Methionine (Met) is the third limiting amino acid (AA) in typical swine diets. Besides protein synthesis as the main role, Met plays an important role as a methyl donor for cellular metabolism and formation of co-enzyme S-adenosylmethionine (Bunchasak, 2009), and involved in immune function and a precursor for the synthesis of glutathione and taurine in animals (Li et al., 2007). The dietary requirement of Met could be supplied by using plant or animal protein feed ingredients (Htoo and Morales, 2016), however, by increasing the inclusion rate of these raw materials for meeting Met requirement increases the levels of other amino acids in the diet, and consequently results in imbalanced or excess AA. Different Met sources can be used in pig diets. The currently available Met sources for swine and poultry are the dry DL-Methionine (DLMet; a 50:50 mixture of D- and L-isomers; 99% pure), liquid Methionine Hydroxy Analog-Free Acid (MHA-FA, 88% active substance), calcium salt of hydroxy analog of Methionine (MHACa, 84% active substance) and L-Methionine (L-Met; 99% pure).
Bioavailability is a relative value that compares the nutritional value of a given nutrient with a defined standard. There has been ongoing debatable discussions on the bioavailability of these Met sources. Adequate determination of relative bioavailability (RBV) of these Met sources is important for precise and cost effective diet formulation, optimum performance and to minimize nitrogen excretion (Opapeju et al., 2012). In general, the nutritional value of supplemental Met sources in the market is typically compared to DL-Met, which is a gold standard. The objective of this article was to review and summarize some recently published data related to the RBV of different Met sources in pigs.
Bioavailability of MHA-FA or MHA-Ca is lower than DL-Met in pigs
Numerous studies have been conducted to compare the RBV of MHA-FA to DL-Met in pigs. These previous studies have been consistently demonstrated that the bioavailability of MHA-FA ranges between 63 and 71% compared to DL-Met on product basis (Schmidt, 2000; Zimmermann et al., 2005; Kim et al., 2006; Feng et al., 2006). Opapeju et al. (2012) conducted N-balance experiment with 42 barrows (average initial BW of 19 kg) and estimated the RBV of MHA-Ca relative to DL-Met to be 71% on product-to-product basis based on N retention, % of intake (Figure 1). Results of another performance trial with growing pigs (initial BW of 15.5 kg) demonstrated that ADG and FCR were not different among pigs fed diets supplemented with two Met sources at DL-Met (MetAMINO®) to MHA-Ca ratio of 65:100 on a product basis, indicating that 100 parts of MHA-Ca can be replaced with 65 parts of DL-Met on a product-to-product basis, in practical swine feed formulation (Figure 2).
Recently, European Food Safety Authority (EFSA, 2018) indicated in the report that the lower estimated bioavailability of MHA and its salts for pigs could be attributed to the following reasons (i) the gut microbiota of the non-ruminants compete with the host for MHA and its salts compared to DL-Met, and (ii) commercial MHA and its salts may contain significant amounts of dimers, trimers and oligomers, which is considered to have lower bioavailability. Based on these aforementioned studies, a bioavailability value of 75% on equimolar/isosulfurous basis or around 65% on a product-to-product basis for liquid MHA-FA as well as for MHA-Ca relative to DLMet is recommended for practical swine feed formulation.
Bioavailability of Met isomers in pigs
Feed-grade L-Met has been available in some countries as a new Met source for animal feeding. Based on spatial configuration of carboxyl and amino groups around central carbon atom, D and L are the two isomeric forms of Met. In order to be utilized by the animals for body protein synthesis and other biological functions, dietary D-Met must be converted to L-Met via biochemical process (Figure 3). The conversion of D-Met to Keto-Met by D-amino oxidase is not a rate-limiting factor because of the presence of significant D-amino oxidase activity in tissues of pigs (Fang et al., 2010). Furthermore, the transamination of Keto-Met to L-Met by aminotransferases is very efficient in pigs (Dilger et al., 2007) such that D-Met is completely utilized by the pigs (Baker, 1994; 2006).
L-Met and DL-Met are equally bioavailable for pigs
Research data on the RBV of L-Met compared with DL-Met for pigs is scarce. Recently, Htoo and Morales (2016) conducted a study to determine RBV of L-Met compared with DL-Met for young pigs (8 to 15 kg) based on ADG as response criteria of Met intake. Growth performance results of pigs fed diets containing graded level of DL-Met or L-Met is shown in Table 1. Using a slope-ratio regression, the RBV of L-Met relative to DL-Met was 100% based on ADG as a response of supplemented Met intake indicating that DL-Met and L-Met are equally bioavailable for the young pigs (Figure 4). These results are consistent with previous studies (Chen et al., 2013; van Milgen et al., 2013) who also reported that the RBV of L-Met and DL-Met is not different in pigs.
Nitrogen balance or N retention indicates the efficiency of N utilization for body protein deposition as well as N excretion. Recently, Kong et al. (2016) determined the RBV of DL-Met compared to L-Met in nursery pigs using nitrogen balance as response criteria and concluded that RBV of DL-Met and L-Met are not different from 100%.
Tian et al. (2016) conducted a study to determine the effect of L-Met as a source compared with DL-Met for N balance, serum protein and amino acid profile in starter (15 kg BW) pigs. They demonstrated that starter pigs (15 kg BW) fed diet supplemented with either DL-Met or L-Met increased N retention and N retention rate when compared with basal diet (Figure 5). There was no difference in N retention between DL-Met and L-Met indicating that DL-Met and L-Met are equally bioavailable as Met sources for starter pigs. Moreover, serum urea nitrogen, albumin and total protein concentrations were not different among pigs fed diets supplemented with DL-Met or L-Met.
Based on the aforementioned published data, the relative bioavailability of liquid MHAFA or MHA-Ca is around 65% on a product-to-product basis compared with DL-Met in pigs. Furthermore, pig performance was not different among pigs fed diets supplemented with DLMet to MHA-Ca ratio of 65:100 on a product basis. Results of more recent studies also indicated that DL-Met and L-Met are equally bioavailable as Met sources to optimize performance of pigs. Overall, selecting an appropriate Met source as well as applying the correct bioavailability is important in pig nutrition for cost-effective and accurate diet formulation to optimize pig production.