{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,22]],"date-time":"2026-07-22T05:02:46Z","timestamp":1784696566586,"version":"3.55.0"},"reference-count":91,"publisher":"MDPI AG","issue":"14","license":[{"start":{"date-parts":[[2026,7,20]],"date-time":"2026-07-20T00:00:00Z","timestamp":1784505600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Cancers"],"abstract":"<jats:p>Background: Vitamin D deficiency is common in patients with gastroenteropancreatic neuroendocrine neoplasms (GEP-NENs). Whether vitamin D status influences tumor aggressiveness and clinical outcomes remains uncertain. This review examines current evidence on the prevalence, determinants, clinical associations, and clinical relevance of vitamin D deficiency in GEP-NENs. Methods: A narrative and critical review of the literature was conducted using the PubMed\/MEDLINE, Scopus, and Web of Science databases. Clinical studies, observational cohorts, translational research, selected mechanistic studies, and current clinical guidelines addressing vitamin D metabolism and neuroendocrine neoplasms were evaluated. Results: Vitamin D deficiency has been reported in approximately 60\u201380% of patients with GEP-NENs. Low serum 25-hydroxyvitamin D concentrations likely reflect multiple disease-related factors, including malabsorption, pancreatic exocrine insufficiency, chronic diarrhea, previous gastrointestinal surgery, and nutritional impairment. Several observational studies have linked lower vitamin D status with markers of more aggressive disease, including higher Ki-67 proliferation index values and shorter progression-free survival. Nevertheless, the available data are heterogeneous, predominantly observational, and do not support a causal relationship between vitamin D deficiency and tumor progression. At present, the main clinical rationale for assessing vitamin D status is to support metabolic care, preserve bone health, and prevent osteoporosis. Conclusions: Vitamin D deficiency is a frequent and clinically relevant comorbidity in patients with GEP-NENs. Although lower vitamin D status has been associated with markers of more aggressive disease, current findings do not support vitamin D supplementation as an anticancer treatment strategy. Prospective clinical and translational studies are needed to clarify the biological and clinical significance of vitamin D signaling in GEP-NENs.<\/jats:p>","DOI":"10.3390\/cancers18142346","type":"journal-article","created":{"date-parts":[[2026,7,20]],"date-time":"2026-07-20T16:19:27Z","timestamp":1784564367000},"page":"2346","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Vitamin D Status and Gastroenteropancreatic Neuroendocrine Neoplasms: Biological Rationale, Clinical Associations and Limitations of Current Evidence"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0009-0009-8300-0674","authenticated-orcid":false,"given":"Bartosz","family":"Basiaga","sequence":"first","affiliation":[{"name":"Department of Endocrinology and Neuroendocrine Tumors, ENETS Centre of Excellence, Department of Pathophysiology and Endocrinology, Medical University of Silesia, 40-514 Katowice, Poland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5238-9287","authenticated-orcid":false,"given":"Violetta","family":"Rosiek","sequence":"additional","affiliation":[{"name":"Department of Endocrinology and Neuroendocrine Tumors, ENETS Centre of Excellence, Department of Pathophysiology and Endocrinology, Medical University of Silesia, 40-514 Katowice, Poland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3755-1722","authenticated-orcid":false,"given":"Beata","family":"Kos-Kud\u0142a","sequence":"additional","affiliation":[{"name":"Department of Endocrinology and Neuroendocrine Tumors, ENETS Centre of Excellence, Department of Pathophysiology and Endocrinology, Medical University of Silesia, 40-514 Katowice, Poland"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,7,20]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"61","DOI":"10.1016\/S1470-2045(07)70410-2","article-title":"Gastroenteropancreatic neuroendocrine tumors","volume":"9","author":"Modlin","year":"2008","journal-title":"Lancet Oncol."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"1114","DOI":"10.5858\/arpa.2024-0315-RA","article-title":"Endocrine and neuroendocrine tumors: Updates from the 5th edition of the World Health Organization \u201cBlue Book\u201d","volume":"149","author":"Riddle","year":"2025","journal-title":"Arch. 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