INTRODUCTION: The natural alignment of the lower extremities shifts rapidly from genu varum to neutral during early childhood. However, persistent or worsening genu varum, termed physiological genu varum, may be associated with vitamin D (VitD) deficiency. This study aimed to evaluate the effect of low-dose VitD supplementation on genu varum improvement in toddlers, given the prevalence of VitD deficiency and lack of supplementation practices in Japan. MATERIALS AND METHODS: This retrospective study included 90 toddlers aged 10-30 months with genu varum (tibiofemoral angle >
5°). The supplement group (n = 30) received 400 IU/day of VitD and moderate sun exposure, while the non-supplement group (n = 60) received only follow-up care. Radiographic parameters, including tibiofemoral angle (TFA), metaphyseal-diaphyseal angle (MDA), and metaphyseal-metaphyseal angle (MMA), were measured at baseline, 6 months, and 12 months. Seasonal variations in ultraviolet (UV) exposure were also considered. Statistical analyses used the Kruskal-Wallis and Mann-Whitney tests. RESULTS: Both groups showed significant radiographic improvements over 12 months. However, the supplement group demonstrated greater reductions in TFA (p = 0.04), MDA (p <
0.01), and MMA (p <
0.01) at both 6 and 12 months. Seasonal UV variations influenced outcomes, with moderate-high UV seasons showing more pronounced improvements. Despite a low dose of VitD, the supplement group experienced significantly enhanced alignment compared to the non-supplement group. CONCLUSION: Low-dose VitD supplementation may accelerate the natural improvement of genu varum, particularly in populations with prevalent VitD deficiency. Genu varum may serve as a visible indicator of VitD deficiency. A prospective randomized-controlled trial is warranted to validate these findings.