Optimum BMI and Healthy Weight

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The concept of optimum BMI can be applied to populations. For countries such as the United States, where undernutrition is not as common as in developing countries,3 a BMI-distribution median of around 21 kg/m2 may be optimal (WHO, 2000). Population weight goals for obesity prevention in adults can also be stated in terms of decreasing the proportion that exceed the threshold of 30 kg/m2, although this goal includes both preventing new cases of obesity and reducing weight among those already over the threshold.

The same principles are appropriate for assessing the population of children in the United States in pursuit of the committee's primary objective: to stop, and eventually reverse, current trends toward higher BMI levels. Also, as discussed in Chapter 2, there are particular concerns about the population of obese children becoming heavier. Achieving this objective would have the effects of reducing the mean BMI as well as decreasing the proportion of children and youth in the population that exceeds the threshold definition of obesity.

Available research does not currently allow the committee to define an optimum BMI for children and youth. It suggests, however, that future research toward this aim should be focused on defining the associations between BMI and objective measures of concurrent and future growth and between BMI and physiological and psychological morbidity, mortality, and health (Robinson, 1993; Robinson and Killen, 2001).

Analogous to the current practice for adults, the committee recommends the use of BMI for assessing individual and population changes in children and youth over time and in response to interventions. Population weight goals for childhood obesity prevention should be stated in terms of changes in the mean BMI and in the shape of the entire BMI distribution. Alternatively, goals can be stated in terms of decreasing the proportion of children or youth who exceed particular thresholds—e.g., 75th, 85th, 90th, 95th, or 97th percentiles of BMI for age and gender on the CDC BMI charts. In the absence of an appropriate evidence base, however, threshold goals are necessarily somewhat arbitrary and sacrifice substantial information about the rest of the distribution as well as substantial statistical power to detect differences between groups and over time (Robinson and Killen, 2001).

3Hunger and food insecurity persist in the United States. In 2002, 35 million individuals including 13.1 million children lived in food insecure households (an estimated 11 percent of all U.S. households); 3.5 percent (3.8 million) of U.S. households were food insecure with hunger (Nord et al., 2003). Additionally, rates of micronutrient deficiencies remain unaccept-ably high in certain subgroups of the U.S. population (Wright et al., 1998; Ballew et al., 2001; Ganji et al., 2003; Hampl et al., 2004).

The current CDC guidelines for healthy weight in children and youth are in the range of the 5th to 85th percentiles of the age- and gender-specific BMI charts. Therefore, a child whose weight tracks in that range—that is, he or she does not cross to lower than the 5th or higher than the 85th percentiles—would be considered to be in the healthy weight range according to these definitions.

The CDC BMI charts are mathematically smoothed curves of the pooled growth parameters of children and adolescents sampled in cross-sectional national health surveys conducted from 1963 to 1994. An analogy would be to consider the curves as compiled from a series of "snapshots" of large national samples made at different times over three decades. But because the sample sizes at each age level get much smaller at the extremes of the distributions, the growth curves may be more prone to errors at the upper and lower ends.

Because of the increases in body weight that occurred in the 1980s and 1990s—after the second National Health and Nutrition Examination Survey (NHANES II) conducted in 1976-1980—a decision was made not to include the NHANES III (1988-1994) body-weight data in the revised 2000 BMI charts for children aged 6 years or older. The NHANES III data would have shifted the affected curves (weight-for-age and BMI-for-age) upward, which was considered to be biologically and medically undesirable. However, the fact that the CDC BMI charts were developed from data for a prior time period in which children were leaner, on average, leads to an occasionally confusing situation—for example, where more than 5 percent of the population is above the 95th percentile—but this is readily clarified in the context of the charts' historical source.

The CDC BMI charts are derived from cross-sectional samples of children (data for different age groups are based on different children). That is, they do not directly represent the longitudinal growth trajectory for the same set of children who have been measured as they age.4 Therefore, it is not known whether an individual child's height, weight, or BMI should be expected to follow along the same percentile curve over time in order to maintain health or whether there are health implications of variations throughout childhood (e.g., crossing percentiles by going from the 20th percentile at age 1 to the 60th percentile at age 5 to the 40th percentile at age 12). Mei and colleagues (2004) found that shifts in growth rates were

4The latter approach has been used to develop longitudinal growth charts that are used in several other countries (Tanner and Davies, 1985; Cameron, 2002). These types of charts are generally developed from smaller, and potentially less representative, samples.

common during birth to 6 months and less common in children aged 2 to 5 years. More research is needed to determine whether there is an increased prevalence of "crossing" percentiles in different populations or during different age intervals and whether there are associations between crossing percentiles and health-related outcomes.

The problem is how to proceed despite this lack of certainty. The committee concluded that because the CDC BMI charts are based on large national samples of the U.S. population of children and youth, they are the best available tools for assessing growth in clinical and public health settings. Although there are many unknowns about how to apply this information to individual children, and clinicians face difficulties in making generalizations regarding normal growth trajectories, experience suggests that children who demonstrate rapid changes—that is, frequently crossing up or down percentiles—may require special health-care attention. Health-and medical-care professionals should be consulted regarding growth-related questions for individual children as they can assess a child's own growth trajectory in context (see Chapter 6).

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