The influence of diet on the vitamin B12 activity in the serum, urine and faeces of rabbits.

In this paper the term vitamin B,, is used to signify vitamin B,, activity measured by microbiological assay with Lactobacillus leichmannii as test organism. The concentration of vitamin B,, in the serum of the rabbit is remarkably high compared with that of some other species. In man and the rat the highest normal values found in this laboratory have been 0.001 and 0-00088 pg/ml respectively (Spray & Witts, 1958; Booth & Spray, 1960), whereas in normal rabbits we have found levels as high as 0*156pg/ml. In spite of this difference the levels in tissues are of the same order as those in other species (Kulwich, Struglia & Pearson, 1953 ; Rosenthal & Cravitz, 1958). Rabbits ingest much more vitamin B, than other species, as a result of coprophagy (Kulwich et aZ. 1953). The high levels of vitamin B,, in the serum, and also in the urine, are probably due to the absorption from the gut of large amounts of vitamin B,, as a result of the high intake. The mechanism by which this absorption takes place has not been elucidated. Rosenthal (1959), using doses of cyanocobalamin much smaller than the quantities of vitamin B,, that rabbits normally ingest, concluded that the mechanism of absorption is essentially the same as in man. However, the possibility cannot be excluded that under physiological conditions comparatively large amounts of vitamin B,, are absorbed by another mechanism. To study further the significance of the level of vitamin B,, in the serum of rabbits we have placed them on various diets. It seems that the level is dependent on the diet, and thus it is impossible to establish a normal range for rabbits. We found that the nature of the diet, particularly its cobalt content, also influenced the faecal excretion of vitamin B,,, and that differences in diet were reflected in the vitamin B,, levels of both serum and urine.