The furor over our nation’s weight crisis seems to have died down, and schools and hospitals have started to implement elements of the First Lady’s “Let’s Move!” campaign. But we should not let this lull lead us to believe that Americans-especially our children-are now healthier and normal weight. We know that approximately one-third of children in the U.S. are overweight or obese, a staggering statistic that urges researchers, policy makers, pediatricians and parents to better understand how we can raise healthier children.

While many studies have examined the rates of overweight and obesity in children in the U.S., few have tried to pinpoint a specific time when children become fat. A recent study published in the December 2011 issue of Pediatrics sought to determine the critical period, if any, when children experience a significant body mass index (BMI) gain.

Body mass index is a mathematical calculation that measures a person’s body fat to determine his weight status (underweight, healthy weight, overweight or obese). Using English units, BMI = weight in lbs ÷ (height (in.) x height (in.)) x 703. A child’s BMI is compared to that of other children of the same age and gender. Underweight (BMI < 18.5) is considered less than the 5th percentile, healthy weight is considered 5th-84th percentile (BMI 18.5-24.9), overweight is considered 85th to less than 95th percentile (BMI 25.0-29.9), and obese is considered 95th percentile and higher (BMI 30 or higher). For example, an 11-year-old female that is 5’1″ and weighs 99 pounds has a BMI of 18.7, which is in the 62nd percentile and falls within the healthy weight category.

Since an overweight or obese child has a greater risk than a healthy weight child of being obese as an adult, public health efforts are being directed to prevent overweight in young children. The study looked at data from the Early Childhood Longitudinal Study-Kindergarten Class (ECLS-K), which followed a single, nationally representative cohort of kindergartners for nine years (1998-9 through 2006-7). The researchers from Pardee RAND Graduate School and RAND Corporation specifically looked at changes across BMI distributions to determine where BMI gains might be concentrated and if there are “critical periods” when
there might be a large gain in BMI percentile.

Without even looking at the results, it is surprising that nearly 40% of the kindergartners had BMIs in the 75% percentile or higher (25% should have BMIs in that quartile). Just 14% of the kindergartners had BMIs less than 25% quartile. The researchers found that the largest BMI gains were among the top quartile children in 1st through 3rd grade (5.8 percentage point gain); those that were already overweight or obese as kindergartners, as well as those were normal weight, gained significant weight between 1st and 3rd grade, and gained marginally more weight from 3rd through 5th grades. Further, obesity prevalence increased nearly 50% from kindergarten through 3rd grade, from 11.9% to 17.6%.

The starting weight status of the kindergartners may be as eye opening as their change in BMI status over nine years. The data showed that 50% of Hispanic boys were in the top BMI quartile (75th-100th percentile) in kindergarten, double the historical growth chart rate. From kindergarten to 5th grade, there were significant changes in BMI distribution for black, white and Hispanic boys, but by middle school there were still significant BMI changes for white and Hispanic boys, but not black boys.

Different from boys, the data showed that in kindergarten there were more black girls (45.5%) in the top quartile than Hispanic (43.4%) or white (35.2%) girls. Researchers found that there were increases in proportions of girls in the third quartile (50%-75% percentile) among white, but not black or Hispanic, girls. Most notably, they found that similar to boys, 1st through 3rd grades seemed to be a “critical time” for excess weight gains for Hispanic girls, but not white or black girls.

The researchers concluded that early elementary school may be an excellent time to focus on weight intervention, given that 1st through 3rd grades seem to be a critical period of weight gain for children. But, as the data revealed, too many children are overweight even by age five, before they enter formal education where such intervention programs may be in place. Public health efforts should also stress to parents the importance of providing a healthy, balanced diet and encouraging daily exercise that should begin as soon as their child can walk, not at age five when it may already be too late.

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From issue: 27/11-12