In the current study, gene expressions of enzymes
closely involved in the methylation cycle were modified
by Met supply. Cellular retention of folates and vitamin
B12 in liver was also modified when the supply in labile
preformed methyl groups was limited. Supplementary
folic acid and vitamin B12 also affect MUT mRNA
abundance, an enzyme related to gluconeogenesis. In
the cows studied in the present experiment, Preynat et
al. (2009a) observed that supplementary folic acid, but
not vitamin B12 or RPM, decreased plasma concentrations
of Hcy although there was no effect of folic acid
given alone or RPM on lactational performance. These
observations combined with those of the present experiment
indicate that an increase in methyl group supply,
preformed or from methylneogenesis, even if it affected
efficiency of the methylation cycle, had by itself a limited
effect on milk component yields. Therefore, the
increases in milk production and milk component yields
reported in cows supplemented with folic acid and vitamin
B12 together (Preynat et al., 2009a) seem to be
related to the effect of the vitamins on the increased
gene expression of MUT. This observation is further
supported by the increased in whole-body flux of glucose
observed in cows supplemented with folic acid and
vitamin B12 together compared with unsupplemented
cows (Preynat et al., 2009b). Therefore, it seems that
the effects of the combined supplement of vitamins on
lactational performance were caused by an effect on
energy metabolism rather than on methylneogenesis.