Severe vitamin B12 deficiency (<125 pg/mL) in exclusively breastfed infants born to vegan mothers is associated with macrocytic anemia, hypotonia, developmental regression, and neurological symptoms, and supplementation with vitamin B12 is associated with improvement in these symptoms.

When a baby gets too little vitamin B12 from breast milk, its body cannot make DNA properly or build the protective coating around nerve fibers. This stops blood cells from growing normally, making them large and dysfunctional, and prevents the brain and nerves from developing correctly. The baby becomes weak, sluggish, and loses skills it had already learned. Giving vitamin B12 fixes both the blood and the nerves by restoring the chemical reactions needed for DNA and myelin production.