Plant foods contain a negligible amount of vitamin B 12 ; thus, vegans have difficulty meeting the recommended daily allowance (2.4 μg) and maintaining adequate serum concentrations of B 12 . In a cross-sectional study of 689 men, 121 of 232 vegans (52%) were B 12 deficient (<118 pmol/L), whereas only 1 of 226 omnivores (0%) was B 12 deficient. 15 A prospective study of 20 omnivores who adopted a vegan diet over 5 years showed that B 12 levels were only maintained in the patients who consumed B

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... 12 -fortified products. 16 Vitamin B 12 deficiency can cause cognitive disorders, neuropathies, weakness, and macrocytic anemia, which can result in fatigue and low energy. 17 Deficiency also affects digestion and absorption of nutrients and metabolism of serotonin and melatonin. 17 Vegetarians have a lower risk of death from ischemic heart disease 5,18 ; however, low B 12 levels have been associated with worsening of some cardiovascular markers. A cross-sectional study of 154 vegans revealed an elevated homocystine level in 66% of participants, of whom 93% also had low B 12 levels (<250 pmol/L). 19 A 2014 review 6 identified elevated homocysteine level, impaired brachial flow-mediated endothelial dilation, and increased carotid intimal medial thickness in patients with low B 12 levels. These effects were improved with B 12 supplementation. 6