Revisiting trajectories of BMI in youth: An in‐depth analysis of differences between BMI and other adiposity measures

OBJECTIVE: Body mass index (BMI) is used to identify trajectories of adiposity in youth, but it does not distinguish fat‐ from fat‐free‐mass. There are other inexpensive measures of adiposity which might better capture fat‐mass in youth The objective of this study is to examine differences between sex‐specific trajectories of BMI and other adiposity indicators (subscapular and triceps skinfold thickness, waist circumference, waist‐to‐height ratio) which may better capture fat‐mass in youth. METHODS: Data come from four cycles of a longitudinal cohort of 1293 students in Montréal, Canada at ages 12, 15, 17 and 24. Group‐based trajectory models identified sex‐specific adiposity trajectories among participants with data in ≥3 cycles (n = 417 males; n = 445 females). RESULTS: There were six trajectory groups in males and females for all five indicators, except for waist circumference (seven) in both sexes and triceps skinfold thickness (four) and waist‐to‐height ratio (five) in females. Most trajectories indicated linear increases; only the skinfold thickness indicators identified a decreasing trajectory. While all indicators identified a trajectory with high levels of adiposity, they differed in the number and relative size of trajectories pertaining to individuals in lower half of the adiposity distribution. CONCLUSION: BMI is a satisfactory indicator of adiposity in youth if the aim of the trajectory analysis is to identify youth with excess adiposity, a known risk factor for cardiometabolic outcomes in adulthood.