The site of intestinal disappearance of DL-methionine and methionine hydroxy analog differs in pigs

Diets are routinely supplemented with DL-methionine (DLM) or with the methionine (Met) hydroxy analog, DL-2-hydroxy-4-methylthio butanoic acid (HMTBA). The little difference between them results in DLM and HMTBA having very different routes of absorption. DLM is absorbed by a combination of passive and sodium-dependent tertiary active transport in the small intestine, with increasing transport capacity as digesta moves from the duodenum to the terminal ileum. However, the hydroxy analog is a free acid that is absorbed via the L-lactate-associated monocarboxylate transporter along the entire gastrointestinal tract. In addition, diets high in fiber have been shown to decrease the apparent digestibility and retention of N and AA in swine. We hypothesized that HMTBA digestibility would be less susceptible than DLM to the antinutritional effect of fiber. Therefore, the objectives of our study were to determine nutrient concentrations in the duodenum and ileal digestibility of supplemental DLM and HMTBA in pigs fed a Met-deficient corn-soybean meal-based diet with or without supplemental fiber from wheat middlings (WM). Twelve 25-kg barrows were used in a replicated 6 x 4 incomplete Latin-square design with 6 diets and 4 periods per pig. Each pig was surgically fitted with simple T-cannulas in the terminal duodenum and ileum 2 wk before the initiation of the experiment and housed in a stainless-steel metabolism crate. The 6 diets were arranged in a 2 x 3 factorial of WM at 0 (CON) or 8% and Met source including none, DLM, or HMTBA.

Supplemental DLM doubled the duodenal free Met concentration from 9.6 to 19.6 g/100 g of DMI. Supplemental DLM also increased the apparent ileal digestibility (AID) of free Met from 52 to 92% (P < 0.05). Adding 8% WM increased (P < 0.01) duodenal NDF concentrations by 2 g/100 g of DMI and decreased (P < 0.05) AID of free Met. No HMTBA was detected in digesta collected from the duodenum or ileum of pigs fed the CON + HMTBA or the WM + HMTBA diets, indicating complete absorption before the terminal duodenum regardless of WM inclusion. Interactions between Met supplementation and WM were observed for the digestibility of all other nutrients (P < 0.05). Fiber and AA digestibility in the ileum were the greatest for the CON + HMTBA diet, and least for the WM + HMTBA diet. The CON + HMTBA diet had greater AID of ADF, NDF, CP, and Thr, and greater AID of ADF, NDF, ash, Arg, Ile, Gly, and Pro when compared with the CON and CON + DLM diets, respectively (P < 0.05). The WM + HMTBA had less AID of ADF, CP, and all AA with the exception of Lys, Trp, and Val when compared with the CON + HMTBA (P < 0.05).

In summary, absorption of HMTBA in the pig is complete by the end of the duodenum, but negative interaction between HMTBA and WM can decrease the AID of most AA.