Secular Trend in Thinness Prevalence for 26 Years (1989 - 2014) among High School Runners in Japan

Female distance runners are predisposed to thinness for performance reasons and at greater risk for health-related consequences than the general population. The aim of the study was to evaluate the secular trends in thinness among competitive distance runners in Japanese high schools with sex-specific comparison. Body mass index (BMI) were analysed in 9,881 female and 12,786 male runners participating in the annual national road relay races between 1989 and 2014 using the directory of a track and field magazine. The age-specific trends of these variables and the prevalence of thinness were tested by linear and logistic regression, respectively. Although BMI declined significantly among both male and female runners (coefficients: boys, -0.033, -0.030, and -0.032; girls, -0.033, -0.034, and -0.034 [all P < 0.001] in 15.5-, 16.5-, and 17.5-year-olds, respectively), the prevalence of severely thin runners significantly increased only among girls (odds ratios for the yearly change = 1.05 and 1.06 [both P < 0.01] for 16.5- and 17.5-year-olds, respectively). Only 0.13% of male runners were regarded as severely thin throughout the study period. The results suggested that severely thin girl runners has steadily increased. Surveys on the health consequences of extreme thinness are necessary for the female adolescent runners.