OBJECTIVES: Vitamin D (VitD) deficiency affects the cardiovascular system via endocrine, paracrine, and autocrine pathways. Limited data are available regarding cardiac autonomic dysfunction in VitD deficiency. The aim of this study was to assess the cardiac autonomic functions by using heart rate recovery index (HRRI) and heart rate variability (HRV) in apparently healthy subjects with VitD deficiency. METHODS: A total of 24 VitD deficient and 50 age-, gender-, and body mass index-matched VitD sufficient healthy participants who admitted to outpatient clinics at a tertiary centre were enrolled. All study participants underwent Treadmill exercise test and 24-hour Holter recording to assess cardiac autonomic functions. HRRIs were calculated by subtracting first, second, and third minute heart rates during recovery period from maximal heart rate. RESULTS: Mean HRR1 (28.0 +/- 8.3 vs 42.8 +/- 6.4, P < 0.001), HRR2 (41.1 +/- 11.2 vs 60.8 +/- 10.4, P < 0.001), and HRR3 (44.9 +/- 13.3 vs 65.9 +/- 9.8, P < 0.001) were significantly higher in VitD sufficient group compared to VitD deficient group. HRV parameters as, SDNN (P = 0.040), SDANN (P < 0.001), RMSSD (P < 0.001), PNN50 (P < 0.001), and HF (P < 0.001) were significantly decreased in patients with VitD deficiency; but LF (P < 0.001) and LF/HF (P = 0.003) were significantly higher in VitD deficient group. Serum 25(OH)D level was positively correlated with HRRIs (P < 0.001), PNN50, RMSSD, SDANN, and HFnu; negatively correlated with LFnu and LF/HF (P < 0.05). Also, multivariate linear regression analysis showed that serum 25(OH)D level was significantly associated with HRRIs and HRV parameters (P < 0.001). CONCLUSION: Our study results suggest that cardiac autonomic functions are impaired in patients with VitD deficiency despite the absence of overt cardiac involvement and symptoms. Further studies are needed to elucidate the prognostic significance and clinical implications of impaired autonomic functions in patients with VitD deficiency.