Table 3.

Diseases that are linked to vitamin B12 homeostasis and their association with AD.

Link to Vitamin B12		Link to Alzheimer’s Disease
Author	Main Finding	Author	Main Finding
Inflammatory Bowel Disease (IBD)
Weisshof et al. (2015) [198]	Micronutrient deficiencies are common (>50%) in patients with IBD with vitamin B12 deficiency belonging to the most common ones.	Zingel et al. (2021) [203]	This study analyzing 3850 patients with an initial diagnosis of inflammatory bowel diseases (IBD; Crohn’s Disease, ulcerative colitis) and 3850 patients without IBD reported that IBD is associated with a 1.22-fold increase in the risk of developing dementia.
Yakut et al. (2010) [199]	Patients with Crohn’s disease common have a serum vitamin B12 deficiency.	Zhan et al. (2021) [204]	An increase in the risk of developing AD was reported in IBD patients in a 16-year longitudinal study including 1742 patients with IBD.
Bermejo et al. (2013) [200]	15.6% (95% CI 9.7–20%) of patients with Crohn’s disease suffer from vitamin B12 deficiency.
Park et al. (2021) [201]	Crohn’s disease patients are more often deficient in micronutrients like vitamin B12.
Ward et al. (2015) [202]	The prevalence of vitamin B12 deficiency is common in patients with Crohn’s disease.
Gastritis
Porter et al. (2021) [207]	Atrophic gastritis was associated with significantly lower serum total vitamin B12 levels and higher prevalence of vitamin B12 deficiency.	Li et al. (2018) [210]	The risk of dementia and AD is increased in patients with many types of autoimmune disorders, like pernicious anemia.
Green (2017) [205]	Pernicious anemia (autoimmune gastritis) is a cause of vitamin B12 deficiency.	Metzler et al. (1991) [212]	Specific clinical entities of a vitamin B12 deficiency include, amongst others, dementia.
Sipponen et al. (2003) [208]	Association of low vitamin B12 serum levels and atrophic gastritis in an elderly male cohort.	Kountouras et al. (2006) [213]	There is a link between an infection with Helicobacter pylori and Alzheimer’s disease.