Body Weight And Body Fat Classification Of School-Age Swimmers

Nutrition Research Newsletter, March, 2000

There is strong evidence that overweight children also tend to be overfat. Obesity is defined as excess body fat in relation to lean body mass. Overweight is defined as body weight above an acceptable weight in relation to height. Athletic children with a low ratio of body fat to lean body mass, however, may exceed their recommended weight range without being overfat. It is therefore possible that athletic children with low proportions of body fat may be misclassified as obese when age-specific height and weight reference data are used. Recently, researchers hypothesized that given the rigorous year-round workout routine of swimmers, a significant proportion of athletic children who are within a desirable range for body fat may exceed their recommended weight classification--based on the National Center for Health Statistics (NCHS) height-weight tables and body mass index (BMI) standards--and therefore be misclassified as overweight. To explore this hypothesis, a study was conducted to determine the frequency in which athletic children may be classified into inappropriate weight categories by comparing weight classifications with estimated body fat classifications.

A convenience sample of 114 competitive school-age club swimmers was recruited from swim teams in eastern Washington and northern Idaho. The average age of the female subjects was 12.7 years and the males averaged 12.5 years. The sample was more than 96% white. The subjects spent an average of 8.0 hours a week training. Information regarding age, race, hours of training per week, and level of swimming achievement was collected. Anthropometric measures included weight, height, a triceps skinfold thickness, and medial calf skinfold thickness. Subjects exceeding the 85th percentile for BMI, weight for age, and weight for height were classified as overweight. The male subjects with greater than 25% body fat and the female subjects with greater than 32% body tat were classified as obese.

Based on NCHS weight-for-age guidelines, 30 girls (35.3%) and 22 boys (37.3%) were classified as overweight. When the NCHS weight-for-height chart was used, the number of subjects classified as overweight was somewhat lower for girls than for boys. Based on BMI values, 19 girls (22.4%) and 16 boys (27.1%) were classified as overweight. In regard to weight for age and body fat, the majority of girls and boys were classified as not overweight and not overfat. However, 20 girls (24.1%) and 8 boys (13.5%) were classified as overweight but not overfat. In regard to weight for height and body fat, 78.3% of girls and 69.5% of boys were classified as not overweight and not overfat. The same index classified 10.8% of girls and 1.7% of boys as overweight but not overfat. According to BMI, 78.3% of girls and 55.9% of boys were not overweight and not overfat. Nine girls (10.8%) and one boy (1.7%) were classified as overweight but not overfat. Only one girl was classified as overfat but not overweight, as compared with 10 boys (16.9%).

The results indicate that most subjects classified as not overweight were also classified as not overfat. Less than 11% of the girls and less than 2% of the boys were classified as overweight but not overfat. Weight for height was the body weight index that classified the greatest number of subject consistently. There were no trends observed in terms of age, training hours, or level of swimming achievement among subjects classified as overweight but not overfat. Obesity among girls in the study was less than reported in other general population studies, possibly explained by the effect of athletic training on body composition. The boys, however, had a higher rate of obesity than the national average, and a lower percentage of overweight than the national average. In addition, more male than female swimmers were classified as overfat. It is possible that the criterion used to classify obesity in prepubescent boys is too low. It is currently 7% lower than the criterion for prepubescent girls. Ninety percent of the male swimmers that were classified as overfat were younger than 12 years. The mean percentage body fat in male swimmers older than 13 years was significantly lower than mean percentage body fat in female swimmers. Weight-for-age guidelines classified more subjects as overweight compared with BMI and weight for height. Therefore weight for age, a useful tool for longitudinal assessment, may be limited as a normative assessment.

J. Richardson, K. Beerman, C. Heiss, J. Shultz, Comparison of Body Weight and Body Fat Classification of Competitive School-Age Club Swimmers, JADA 100 (2): 237-239 (February 2000) [Correspondence: Kathy Beerman, PhD, Dept. of Food Science and Human Nutrition, Washington State University, Pullman, WA 99164-6376.]