In the United States there is a trend toward increasing physical inactivity, television viewing, use of electronic devices, and consumption of convenience foods which are high in fat and sugars. The culmination of these trends may be contributing factors to the increased prevalence of obesity throughout the United States. Body mass index (BMI) is often used as an indicator of obesity. BMI is calculated as weight in kilograms divided by height in meters squared. Adults are classified as overweight or obese when their BMI is > 25 and > 30, respectively. Children and adolescents are classified as overweight or at risk for being overweight when their BMI is greater than the 95 th or 85 th percentile, respectively, as determined by the Centers for Disease Control (CDC) BMI-for-age growth charts.
According to the 1999 – 2000 National Health and Nutrition Examination Survey (NHANES) data released in 2002, 64% of adults 20 years of age and older and 15% of children and adolescents 6 – 19 years of age are overweight. This is a 10% increase for children and adolescents and a 16% increase for adults classified as overweight since the NHANES II survey conducted between 1976 – 1980. 5 Of the 64% of adults who are overweight, 31% (about 59 million people) are classified as obese. There are genetic and environmental (behavioral) factors that contribute to body composition of an individual.
Studies demonstrate that genetics accounts for about 60 – 70% of body fat and BMI within twin pairs. In addition, there appears to be more heterogeneity in percent body fat between pairs of twins than within pairs of twins when levels of physical activity are controlled for within and between pairs of twins. It has also been shown that within a family there is a positive correlation in percent body fat between parents and children and between siblings. In addition, it has been reported that 62% of percent body fat trends within families is determined by genetics and familial environment. In spite of a significant genetic component to body composition, lifestyle factors also significantly contribute to body composition. For example, recent studies have reported that television viewing time and participation in physical activity appear to have a significant impact on body composition.
Most studies estimated percent body fat using bioelectrical impedance or skinfolds measurements, or used BMI as an indicator of obesity. Few studies have used dual-energy x-ray absorptiometry (DXA) for assessing body composition. It would be beneficial to estimate body composition of adolescents using a reference method such as the DXA.
Knowledge of factors that contribute to differences in body composition between individuals and between families could lead to a greater understanding of current trends in body composition and appropriate interventions. The variance between and within families has not yet been reported. Therefore, the purpose of this study was to determine if the variance in percent body fat as measured by DXA
between families is similar to the variance within families. In addition, a secondary purpose was to determine the contribution of physical activity and television viewing to the variance in body composition.
Contents
Variance in Percent Body Fat Between and Within Families as Measured by Dual-Energy X-Ray Absorptiometry
- Abstract
Introduction
Methods
Results
Discussion
References
Appendix A Prospectus
- Introduction
Review of Literature
Methods
References
Appendix A-1 Assent for Participation in Research
Appendix A-2 Physical Activity Questionnaire
Appendix B Summary of Statistical Results
Download
Variance in Percent Body Fat Between and Within Families
