Determination of Plasma Metabolites and Basal Ileal Endogenous Amino Acid Losses in Growing Pigs Fed a Nitrogen‐Free or Casein Diet

The nitrogen-free diet (NFD) method for determining basal ileal endogenous losses (BEL) of amino acids (AA) has been associated with different metabolic abnormalities that can affect the accurate determination of BEL. Consequently, the use of highly digestible proteins has been suggested. This study aimed to determine the metabolic status and BEL of AA in pigs fed either an NFD or a casein (CAS) diet. Eight cannulated, castrated male pigs (39.8 kg) were randomly assigned to a 2 × 2 crossover design. An NFD diet based on corn starch, dextrose, cellulose, oil, vitamins and minerals was used. The CAS diet was equivalent, but 18% of the corn starch was replaced with casein. Pigs were fed one of the diets for a 7-day period, and blood samples were collected at the beginning and end of each period to determine plasma metabolites. Ileal digesta samples were collected on Days 6 and 7 to estimate the BEL of the AA. Results indicated that plasma albumin was significantly higher (p < 0.05) in pigs fed the CAS diet, whereas creatinine and LDL levels were higher (p < 0.03) in pigs fed the NFD. No significant differences were observed in the levels of other plasma metabolites. The BEL of protein did not differ between diets. However, in pigs fed the CAS diet, the BEL of glutamic acid, aspartic acid, serine, glycine, histidine, threonine, alanine, tyrosine and valine significantly increased (p < 0.05), while isoleucine showed a tendency to increase (p = 0.06). In conclusion, NFD did not significantly affect energy and lipid metabolism in pigs. However, the decrease in albumin synthesis and increase in plasma creatinine levels indicate that pigs fed NFD have a negative protein balance, affecting the estimation of the BEL of AA. Therefore, it is essential to consider the metabolic state of animals when estimating the BEL of AA.