Although these trials were not designed to study differences in productive performance due to the short period of study and low sample size, productive performance was monitored and there were some interesting findings. In S1, there were no differences in productive performance among diets. Thus, for S2, the authors decided to use younger pigs which are more likely to be affected by reductions in dietary AA levels similar to those used in S1 (from 1.15 to 0.75 g SID lysine per MJ). Pigs fed LA2 diet were the lightest and had the worst efficiency of all the diets in S2 probably because the levels of AAs were not enough to meet the requirements of the pigs (26). Pigs in S2 were also followed for a slightly longer period of time which may have allowed them to show differences in productive performance. It is interesting to note that productive performance of pigs fed LP2 diet was not affected compared to HP2. This diet was supplemented with AAs and both LP2 and HP2 had the same level of SID lysine (9.5 g per MJ). Thus, LP2 would achieve a lower risk of abnormal protein fermentations and lower emissions than HP2 with no negative effects in performance.
Serum metabolites have been used in pigs for research purposes but the literature on their use for clinical purposes is scarce and reference values are needed considering different factors such as age, breed, sex, diet, and methods of sample collection and analysis (34). In this study, the authors aimed to use serum metabolites as biomarkers to discriminate between diets differing on CP, AA, and NE levels with the final intention to use these biomarkers in daily practice as indicator of suboptimal diets. In S1, SUN was the clearest indicator of differences between diets with high and low CP discriminate in the ROC curve analysis with AUC close to 1. The principal end product of protein catabolism is SUN (35), thus it makes sense that SUN increases when the diet has an excess of CP that cannot be used by the animal due to CP excess or AA imbalances. Diets LP1 and HP1 differed in both, the level of CP and the level of AAs. To separate these 2 effects, in S2, LP2 and HP2 were formulated to have different levels of CP but the same levels of AAs and LA2 and HA2 were formulated to differ on AA levels. As expected, diet HP2, with an excess of CP, resulted in an increased SUN level compared to LP2. However, diet HA2 compared to LA2 did not induce the same increase in SUN that HP2 compared to LP2, despite having a similar increase in CP. The increase in CP in diet HA2 could be used by the pig for growth because it was achieved by increasing AAs levels according to ideal protein profile. Overall, SUN may be a useful indicator of protein efficiency at farm level when pigs are fed suboptimal dietary CP diets not balanced in AAs. The use of SUN is also an advantage because of its short time to achieve a constant concentration in blood after changing the diet (20).
Total protein and albumin are also involved in protein metabolism and were studied as interesting biomarkers. There were no differences in S1 in total protein and albumin between diets. The latter is in agreement with Regmi et al. (17), who did not observe differences in serum total protein concentration in finishing pigs, of similar age to those in S1, fed insufficient (0.32%), adequate (0.60%) or excessive (0.87%) SID lysine diets during 4 weeks, and only observed reduced plasma albumin concentration in finishing pigs when fed the 0.32% SID lysine diet which is far below the SID lysine levels of the dietary treatments of the present study. Nevertheless, in S2, pigs fed higher amounts of CP showed an increase in serum total protein and albumin levels. These findings are in accordance with some of the previous literature (16,18). Thus, the age of the pig may affect serum total protein and albumin levels and finishing pigs may be able to show a homeostatic control besides the dietary CP content. Moreover, finishing pigs have already reached the maximum protein deposition (36–38) and their metabolism may not be focused on protein turn-over contrary to early stages of the grower-finisher period. Albumin was a good biomarker to differentiate between LA2 and HA2 in S2, but it was not as consistent as SUN. Further research is needed to explore its use in multivariable biomarkers in combination with SUN.
Serum creatinine is also related to protein metabolism because is the product from muscle metabolism (34) and has a positive correlation with total and striated muscle (39). In the current studies, creatinine did not show any clear patterns and may not be as good as SUN as a biomarker.
Concerning energy, serum glucose did not show clear patterns between dietary treatments either which agrees with previous literature (15–17) and shows a good homoeostatic control of serum glucose concentration by either growing and finishing pigs in commercial conditions fed ad libitum. Triglycerides and cholesterol are both metabolites involved in lipid metabolism (34). They did not show any consistent pattern despite showing some differences between diets differening in NE levels. The incosistency in the results may be related to the age of the animals. The hypercholesterolemic effect observed in S2 pigs but not so clear in S1 is in agreement with previous literature (16–18), although the exact mecanishim of this effect is not clear yet. Further research should be carried out in order to further define these results.
No differences in total VFAs concentrations were observed between the dietary treatments in any of the two trials, and none of the individual VFAs showed difference worth discussing except for BCFA. The authors hypothesised that BCFA would show CP excess in the diets. However, the pattern differed between trials. Growing pigs in S2 fed HP2 diet showed a higher percentage of BCFA than pigs fed the HA2 and LE2 diets. Moreover, HP2 pigs had numerically greatly amounts of BCFA compared to those pigs fed the LP2 and LA2 diets. ROC curve analysis showed that BCFA has a moderate-high accuracy to differentiate LP2 and HP2 diets in growing pigs. These findings agree with previous literature that reported an increased production of BCFA in manure in grower-finisher pigs fed high CP diets (23,40). The low production of BCFA in pigs fed the HA2 diet compared to those fed the HP2 diet might be related to a fast absorption rate of free AA and a lower level of CP available for fermentation which agrees with the pattern observed for SUN. In S1, finishing pigs fed the HP1 diet did not show higher percentage of BCFA than any other dietary treatment. This absence of differences in BCFA might be explained by the fact that older pigs have a more developed gastrointestinal tract with a high fermentation capacity that makes it difficult to observe differences between the dietary treatments at faecal level. The differences may exist in cecum or proximal colon but are not present in faeces. On the other hand, a higher percentage of BCFA between pigs was found in pigs fed diet HE1 when compared to LE1. This difference could be related to the added fibre in the LE1 diet. In this line, a recent study reported that the increased body weight and age of the pigs resulted in an improved digestibility of dietary fibre fractions (41), which will influence the VFA profile as it is positively correlated with the apparent total tract digestibility of insoluble dietary fibre and cellulose (42). Therefore, the fermentation of soybean hulls could have produced a shift in the VFA profile reducing the BCFA production by the microbial population. Although BCFA did not show the same consistency as SUN, more research is warranted. The deamination of branched AAs may also cause a shift in the microbiome population to increase production of BCFA (25) which may be a more sensitive biomarker.