vitamin b12 deficiency reasonsside effects of lack of b12vitamin b12 deficiency diseasessevere vitamin b12 deficiencyvitamin b12 deficiency leads toanemia caused by b12 deficiencyb12 deficiency symptomslack of b12 can causeanemia due to vitamin b12 deficiencylack of vit b12 symptomsvitamin b12 resultssevere b12 deficiency