Patterns of vitamin D deficiency and functional resistance in cancer: a brief observational report.

Background: Although vitamin D deficiency is common in cancer populations, responses to vitamin D supplementation remain inconsistent. This suggests that circulating vitamin D levels may not fully capture its biological activity and that impaired vitamin D responsiveness, or functional vitamin D resistance, may be present in a subset of patients. Methods: A retrospective observational analysis of 220 patients with solid and hematological malignancies evaluated at an integrative oncology center in India. Serum 25-hydroxyvitamin D [25(OH)D] and intact PTH (iPTH) levels were assessed. Vitamin D deficiency (<20 ng/mL), insufficiency (20-29 ng/mL), and sufficiency (≥30 ng/mL) were defined using standard thresholds. Functional vitamin D resistance was defined as elevated iPTH (>65 pg/mL) in the presence of sufficient vitamin D. Distribution patterns were examined across cancer types, disease stage, metastatic status, and metabolic comorbidities. Results: Among 220 patients (mean age 57.6 ± 14.8 years), mean serum 25(OH)D was 22.2 ± 16.0 ng/mL, indicating a high prevalence of insufficiency. Serum 25(OH)D was inversely correlated with iPTH (p = 0.005). Advanced tumor stage was independently associated with vitamin D deficiency, while dyslipidemia was paradoxically associated with lower odds of deficiency. Functional vitamin D resistance was identified in 8 of 206 evaluable patients (3.9%) and was significantly associated with type 2 diabetes mellitus and dyslipidemia, but not with tumor stage or metastatic status. Conclusion: Vitamin D deficiency and insufficiency are common in cancer, whereas functional vitamin D resistance represents a distinct and less frequent phenotype associated with metabolic comorbidities. Assessment of iPTH alongside serum vitamin D may provide additional insight into vitamin D endocrine responsiveness in oncology settings.