Supplemental dietary methionine sources have a neutral impact on oxidative status in broiler chickens

The results of growth performance are presented in Table 3. Methionine supplementation significantly improved the live weight and feed conversion ratio of broilers compared to the methionine deficient basal diet; however, methionine source had no effect on growth performance.

The results of the GSH profile are presented in Table 4. Met sources did not affect the GSH markers in any of the tissues with the exception of a higher concentration of TSGH and rGSH in breast tissues for birds fed L-Met compared to broilers fed the control and DL-Met diet. However, there were no differences between broilers fed DL-Met and DL-HMTBA. The ratio of reduced to total glutathione was not significantly affected by Met source and oxidized GSH was not detectable in breast tissue from any treatment group. There were a few significant interaction effects between Met source and broiler age observed (Figure 1). In the Jejunum the L-Met treatment group had higher oxidized glutathione (GSSG) than the control group or DL-Met treatment at d 26 but not at d 10 (Figure 1B). Conversely, in the ileum the L-Met treatment group had higher total and reduced glutathione than the DL-Met on d 26 but no differences were observed on d 10 (Figure 1D and E). However, none of the treatments affected the ratio of reduced to total glutathione in either jejunum or ileum at either time point (Figure 1C and 1F, respectively). Therefore, it can be concluded that dietary Met source did not affect the balance of the glutathione system in breast, liver, duodenum, jejunum or ileum in broilers at either d10 or d26.

Oxidative stress can lead to tissue damage due to oxidation of cell proteins, lipids and DNA. Therefore, biomarkers of protein oxidation (protein carbonyl) and lipid peroxidation (thiobarbituric acid reactive substances, TBARS), were measured in various tissues (Table 5). Additionally, the antioxidant potential of the tissues was determined by measuring the ferric reducing antioxidant power (FRAP) of tissues from the broilers fed different dietary Met sources. Neither the lipid peroxidation marker TBARS, nor the antioxidant power FRAP were affected by dietary treatment. However, dietary treatment affected protein carbonyl levels in the jejunum and breast muscle. DL-HMTBA treatment decreased protein carbonyl in jejunum compared to DL-Met, but neither differed from control. In breast muscle there was a diet x age interaction effect for protein carbonyl showing no dietary effects at d 10 but significant effects of diet at d 26. L-Met increased protein carbonyl in breast muscle at d26 compared to Met deficient control and DL-HMTBA, but not DL-Met. These findings were not consistent over tissues and at different time points, as there were no effects of dietary treatment observed for liver, duodenum or ileum at any time point and no treatment effects observed at d10 in any tissue.

Overall, the results show that dietary methionine source (DL-Met, L-Met or DL-HMTBA) did not affect antioxidant status of broiler birds in physiologically normal conditions as there were no significant effects of dietary met source observed on antioxidant to oxidant ratios. Additionally, there were no consistent effects of dietary Met source on antioxidant or oxidative stress biomarkers when comparing different tissues or time points.

Unexpectedly, sub-optimal Met+Cys did not impact antioxidant status in the tissues of broilers in this trial. Although, a recently published study has shown that when Met supplementation is tested in a more sensitive dose response experimental design, sub-optimal dietary Met+Cys does increase oxidative stress in tissues (Zeitz et al. 2018). It can also be speculated that the lack of observable effects of the sub-optimal Met+Cys basal diet in this study could be partially attributed to the lack of oxidative stress challenge. If the broilers had been exposed to challenging conditions causing oxidative stress (such as heat stress or a disease challenge), the need for TGSH production would have been higher. Therefore, the negative impact of a deficiency in Met+Cys as a precursor for TGSH synthesis, on the GSH system and antioxidant capacity of the tissues may be more easily observed in oxidative stress conditions. Future research, will investigate the effect of Met dose and source in challenging conditions associated with oxidative stress.

In conclusion the study shows that DL-Met, L-Met and DL-HMTBA (formulated at 65 % BE) did not differentially affect growth performance or the oxidative status in the digestive, metabolic and meat tissues of healthy broilers.