Body Mass Index as a Predictor of Percent Fat in College Athletes and Students

Abstract

Despite recent concern for adiposity-related problems in football, no universally established classification system for body fatness (% fat) exists in athletes. The body mass index (BMI) classification system for overweight and obesity is derived from cut points that apply to the general adult population and may not be suitable for athletes. PURPOSE: To determine the accuracy of BMI as a measure of % fat in college athletes and compare findings to a reference group of non-athlete students. METHODS: A total of 226 (149 male, 77 female) athletes and 213 (78 male, 135 female) non-athletes participated. Based on similarities in BMI and % fat among athletes, three male groups: 1) Athletes, 2) Football Linemen, and 3) Non-athletes, and two female groups: 1) Athletes, and 2) Non-athletes, were created. Linemen were grouped separately due to their extremely high and homogeneous BMI values. BMI was calculated from participants' measured height and weight. Body fatness was determined via air displacement plethysmography. A BMI cut point of 25 was used to define overweight. Cut points of 20% fat for males, and 33% fat for females were used to define overfat (ACSM, 2005). Using % fat as the criterion measure, sensitivity and specificity of BMI were calculated. Receiver operator characteristic curves were used to derive optimal BMI cut points for % fat. RESULTS: For male athletes, sensitivity of BMI >25 was high (1.0), but specificity was low (.27). Sensitivity was also high in football linemen (1.0). Specificity was not estimated for linemen because only one individual was below 20% fat. Sensitivity and specificity of BMI for male non-athletes were .83 and .60, respectively. Optimal BMI cut points for male athletes, linemen, and non-athletes were 27.9, 34.1, and 26.5, respectively. Sensitivity of BMI >25 for female athletes was 1.0 and specificity was .66. Sensitivity was .56 and specificity was .90 for female non-athletes. Optimal BMI cut points for female athletes and non-athletes were 27.7 and 24.0, respectively. CONCLUSIONS: BMI should be used cautiously when classifying fatness in college athletes and non-athletes. With the exception of female non-athletes, BMI >25 has high sensitivity and low specificity for predicting overfatness. This suggests few overfat individuals are classified as normal weight and many normal fat individuals are classified as overweight. A BMI cut point of 25 appears low for predicting overfatness in all male and female athletes, and male non-athletes. However, a BMI cut point of 25 is high for predicting fatness in female non-athletes. Our results support including the use of % fat, in addition to BMI, when evaluating the health status of young adults.