Yost et al. BMC Public Health 2010, 10:465 http://www.biomedcentral.com/1471-2458/10/465 Background Due to the increased worldwide prevalence of over- weight and obesity, health care officials consider over- weight and obesity a global epidemic [1,2], especially among young people. The latest estimates among chil- dren and adolescents in 34 countries indicate a preva- lence of overweight ranging from 5.1% to 25.4% and obesity from 0.4% to 7.9%. Overweight and obesity also represent a national epidemic in the United States, where the prevalence of overweight and obesity has almost tripled among children and adolescents in the last 30 years. Data from the National Health and Nutri- tion Examination Survey indicate the prevalence of over- weight U.S. adolescents (12 to 19 years old) increased from 6.1% in 1971-1974 to 18% in 2005-2006, placing the United States second among countries with the highest incidence of overweight and obesity (25.1%) [1,3] Under the influence of numerous factors, overweight and obesity develop from an imbalance between energy intake and expenditure [4,5]. Overweight and obesity in childhood and adolescence can lead to consequences extending into adulthood, such as type 2 diabetes, hypertension, atherosclerosis, and poor quality of life [4]. To avoid such consequences, developing effective interventions for overweight and obese children and adolescents has become a major public health issue [6]. Results from recent systematic reviews demonstrate that no single, particular intervention or strategy successfully assists overweight or obese adolescents in losing weight [7,8]. Instead, researchers suggest that effective interven- tions for adolescents consist of multiple components, including behaviour skills, behaviour change, and paren- tal involvement to promote healthy diet and nutrition and to increase physical activity (PA)/exercise [7-9]. Despite recommendations for healthy weight-loss behaviours, adolescents engage in numerous unhealthy weight-loss behaviours [10], such as taking diet pills, laxatives, and diuretics, employing self-induced vomit- ing, and skipping meals [11-14]. Evidence suggests that female adolescents engage in unhealthy weight-loss behaviours more frequently than do male adolescents [11,13]. In addition, unhealthy weight-loss behaviours are more likely to occur among overweight or obese female adolescents [15,16] and among female adoles- cents who perceive themselves to be overweight or obese [17,18]. Consequently, female adolescents need effective, multi-component, weight-loss interventions that promote healthy weight-loss behaviours. An increased understanding of factors that influence female adolescents’ desire to lose weight and their choice of health-promoting weight-loss behaviours will help develop more effective weight-loss interventions. Therefore, the purpose of the present study was to Page 2 of 11 determine whether body-mass index (BMI) and weight perception are associated with trying to lose weight and engaging in exercise as a weight-loss method among U.S. female adolescents. Review of the Literature The following literature review was conducted in order to identify what is known and not known about the variables under consideration in this secondary analysis study. The strengths of previously conducted studies considering similar variables included the use of large probability samples representative of U.S. and interna- tional populations of adolescents, objective measure- ments of height and weight, and consideration of associations between variables and trying to lose weight among female adolescents. However, these previous relevant studies also included the following limitations: convenience samples that limited generalizability beyond the studies’ population; participants’ self-reports of height and weight; and the researchers’ use of tools with insufficiently reported and/or lack of validity and reliability, which undermined external and internal validity and included various classifications of actual weight (BMI) and weight perception. Considering such limitations and in an effort to improve the ability to synthesize previous findings, only studies that used probability-sampling methods were included in the fol- lowing literature review. Researchers report a significant percentage of adoles- cents are trying to lose weight, ranging from approxi- mately 34% to 42% in the United States [16,19] to 61% in Quebec, Canada [20]. The most consistent demo- graphic finding is that female adolescents in the United States [15,21,22] and worldwide [22-24] are significantly more likely to be trying to lose weight than are their male counterparts. For example, Ojala et al. [22]. esti- mated that, among their sample of adolescents from over 30 countries, 19.8% female adolescents were trying to lose weight compared to 7.5% male adolescents in their cross-national 2007 survey. A limited number of studies have considered demo- graphic variables other than nationality, such as race/ ethnicity, age, and socioeconomic status (SES), among adolescents trying to lose weight. Research findings of U.S. adolescents demonstrate either that no relationship exists between race/ethnicity and trying to lose weight [21] or that Black/African American female adolescents are less likely to be trying to lose weight than their His- panic and White/non-Hispanic White counterparts [11,25]. In addition, study findings show a significant positive relationship between age and trying to lose weight in U.S. adolescents [26] and worldwide [22], in that older adolescents are more likely to be trying to