The Longitudinal Relationship Between Seasonal Variation and Body Shape Indices (Conicity Index, Body Shape Index and Body Roundness Index) in Ellisras Rural Population

Seasonal variation is an important environmental determinant of human health and body composition. Traditional measures are widely used as indicators for body composition—body shape indices such as conicity index (CI), body roundness index (BRI), and a body shape index (ABSI)—provide better insights into fat distribution and cardiometabolic risk. This study aimed to determine the longitudinal relationship between seasonal variation and CI, BRI, and ABSI in children from Ellisras. A longitudinal design was employed using secondary data from the Ellisras Longitudinal Study collected between 1997 and 2003. Height, weight, and waist circumference (WC) were obtained using standardized procedures, and CI, BRI, and ABSI were calculated. Measurements were taken in spring and autumn each year. Linear mixed-effects models were used to assess seasonal differences over time. Conicity index and BRI showed a decline over time in both seasons, while ABSI increased initially before stabilizing. Mixed-effects model results indicated a significant (p < 0.001) overall seasonal effect, with higher values observed in spring—CI: (0.0154 95% CI 0.014–0.017), BRI: (0.087 95% CI 0.077–0.097), and ABSI: (0.008 95% CI 0.008–0.009). These results highlight the importance of seasonally targeted interventions to promote healthy lifestyles in rural communities.