Prolonged exclusive breast-feeding from vegan mother causing an acute onset of isolated methylmalonic aciduria due to a mild mutase deficiency

We describe a case of methylmalonic aciduria (MMA) occurred in a 22-month-old boy whose diet was exclusively based upon breast-feeding from a mother following a long-lasting strict vegetarian diet. Clinical picture showed a dramatic onset, with a profound drowsiness associated with a severe metabolic acidosis, hyperammonemia, macrocytic anemia, ketonuria, and massive methylmalonic aciduria without homocystinuria. Both symptoms and biochemical findings quickly improved thanks to prompt vitamin B12parenteral therapy. Biochemical and enzymatic findings allowed a diagnosis of mild mutase deficiency, which only and inadequate dietary B12contribution might have revealed. Our case highlights the risk of a prolonged strictly vegetarian diet of lactating mother for providing inadequate amounts of some nutrients to the breast-fed baby. Moreover, such a dietary behaviour could dramatically unmask otherwise clinically unapparent metabolic defects in the baby.