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The effects of dietary methyl donors on methionine partitioning in the neonatal piglet.

The metabolism of the indispensable amino acid methionine is critical during development. Methionine is used to synthesize protein for growth and, using the methionine cycle, it is the precursor of >50 critical nutrients and contributes to epigenetic regulation. Therefore, the dietary methionine requirement must factor all the potential roles of methioine. Three major processes summarize the methionine cycle: transmethylation (TM), which transfers methyl groups to nutrient precursors and DNA; transsulfuration (TS), which represents methionine disposal; and remethylation (RM), which resynthesizes methionine using the dietary methyl donors folate and choline (via betaine). Dietary intakes of folate vary drastically, and choline intakes are often below the adequate intakes during pregnancy and in early life, which we hypothesized would influence the methionine requirement. To test our hypothesis, we fed 4- to 8-day-old neonatal piglets a low-methionine diet that was either deficient (MD-) or replete (MS+) in dietary methyl donors. We evaluated how methionine was balanced between the major TM reactions and protein synthesis. The MD- group exhibited marked differences in TM as creatine synthesis was ≈30% less (p < 0.05) and phosphatidylcholine synthesis was ≈60% more (p < 0.05) during MD- feeding. Interestingly, while MD- feeding did not affect liver protein synthesis, the methionine availability and protein synthesis were lower in skeletal muscle of the MD- versus MS+ animals (p < 0.05). Furthermore, whole-body protein turnover was also reduced during MD- feeding (p < 0.05), which is significant as protein turnover is especially critical during infancy. Next, we measured the effect of methyl donors on the rates of TM, TS, and RM. The rates of RM and TM were reduced by ≈75% in the MD- group (p < 0.05), while TS was unchanged. To evaluate the effectiveness of individual methyl donors on RM, we "rescued" a second group of MD- animals with betaine (MD + B), folate (MD + F) or both (MD + FB). The rate of RM and TM increased by ≈2-fold after rescue (p < 0.05) and reduced protein breakdown (p < 0.05). These studies showed that dietary methyl donors affect neonatal methionine metabolism, which should be considered when defining the dietary requirements of methionine during development.