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Prolonged low intakes of vitamin B12, such as seen in those adhering to a vegan diet, lead to physiological deficiency of vitamin B12 and an elevated risk of B12\u2010related morbidity. However, while serum B12 serves as a conventional biomarker for assessing B12 status, its utility is limited given its sensitivity and specificity in ascribing physiological deficiency of B12 and the functional vitamin B12 status of those adhering to vegan diets is unclear. We conducted a systematic review and meta\u2010analysis using data based on the full panel of biomarkers of vitamin B12 status to test whether adherence to a vegan diet is associated with an elevated risk of functional vitamin B12 deficiency compared to vegetarian or omnivorous diets. In addition, subgroup analysis was carried out to look at the effect of vitamin B12 supplement use on B12 status among vegans. Our search identified 4002 records, of which 19 studies met the inclusion criteria for the systematic review and 17 studies were taken forward for the meta\u2010analysis. Meta\u2010analysis results revealed significantly lower serum B12, pmol\/ (\u22120.72 [\u22121.26, \u22120.18]; <jats:italic>p<\/jats:italic>\u2009=\u20090.01) and elevated total homocysteine, \u03bcmol\/L (tHcy) (0.57 [0.26, 0.89]; <jats:italic>p<\/jats:italic>\u2009&lt;\u20090.001) concentrations, alongside elevated methylmalonic acid, nmol\/L (MMA) (0.28 [\u22120.01, 0.57]; <jats:italic>p<\/jats:italic>\u2009=\u20090.06) and lower holotranscobalamin, pmol\/ (HoloTC) (\u22120.42 [\u22120.91, 0.07]; <jats:italic>p<\/jats:italic>\u2009=\u20090.09) levels among vegan adults compared to omnivores, indicating increased functional B12 deficiency in addition to low vitamin B12 status in vegan adults. There were no differences between vegans and vegetarians in HoloTC (0.04 [\u22120.28, 0.35]; <jats:italic>p<\/jats:italic>\u2009=\u20090.814) or MMA (\u22120.05 [\u22120.29, 0.20]; <jats:italic>p<\/jats:italic>\u2009=\u20090.708), but differences were found in serum B12 (\u22120.25 [\u22120.40, \u22120.10]; <jats:italic>p<\/jats:italic>\u2009=\u20090.001) and for tHcy (0.24 [0.09, 0.39]; <jats:italic>p<\/jats:italic>\u2009=\u20090.002) concentrations. Subgroup analyses indicated that the use of vitamin B12 supplements among vegans contributes to significant improvements in all biomarker concentrations compared to their unsupplemented counterparts. Our findings underscore the need for improved strategies to redress poor vitamin B12 status with appropriate B12 supplementation use among those adhering to vegetarian and vegan diets.<\/jats:p>","DOI":"10.1111\/nbu.12712","type":"journal-article","created":{"date-parts":[[2024,10,7]],"date-time":"2024-10-07T11:40:05Z","timestamp":1728301205000},"page":"463-479","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":28,"title":["A systematic review and meta\u2010analysis of functional vitamin <scp>B12<\/scp> status among adult vegans"],"prefix":"10.1111","volume":"49","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-8986-0573","authenticated-orcid":false,"given":"Ali","family":"Niklewicz","sequence":"first","affiliation":[{"name":"Department of Nutritional Sciences, School of Biosciences and Medicine University of Surrey  Guildford UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Luciana","family":"Hannibal","sequence":"additional","affiliation":[{"name":"Laboratory of Clinical Biochemistry and Metabolism, Department of General Paediatrics, Adolescent Medicine and Neonatology, Faculty of Medicine, Medical Centre University of Freiburg  Freiburg Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Martin","family":"Warren","sequence":"additional","affiliation":[{"name":"Quadram Institute Bioscience, Norwich Research Park  Norwich UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kourosh R.","family":"Ahmadi","sequence":"additional","affiliation":[{"name":"Department of Nutritional Sciences, School of Biosciences and Medicine University of Surrey  Guildford UK"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"311","published-online":{"date-parts":[[2024,10,7]]},"reference":[{"key":"e_1_2_10_2_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.clnu.2020.11.035"},{"key":"e_1_2_10_3_1","unstructured":"BDA. 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