The upregulation of MTR abundance with Met or Chol along with upregulation of SAHH abundance with Met indicated that homocysteine and potentially 5-methyltetrahydrofolate synthesis increased (Tavares et al.,2016). In vivo, both MTR and BHMT can catalyze the remethylation of homocysteine to Met (Pellanda et al.,2012). However, judging from in vivo data with dairycows, there have been mixed results on hepatic MTR abundance in response to greater Met supply [i.e., upregulation of its mRNA (Osorio et al., 2014a) or lack of change (Zhou et al., 2017)]. In a recent study, increasing concentrations of exogenous Met in vitro (16, 30, 100, or 300 μM) decreased MTR expression linearly in neonatal calf hepatocytes (Chandler and White, 2017), underscoring the potential for feedback inhibition ofthe enzyme. Whether such effect is relevant in vivo is unknown. However, it seems unlikely that during lactation when the mammary gland is a prime user of Met its intracellular concentrations within hepatocytes would reach an inhibitory concentration.