Vitamin D Deficiency and Replacement Challenges in Type 1 Gastric Neuroendocrine Tumors: A Comparative Study

<b>Background/Objectives</b>: Type 1 gastric neuroendocrine tumors (gNET) arise in the setting of autoimmune chronic atrophic gastritis and secondary hypergastrinemia. Vitamin D deficiency (VDD) has been associated with bone impairment and adverse outcomes in patients with neuroendocrine tumor (NET); however, data specifically addressing gNET remain limited. This study aimed to evaluate vitamin D status, supplementation requirements, and bone involvement in patients with type 1 gNET compared with those with entero-pancreatic NET (EP-NET). <b>Methods</b>: This retrospective study included patients with type 1 gNET followed at a tertiary referral center between 2010 and 2025 and an age- and sex-matched EP-NET cohort. VDD prevalence, time and dose required for normalization, supplementation formulations, bone status, and dietary habits were analyzed. <b>Results</b>: Twenty-six patients were included (thirteen gNET and thirteen EP-NET). VDD was significantly more prevalent in the gNET group compared with the EP-NET group (92.3% vs. 46.2%, <i>p</i> = 0.03, OR: 14). gNET required significantly higher daily cholecalciferol doses (3198.9 ± 1629 vs. 1580 ± 1121 IU/day, <i>p</i> = 0.008) and more frequently required multiple supplementation formulations (38.5% vs. 0%, <i>p</i> = 0.04). Multivariable linear regression analysis restricted to VDD patients confirmed that gNET was independently associated with higher daily cholecalciferol dose requirements (<i>p</i> = 0.037). Bone impairment, defined as osteoporosis or osteopenia, was significantly more common in the gNET group (61.5% vs. 15.4%, <i>p</i> = 0.04, OR: 8.8). Dietary adherence did not differ between groups. <b>Conclusions</b>: Type 1 gNET show a higher burden of VDD, increased vitamin D supplementation requirements, and a higher prevalence of bone impairment compared with EP-NET, irrespective of dietary habits. These findings suggest disease-specific mechanisms and support the need for tailored management in these patients.