BMI-referenced standards for recommended pedometer-determined steps/day in children
Introduction
The worldwide obesity epidemic includes a trend towards increased prevalence of overweight and obesity in youth [1], [2]. Although obesity is emerging as a problem in developing countries [3], [4] as well, the very high and increasing prevalence (regardless of specific standards used to define overweight/obesity) reported in developed countries is cause for more immediate concern. For example, in the USA, direct measures of body mass and height obtained through the National Health and Nutrition Examination Survey (NHANES) indicate that ≈ 15% of 6–19 year olds were classified as overweight (95th percentile of age- and sex-specific body mass index derived from five previous NHANES surveys) in 1999–2000, up ≈ 5% from 1988 to 1994 [5]. Using the 85th percentile (as a cut point for overweight) and the 95th percentile (as a cut point for obesity) derived from international standards [6], 19–23% of Australian children and adolescents were overweight or obese [7]. Using the 91st and 98th percentiles of body mass reference curves for the UK [8] to, respectively, define overweight and obesity from self-reported height and weight, ≈ 11% of Swedish boys and ≈ 5% of Swedish girls ages 12, 15, and 18 years were overweight; the corresponding values for obesity were ≈ 8% and ≈ 4% [9].
A potential contributor to this epidemic is a shift towards ever-diminishing physical activity behaviors [10]. For example, only 21–22% of American youth participate in physical education classes [11], [12] and a 1993 report suggests that fifth grade children are moderately or vigorously active less than 9% of actual class time [13]. Transportation surveys in the US indicate an increase in the use of motorized vehicles, including for purposes of chauffeuring children [14] and the number of trips made by children by foot or by bicycle has declined 37% between 1977 and 1995 [15]. In fact, ≈ 50% of 5- to 15-year-old American school children are driven to school in privately owned motor vehicles compared to only ≈ 10% who walk [16]. Most recently, a report of school transportation modes in a Southern US state indicates that almost 42% of children living ≤ 1 mile from school are usually driven there in a private motor vehicle [17]. Finally, 65–67% of 8- to 16-year-old American youth watch television ≥2 h/day [18], [19]. Such elevated levels of television viewing have been related to youth overweight and obesity [18], [19], [20], [21], [22].
In 1996, the US Surgeon General [23] endorsed public health recommendations [24] that all individuals over the age of 2 years accumulate 30 min or more of at least moderate intensity activity, on most, if not all, days of the week. Similar recommendations were made specifically for adolescents [25]. Accumulating evidence suggests that most young people meet these recommendations, however, leading to questions about their appropriateness [26]. Since that time, researchers have proposed that the recommendations be modified to higher levels of duration [26] and/or intensity [27] for optimal functioning. In 1998, a convened symposium in the UK recommended that all young people should engage in physical activity of at least moderate intensity for 1 h per day [28]. Guidelines established by the National Association for Sport and Physical Education (NAPSE) in 1993 [29] agreed that elementary school children should be active for at least 30–60 min daily and the most recent revision of that document suggests the accumulation of at least 60 min and up to several hours of moderate to vigorous physical activity daily [30]. A recent report from the Institute of Medicine reinforces this higher recommendation (i.e., at least 1 h daily) for both adults and children [31] if body fat maintenance is the objective.
Setting appropriate benchmarks or cut points for health-related physical activity is hampered by imperfect assessment of this behavior. Most of the data previously collected has been based on self-reported surveys, the limitations of which are well-known [32], especially for younger children [33]. In recent years, there has been an increased interest in objective monitoring of daily physical activity using simple and inexpensive pedometers [34], [35], [36]. Pedometers are small, light-weight, unobtrusive instruments that have been accepted by researchers as practical physical activity assessment tools that are particularly sensitive to ambulatory activities [37], [38], [39]. Accumulated evidence indicates that pedometers are acceptably agreeable compared to accelerometers, direct observation, measures of energy expenditure, and self-report [40]. Correct interpretation of steps/day in relation to desired health outcomes demands the development of appropriate and pedometer-specific cut points. Although 10,000 steps/day appears to be readily accepted by the media [41], [42], [43], [44] and can be traced to Japanese health promotion efforts [45], emerging data on children indicate that this value is likely too low to elicit substantial health benefits in this population segment. Limited evidence shows that 8- to 10-year-old UK children take 12,000–16,000 steps/day (lower for girls than boys) [46]. The 2001–2002 President's Challenge Physical Activity and Fitness Awards Program [47] also recognized that the popular 10,000 steps/day cut point was likely too low for children by recommending instead that children accumulate 11,000 steps (for girls) to 13,000 steps (for boys) at least 5 days a week for a standard healthy base. These recommendations were based on norm-referenced standards determined from a US sample [48]; the validity of even this level of steps/day to prevent or decrease overweight and obesity in youth is unknown.
In an attempt to answer, “How many steps are enough?”, we can turn to experts who advocate the use of appropriate statistical techniques for establishing criterion-referenced performance standards on physical fitness tests [49], [50], [51], [52]. Criterion-referenced standards are set based on their likelihood to elicit a specific benefit [53], and are therefore preferable to norm-referenced standards. Simply put, recommended levels of steps/day should emerge from data related to important health outcomes (in this case BMI, an accepted indicator of relative obesity). Therefore, the purpose of the present study was to establish preliminary criterion-referenced standards for physical activity using pedometer-assessed steps/day related to healthy body composition in youth as indicated by international cut points for BMI [6].
Section snippets
Methods
This is a secondary analysis of an existing data set of 1954 children (995 girls, 959 boys; ages 6–12 years) from the USA, Australia, and Sweden. Recruitment and data collection methods have been described previously in detail [54] and are summarized again here. The previous paper focused on between-country differences in steps/day and BMI. The statistical analysis undertaken herein has not been performed previously and is therefore novel.
Results
Mean ± SD steps/day are presented in Table 1 for each sex and age group, stratified for weight status as either normal weight or overweight/obese. The t test results were consistently significant for all sex and age strata with only two exceptions (6- and 7-year-old girls). In both these cases, the number of subjects was small compared to other strata.
A summary of the analytical steps (i.e., probability of correct decisions, misclassification of errors, validity coefficient, utility analysis)
Discussion
In this secondary analysis of an existing international database, we used accepted analytical procedures to establish BMI-referenced standards for pedometer-determined physical activity in youth. Such cut points are necessary to guide surveillance, intervention, and evaluation. For example, the proportion of youth meeting or falling short of set cut points can be reported, cut points can be used as behavior change goals during intervention, and they can be consulted when evaluating the impact
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