Global pork production still under pressure

Gut development

Glutathione is produced from the amino acid cysteine, the production of which is heavily reliant on the dietary supply of methionine (Bauchart-Thevret et al., 2009). It is vital that the cells of the small intestinal epithelium are protected by antioxidants as they are constantly challenged with high levels of oxidative metabolism, toxins, and oxidants arising from the diet, causing oxidative stress. Research has shown that if insufficient quantities of antioxidants are present within the lumen of the small intestine, cellular proliferation is impaired, and apoptosis is elevated. Cysteine and glutathione are essential for gut health (Shen et al., 2015), maintenance, and cell survival (Bauchart-Thevret et al., 2009). In the study conducted by Bauchart-Thevret et al. (2009), piglets with a deficiency in sulphur-containing amino acids (cysteine and methionine) had reduced: intestinal growth, intestinal cell proliferation, glutathione production, and sulphur amino acid production within the small intestines. Therefore, methionine deficiencies can lead to both poor gut development and potential gut damage, which may culminate in production losses, owing to the impaired absorptive capacity of the small intestine. Figure 1 and Figure 2 illustrate the morphological differences in the small intestinal sections of piglets fed a diet either deficient in sulphur amino acids or a control diet.

Owing to the differences in metabolism of L-methionine and DL-methionine, differences in gut morphology were noted by several researchers. In the study conducted by Shen et al. (2015), broiler chicks supplemented with L-methionine had greater levels of glutathione present in the duodenal section of their small intestines, when compared to chicks receiving the same quantity in the DL-methionine form. This implies that L-methionine had a greater capability at maintaining the redox balance within the small intestine of L-methionine-supplemented chicks. In addition, the L-methionine-supplemented chicks had an elevated villus height to crypt depth ratio compared to their DL-methionine counterparts.

Figure 1: Intestinal cross-sections of piglets fed a control diet (control) or those fed a sulphur amino acid (SAA)-free diet. Note the impairment in gut development in the piglets from the SAA group (Source: Bauchart-Thevret et al., 2009).

Figure 2: Graphs showing some morphological parameters of the small intestine of both groups of piglets (source: Bauchart-Thevret et al., 2009).

Growth

Growth is essential in animal production systems, as the producer is paid per kilogram weight carcass or meat delivered, determining the economic viability of the business. Shen et al. (2015) calculated the relative bioavailability (RBA) of the two forms of methionine in their study, for the entire growing period of broiler chicks. They concluded that the RBA of L-methionine to DL-methionine for ADG was 138%; meaning that to achieve the overall ADG produced by 100 units of L-methionine, chicks supplemented with DL-methionine should receive 138 units of this form. The authors made the same calculation for the gain-to-feed (G:F ratio) and concluded that the RBA was 141%; meaning that 141 units of DL-methionine should be supplemented to chicks in order to achieve the same G:F response as chicks fed 100 units of L-methionine. Fascinatingly, higher RBA values were calculated within the first 10 days of the study, when compared to the total period, which may highlight the importance of supplementing the L-methionine form in the diet of younger animals. This difference in growth in chicks, between the two forms, may be attributed to the differences in intestinal cell morphology and development (Shen et al., 2015). In another study conducted by Shen et al. (2014) with nursery pigs, L-methionine-supplemented pigs tended to have improved average daily gain (ADG) values, compared to that of DL-methionine-supplemented pigs. The RBA values the authors calculated of L-methionine to DL-methionine was 144% for overall ADG and 123% for G:F. Therefore, L-methionine is the only functional form of methionine that is biologically available to the intestinal cells of young monogastric animals (Shen et al., 2015).

Feather development

In meat ducks, optimal feathering is essential to ensure high standards of carcass quality (Zhang et al., 2019). Keratin, a key component of feathers, is synthesised from the amino acid, methionine. In their study, Zhang et al. (2019) investigated the effects of L-methionine and DL-methionine on the feathering of Pekin ducks. The authors reported that L-methionine enhanced back feather growth in ducks when compared to DL-methionine, with an efficacy of 153%. In addition, DL-methionine tended to improve the length of the fourth wing feathers in the ducks.