Amino acid digestibility

The content of amino acids supplied by dietary proteins do not represent the amount of amino acids fully available to the pig. The availability of amino acids refers to the amount of amino acids that are digested, absorbed, and available in a suitable form for protein synthesis. Availability is traditionally estimated by the digestibility of amino acids as it provides a fair estimate of the amount of amino acids in protein that is actually available to the pig. The digestibility of amino acids is variable depending on protein sources, anti-nutritional factors, and processing methods, particularly heat treatment. The digestibility of amino acids is expressed as total tract digestibility or ileal digestibility:

• Total tract digestibility: measured by the difference between the amount of amino acids ingested and the amount of amino acids recovered from the feces.
• Ileal digestibility: measured by the difference between the amount of amino acids ingested and the amount of amino acids recovered from the digesta in the ileum.

The ileal digestibility is a more accurate method to estimate the amino acid bioavailability than total tract digestibility because amino acids are exclusively absorbed in the small intestine but the microbial fermentation in the large intestine lowers the amount of amino acids recovered from the feces and overestimates digestibility. The ileal digestibility is estimated in cannulated pigs at the end of the small intestine by collecting samples of ileal digesta. The ileal digestibility is expressed as apparent (AID), standardized (SID), or true (TID) ileal digestibility, depending on how endogenous amino acid losses are considered in the measure of digestibility:

Endogenous amino acid losses: represent amino acids from proteins that are synthesized for metabolic functions by the pig, such as enzymes and sloughed cells, that have not been absorbed at the end of the small intestine and are lost. Endogenous amino acid losses are divided into basal or specific losses. Basal losses represent the amount of amino acids that are inevitably lost, whereas specific losses represent the losses of amino acids abovethe basal losses that are influenced by specific diet composition. Endogenous amino acid losses underestimate the ileal digestibility of amino acids because the endogenous amino acids in ileal digesta are accounted for as non-digested amino acids from the diet.

• Apparent ileal digestibility (AID): measured by the difference between the amount of amino acids ingested and the amount of amino acids recovered from the digesta in the ileum without considering the endogenous amino acid losses.
• Standardized ileal digestibility (SID): measured by the difference between the amount of amino acids ingested, the amount of amino acids recovered from the digesta in the ileum, and the basal endogenous amino acid losses.
• True ileal digestibility (TID): measured by the difference between the amount of amino acids ingested, the amount of amino acids recovered from the digesta in the ileum, and both basal and specific endogenous amino acid losses.

The most widely used method to formulate diets and estimate digestibility of amino acids is SID. The values for SID of ingredients are more likely to be additive in diets and some of the disadvantages and limitations of AID and TID are overcome: whereas the AID does not consider any of the endogenous amino acid losses, there is insufficient information available about TID and the ingredient-specific effects on endogenous amino acid losses.

Fact Sheet: Protein Sources for Swine Diets