Blood and Faecal Biomarkers to Assess Dietary Energy, Protein and Amino acid Efficiency of Utilization by Growing and Finishing Pigs

Background: This study aimed to use the blood serum metabolite (BSM) and faecal volatile fatty acid (VFA) profiles to identify changes in protein, amino acid and energy dietary content in growing and finishing pigs. Results Two studies were conducted. The first study (S1) included 20 pens of 11 pigs (87.0 ± 4.10 kg; 18 weeks old) assigned to 5 diets: control (C1), high and low crude protein (HP1 and LP1), and high and low net energy (HE1 and LE1). The second study (S2) included 28 pens of 11 pigs (41.3 ± 2.60 kg; 12 weeks old) assigned to 7 diets: control (C2), high and low crude protein (HP2 and LP2), high and low amino acid (HA2 and LA2), and high and low net energy (HE2 and LE2). Pigs were followed for 10 (S1) and 20 (S2) days and blood and faecal samples were collected at 20 (S1) and 14 (S2) weeks of age. Data was analysed using general linear models and ROC curve analysis. Urea nitrogen was higher in pigs fed HP1 (13.6 ± 0.95 mg/dL) and HP2 (11.6 ± 0.61) compared to those fed LP1 (6.0 ± 0.95) and LP2 (5.2 ± 0.61; P  < 0.001) showing very good discrimination ability (AUC = 98.4 and 100%, respectively). In S1, creatinine (133.8 ± 3.97 µmol/L), triglycerides (0.42 ± 0.03 mmol/L) and branched-chain fatty acids (BCFA; 6.1 ± 0.38% of total VFA; AUC = 96.9%) were higher in HE1 than LE1 pigs (117.2 ± 3.97; 0.28 ± 0.03; 4.4 ± 0.38, respectively; P  < 0.05). In S2, albumin was lower (32.1 ± 1.08 g/L) and propionic acid was higher (24.5 ± 0.57% of total VFA) in LA2 pigs than HP2, HA2, LE2, and HE2 pigs (37.9 ± 1.08; 21.4 ± 0.57, respectively; P  < 0.001). Cholesterol was higher in HE2 pigs (2.8 ± 0.11 mmol/L) than LE2 pigs (2.4 ± 0.11; P  < 0.05). Finally, BCFA were higher in HP2 pigs (6.6 ± 0.41) than HA2 and LE2 (4.8 ± 0.41; P  < 0.05) and had a moderate accuracy to differentiate between HP2 and LP2 pigs (AUC = 87.5%). Conclusions Urea nitrogen is a good indicator with high accuracy to detect an excess of crude protein with inadequate amino acid profile in growing and finishing pig diets. Other biomarkers like albumin and BCFA showed promising results and need to be further studied.