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Methionine (Met) is one of the important indispensable amino acids (AA) for protein synthesis of pigs and is generally second or third limiting AA. To improve growth of pigs, Met is often supplemented as dl-Met which is a racemic mixture of d- and l-Met. Because only l-Met can be incorporated into naturally occurring proteins, d-Met must be converted to l-Met. Thus, the possibility that there may be difference in the bioefficacy between d- and l-Met for pigs has been suggested. Nitrogen (N) balance has been used as the classical metabolic indicator of protein and may provide more sensitive and straightforward responses for protein metabolism than growth performance which until now has shown controversial response. The objective of the present study was to estimate the bioavailability of dl-methionine (Met) relative to l-Met for nursery pigs using the slope-ratio assay. A total of 35 crossbred barrows with an initial body weight of 13.4 ± 0.5 kg were randomly allotted to 5 dietary treatments in 7 replicates for a nitrogen (N) balance study. The basal diet (BD) was formulated to contain 2.1 g/kg Met. Dietary treatments included (1) BD, (2) BD + 0.3 g/kg dl-Met, (3) BD + 0.6 g/kg dl-Met, (4) BD + 0.3 g/kg l-Met, and (5) BD + 0.6 g/kg l-Met. The experiment consisted of a 5-day adaptation period and 5 days of total but, separate collection of feces and urine.
The amounts of feed consumed by pigs were similar across treatments. A linear decrease was observed for the urinary N output with increasing concentration of dl- or l-Met and consequent linear increase in retained N was observed for both Met sources. In addition, N retention (% of intake) also linearly increased as the concentration of both Met isomers increased. With the slope-ratio assay using retained N or retention of N (% of intake) as dependent variables and supplemental intake of Met isomers as independent variable, the estimates of relative bioavailability of dl-Met compared to l-Met were 87.9 or 89.3%, respectively. However, the differences in the bioavailability between 2 Met isomers were not significant.
In conclusion, with the slope-ratio assay using N balance as the response, no differences were achieved between dl- and l-Met in terms of bioefficacy.
Kong, C., Park, C. S., Ahn, J. Y., & Kim, B. G. (2016). Relative bioavailability of dl-methionine compared with l-methionine fed to nursery pigs. Animal Feed Science and Technology, 215, 181-185. http://dx.doi.org/10.1016/j.anifeedsci.2016.03.011