Background: Fatigue is one of the most distressing symptoms in patients with advanced cancer. Previous studies have shown an association between low vitamin D levels and fatigue.

Objectives: The aim of this study was to investigate the association between vitamin D levels and self-assessed fatigue in cancer patients admitted to palliative care, with focus on possible sex differences.

Design: This is a cross-sectional study. Subjects: Baseline data from 530 screened patients, 265 women and 265 men, from the randomized placebo-controlled trial "Palliative-D" were analyzed.

Measurements: Vitamin D status was measured as 25-hydroxyvitamin D (25-OHD) and fatigue was assessed with EORTC-QLQ-PAL15 and with Edmonton Symptom Assessment System (ESAS).

Results: In men, there was a significant correlation between 25-OHD and fatigue measured with the "Tiredness question" (Q11) in EORTC-QLQ-PAL15 (p < 0.05), where higher 25-OHD levels were associated with less fatigue. No correlation between 25-OHD and fatigue was seen for women. Fatigue measured with ESAS did not show any significant association with 25-OHD levels neither in men nor in women.

Conclusion: Low vitamin D levels were associated with more fatigue in men but not in women. The study underscores the importance of subgroup analysis of men and women when evaluating the effect of vitamin D in clinical trials since the effect may differ between the sexes. The ongoing "Palliative-D study" will reveal whether vitamin D supplementation may counteract fatigue in both men and women.

ClinicalTrial.gov: NCT03038516.