In addition to impaired renal function, other B12-independent causes of increased serum MMA concentration are states of dehydration, impaired thyroid function, inherited methylmalonic aciduria and small-bowel overgrowth with bacteria producing high amounts of propionic acid, the precursor of MMA.39,40 The single nucleotide polymorphism rs291466 in 3-hydroxyisobutyryl-CoA hydrolase (HIBCH) also influences variation in MMA concentration via the valine degradation pathway.41 This evidence concerns the gene HIBCH and Methylmalonic aciduria.