Research Article Expanded Normal Weight Obesity and Insulin Resistance in US Adults of the National Health and Nutrition Examination Survey Keilah E. Martinez, Larry A. Tucker, Bruce W. Bailey, and James D. LeCheminant Department of Exercise Sciences, College of Life Sciences, Brigham Young University, Provo, UT 84602, USA Correspondence should be addressed to Larry A. Tucker; tucker@byu.edu Received 13 January 2017; Accepted 8 June 2017; Published 25 July 2017 Academic Editor: Eusebio Chiefari Copyright © 2017 Keilah E. Martinez et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This study aims to expand the evaluation of normal weight obesity (NWO) and its association with insulin resistance using an NHANES (1999–2006) sample of US adults. A cross-sectional study including 5983 men and women (50.8%) was conducted. Body fat percentage (BF%) was assessed using dual-energy X-ray absorptiometry. Expanded normal weight obesity (eNWO) categories, pairings of BMI and body fat percentage classifications, were created using standard cut-points for BMI and sex- specific median for BF%. Homeostatic model assessment-insulin resistance (HOMA-IR) levels were used to index insulin resistance. Mean ± SE values were BMI: 27.9 ± 0.2 (women) and 27.8 ± 0.1 (men); body fat percentage: 40.5 ± 0.2 (women) and 27.8 ± 0.2 (men); and HOMA-IR: 2.04 ± 0.05 (women) and 2.47 ± 0.09 (men). HOMA-IR differed systematically and in a dose- response fashion across all levels of the eNWO categories ( F = 291 3, P <0 0001). As BMI levels increased, HOMA-IR increased significantly, and within each BMI category, higher levels of body fat were associated with higher levels of HOMA-IR. Both high BMI and high BF% were strongly related to insulin resistance. Insulin resistance appears to increase incrementally according to BMI levels primarily and body fat levels secondarily. Including a precise measure of body fat with BMI adds little to the utility of BMI in the prediction of insulin resistance. 1. Introduction Over the past several decades, the number of adults in America with excess body weight has increased substantially [1]. According to body mass index (BMI) data derived from the National Health and Nutrition Examination Survey (NHANES), approximately 44% of adults were overweight or obese in 1976–1980 [2]. Findings for 2011-2012 indi- cate that the prevalence increased to 69% of adults [3], an increase of over 55%. This rising trend is not without serious consequences. Obesity is a significant risk factor for numerous medical conditions, including insulin resistance and metabolic dis- ease. In a recent paper, Lim et al. showed that obesity, whether measured by BMI or body fat percentage, is highly correlated with insulin resistance, as indexed by elevated homeostatic model assessment (HOMA-IR) levels [4]. Addi- tionally, BMI is a serious risk factor for the development of type 2 diabetes, displaying a dose-response relationship [5]. Research also shows that adults who have normal body weight, but excess body fat, are at increased risk for developing metabolic syndrome and insulin resistance [6]. Though frequently used to classify obesity, BMI does not measure adiposity. BMI is calculated using only height and weight, not body composition [7]. BMI often misclas- sifies those with excess adiposity (high body fat percentage) as normal or healthy [8]. Though BMI has high specificity for predicting high body fat percentage [9, 10], several researchers have found that BMI has low sensitivity for predicting body fat percentage [9–11]. Many individuals assume that because they have a normal body weight, they are metabolically healthy, and those who are overweight may assume that they are metabol- ically unhealthy. Though commonly used, body weight, and particularly BMI, is not a high-quality index of health status. Researchers have tried to remedy the problems associated with using BMI to index overweight and obesity. As a result, the concept of normal weight obesity (NWO) has emerged. Hindawi Journal of Diabetes Research Volume 2017, Article ID 9502643, 8 pages https://doi.org/10.1155/2017/9502643