Taurine/chenodeoxycholic acid ratio as a potential serum biomarker for low vitamin B₁₂ levels in humans

Deficiency of vitamin B₁₂ (B₁₂ or cobalamin), an essential water-soluble vitamin, leads to neurological damage, which can be irreversible and anaemia, and is sometimes associated with chronic disorders such as osteoporosis and cardiovascular diseases. Clinical tests to detect B₁₂ deficiency lack specificity and sensitivity. Delays in detecting B₁₂ deficiency pose a major threat because the progressive decline in organ functions may go unnoticed until the damage is advanced or irreversible. Here, using targeted unbiased metabolomic profiling in the sera of subjects with low B₁₂ levels v control individuals, we set out to identify biomarker(s) of B₁₂ insufficiency. Metabolomic profiling identified seventy-seven metabolites, and partial least squares discriminant analysis and hierarchical clustering analysis showed a differential abundance of taurine, xanthine, hypoxanthine, chenodeoxycholic acid, neopterin and glycocholic acid in subjects with low B₁₂ levels. Random forest multivariate analysis identified a taurine/chenodeoxycholic acid ratio, with an AUC score of 1, to be the best biomarker to predict low B₁₂ levels. Mechanistic studies using a mouse model of B₁₂ deficiency showed that B₁₂ deficiency reshaped the transcriptomic and metabolomic landscape of the cell, identifying a downregulation of methionine, taurine, urea cycle and nucleotide metabolism and an upregulation of Krebs cycle. Thus, we propose taurine/chenodeoxycholic acid ratio in serum as a potential biomarker of low B₁₂ levels in humans and elucidate using a mouse model of cellular metabolic pathways regulated by B₁₂ deficiency.