Persistent cough and vitamin B12

If you are faced with a puzzling case of chronic cough in the absence of infection or allergy, a study published recently in The American Journal of Clinical Nutrition shows that a functional vitamin B12 deficiency should be considered. The authors state:

"Chronic cough is characterized by sensory neuropathy. Vitamin B-12 (cobalamin) deficiency (Cbl-D) causes central and peripheral nervous system damage and has been implicated in sensory neuropathy and autonomic nervous system dysfunction."

The authors set about evaluating whether Cbl-D has a role in chronic, unexplained cough by examining the laryngeal threshold (how much inhaled histamine it took to provoke a 25% decrease in the mid-inspiratory flow), bronchial threshold (how much to provoke a 20% decrease in forced expiratory volume), and cough threshold (how much to cause 5 or more coughs) before and after intramuscular injections of cobalamin in 42 patients with chronic, unexplained cough. (27 of them were cobalamin deficient [Cbl-D] at the beginning of the study and the rest were normal.) What did their data show?

"Cbl-D patients had a higher prevalence of laryngeal hyper-responsiveness than did Cbl-N patients, a thinner oropharyngeal epithelium, a lower number of myelinated nerve fibers, and a higher immunoreactive score for nerve growth factor (NGF). After cobalamin supplementation, symptoms and laryngeal, bronchial, and cough thresholds were significantly improved in Cbl-D but not in Cbl-N patients."

Besides the fact that vitamin B12 may help resolve chronic cough, there are two points particularly worth noting: (1) the degree to which B12 deficiency can damage nerve tissue, and (2) supplementation will make a difference only if there is a deficiency. The last may seem obvious, but it's surprising how many supplementation trials are conducted without discriminating deficient from replete subjects. The authors conclude:

"This study suggests that Cbl-D may contribute to chronic cough by favoring sensory neuropathy as indicated by laryngeal hyperresponsiveness and increased NGF expression in pharyngeal biopsies of Cbl-D patients. Cbl-D should be considered among factors that sustain chronic cough, particularly when cough triggers cannot be identified."

I have not found serum levels of B12 to be a good indicator of cobalamin sufficiency. While not perfect, serum or urine methylmalonic acid (MMA) are more reliable.