

Body mass index (BMI) is a practical measure used to determine overweight and obesity. BMI is a measure of weight in relation to height that is used to determine weight status. BMI can be calculated using either English or metric units. BMI is the most widely accepted method used to screen for overweight and obesity in children and adolescents because it is relatively easy to obtain the height and weight measurements needed to calculate BMI, measurements are non-invasive and BMI correlates with body fatness.6 While BMI is an accepted screening tool for the initial assessment of body fatness in children and adolescents, it is not a diagnostic measure because BMI is not a direct measure of body fatness.

Use of BMI to Screen for Overweight and Obesity in Children

For children and adolescents (aged 2–19 years), the BMI value is plotted on the CDC growth charts to determine the corresponding BMI-for-age percentile.

Overweight is defined as a BMI at or above the 85th percentile and lower than the 95th percentile.
Obesity is defined as a BMI at or above the 95th percentile for children of the same age and sex.
These definitions are based on the 2000 CDC Growth Charts for the United States and expert committee.7 A child's weight status is determined based on an age- and sex-specific percentile for BMI rather than by the BMI categories used for adults. Classifications of overweight and obesity for children and adolescents are age- and sex-specific because children's body composition varies as they age and varies between boys and girls.

Contributing Factors

At the individual level, childhood obesity is the result of an imbalance between the calories a child consumes as food and beverages and the calories a child uses to support normal growth and development, metabolism, and physical activity. In other words, obesity results when a child consumes more calories than the child uses. The imbalance between calories consumed and calories used can result from the influences and interactions of a number of factors, including genetic, behavioral, and environmental factors.10 It is the interactions among these factors – rather than any single factor – that is thought to cause obesity.11

Genetic Factors

Studies indicate that certain genetic characteristics may increase an individual's susceptibility to excess body weight.12, 13 However, this genetic susceptibility may need to exist in conjunction with contributing environmental and behavioral factors (such as a high-calorie food supply and minimal physical activity) to have a significant effect on weight. Genetic factors alone can play a role in specific cases of obesity . For example, obesity is a clinical feature for rare genetic disorders such as Prader-Willi syndrome.14 However, the rapid rise in the rates of overweight and obesity in the general population in recent years cannot be attributed solely to genetic factors. The genetic characteristics of the human population have not changed in the last three decades, but the prevalence of obesity has tripled among school-aged children during that time.8, 46

Behavioral Factors

Because the factors that contribute to childhood obesity interact with each other, it is not possible to specify one behavior as the "cause" of obesity . However, certain behaviors can be identified as potentially contributing to an energy imbalance and, consequently, to obesity .

Energy intake: Evidence is limited on specific foods or dietary patterns that contribute to excessive energy intake in children and teens. However, large portion sizes for food and beverages, eating meals away from home, frequent snacking on energy-dense foods and consuming beverages with added sugar are often hypothesized as contributing to excess energy intake of children and teens.15 In the area of consuming sugar-sweetened drinks, evidence is growing to suggest an association with weight gain in children and adolescents.16, 17, 18 Consuming sugar-sweetened drinks may be associated with obesity because these drinks are high in calories.19 Children may not compensate at meals for the calories they have consumed in sugar-sweetened drinks, although this may vary by age.20, 21 Also, liquid forms of energy may be less satiating than solid forms and lead to higher caloric intake.22, 23
Physical activity: Participating in physical activity is important for children and teens as it may have beneficial effects not only on body weight, but also on blood pressure and bone strength.24 Physically active children are also more likely to remain physically active throughout adolescence and possibly into adulthood.25

Children may be spending less time engaged in physical activity during school. Daily participation in school physical education among adolescents dropped 14 percentage points over the last 13 years — from 42% in 1991 to 28% in 2003.26 In addition, less than one-third (28%) of high school students meet currently recommended levels of physical activity.27
Sedentary behavior: Children spend a considerable amount of time with media. One study found that time spent watching TV, videos, DVDs, and movies averaged slightly over 3 hours per day among children aged 8–18 years.28 Several studies have found a positive association between the time spent viewing television and increased prevalence of obesity in children.29, 30, 31 Media use, and specifically television viewing, may
displace time children spend in physical activities,32, 33
contribute to increased energy consumption through excessive snacking and eating meals in front of the TV,34, 35
influence children to make unhealthy food choices through exposure to food advertisements,15 and
lower children's metabolic rate.36

Help kids stay active.

Children and teens should participate in at least 60 minutes of moderate intensity physical activity most days of the week, preferably daily.11 Remember that children imitate adults. Start adding physical activity to your own daily routine and encourage your child to join you.

Some examples of moderate intensity physical activity include:

Brisk walking
Playing tag
Jumping rope
Playing soccer

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