Effect of School-Based Physical Activity and Multi Micronutrient Supplementation on Micronutrient Concentrations Among Tanzanian Schoolchildren: Secondary Outcomes from the KaziAfya Cluster-Randomized Controlled Trial

Background: Micronutrient deficiencies and physical inactivity can adversely affect child growth and development. This study assessed the effects of school-based physical activity and multi-micronutrient supplementation on micronutrient status among schoolchildren in Kilombero district, Tanzania. Methods: In a cluster-randomized trial, children aged 6–12 years were allocated to physical activity, multi-micronutrient supplementation, combined physical activity plus supplementation, or placebo control. Anthropometric and biochemical assessments were conducted at baseline, 14 months, and 26 months. Dried blood spot samples were available for 923 children at baseline. Complete-case analyses used biomarker-specific subsamples with valid baseline and 26-month measurements. Results: The primary complete-case sample included 243 children with valid paired measurements for zinc and serum transferrin receptor; vitamin D analyses were restricted to 52 children because of missing or invalid samples. At baseline, iron and vitamin D deficiencies were common, affecting 42.8% and 39.9% of children, respectively, while zinc deficiency affected 11.9%. At 26 months, allocation to the physical activity intervention was associated with lower odds of zinc deficiency, both when delivered alone (OR = 0.16) and when combined with supplementation (OR = 0.57). Supplementation alone was not significantly associated with reduced zinc deficiency. Iron status did not differ between intervention groups. Vitamin D findings should be interpreted with caution because analyses were based on a very small biomarker-specific subsample. Conclusions: School-based physical activity, alone or combined with multi-micronutrient supplementation, was associated with lower odds of zinc deficiency among Tanzanian schoolchildren. Supplementation alone showed no clear benefit for zinc or iron status. Vitamin D findings remain inconclusive because of substantial biomarker-specific missingness. Future trials should strengthen adherence monitoring, biomarker follow-up, and repeated assessment of dietary and contextual factors.