Letter re: Pre-treatment serum vitamin D deficiency is associated with increased inflammatory biomarkers and short overall survival in patients with pancreatic cancer: Analysis of the prognostic effect of serum vitamin D on pancreatic cancer: Several confounders - Zhai, Lu-Lu; He, Yi-Ren; Ju, Tong-Fa; Tang, Zhi-Gang (European journal of cancer (1990), 2021)