Stability of vitamin B12 in the presence of ascorbic acid in food and serum: restoration by cyanide of apparent loss.

Ascorbic acid in varying amounts was added to food and serum samples and heated at 37 C. Vitamin B12 was then measured by radioimmunoassay and microbiologically using several extraction methods. B12 values in a cottage cheese meal were lower than controls when concentrations of ascorbic acid greater than but not equal or less than 0.5 mg/ml were added and if KCN was not used during extraction, but when 70 micrograms/ml KCN was added after ascorbic acid exposure B12 was quantitatively recovered. Serum B12 was variably decreased by lesser concentrations of ascorbic acid but was also quantitatively restored by increasing KCN concentration during extraction. In the absence of KCN in the extraction step some loss of B12 at 100 C was observed; the loss was greater with added ascorbic acid. Our results indicate that previous reports on B12 loss in the presence of ascorbic acid are artifacts of the methods used. In view of these in vitro findings B12 destruction by ascorbic acid in vivo seems highly improbable.