Ho FK, So HK, Wong RS, et al. - In this population‐based 4‐year cohort study, researchers examined the longitudinal reciprocal link between body mass index (BMI) categories and physical fitness. This investigation was carried out in 48 elementary schools. Children aged 6 to 9 years at recruitment have been included. Using standard equipment and protocol, BMI categories and physical fitness including handgrip strength, core muscle endurance, flexibility, and cardiorespiratory fitness were measured. Among 26,392 eligible candidates, 19,504 (73.9%) were successfully followed for 3 years. Data reported that baseline obesity prevalence was 5.9%. Better physical fitness was correlated prospectively with normal weight and vice versa. Children who were physically fit were more likely to maintain a healthy weight and those with a healthy weight were more likely to be physically fit, which is vital for healthy development.