XII. Proteins and Amino Acids, Quality

The term protein quality refers to the availability of amino acids that the protein supplies, and digestibility considers how the protein is best utilized. There are various methods that rank the quality of different types of protein, and this chapter describes some of the in vivo animal bioassays and in vitro laboratory analysis commonly used in animal nutrition.

Several factors can affect protein quality. Animals do not have a protein requirement, rather they have an amino acid requirement. The basic function of dietary protein is to supply adequate amounts of required amino acids. Thus the quality of feed protein depends on the amino acid profile of the diet, the content of essential amino acids as well the content of limiting amino acids, and the digestibility and physiological utilization of amino acids after digestion. These factors are most important for nonruminants. Solubility and digestibility are also important for ruminant animals.

Protein Digestibility and Bioavailability

What is meant by the digestibility of protein?

The digestibility of protein can be defined as the fraction of the protein ingested that is absorbed by the animal—that is, not excreted in feces. Digestibility can therefore be calculated by measuring dietary protein input and fecal output as shown in the following equation:

Digestibility assays are a favored technique for measuring the availability of amino acids. Most common and published values on the digestibility of protein and amino acids in monogastric animals are based on fecal or excreta analysis because of its simplicity and sample size, as a large number of animals could be used without sacrificing the animals. Fecal-based digestibility measurements, however, suffer from the modifying and variable effects of the hindgut (microbes’ dietary protein utilization and the contribution of microbial and endogenous proteins to amino acid excretion in feces contributing to fecal amino acid output).

Fecal digestibility assays are done through fecal collection and analysis, while ileal digestibility assays are done through digesta collection at the ileal junction, either using cannulated animals or slaughter in small animals such as poultry.

The term digestibility is often used synonymously with bioavailability. However, this is not true. Bioavailability of amino acid can be defined as an amino acid in a form suitable for digestion, absorption, and utilization. Bioavailability can only be estimated with animal growth bioassays using live animals.

Several animal and feed factors can affect both digestibility and bioavailability. For example, the source of protein, such as animal versus plant protein, and feed processing methods can affect protein digestibility. Animal proteins are more balanced with essential amino acids and nonessential amino acids. Plant proteins, such as soybean meal, can have antinutritional factors (e.g., trypsin inhibitor) that can affect protein digestibility and availability of amino acids.

Feed processing methods such as heating, roasting, and extrusion can disrupt antinutritional factors like trypsin inhibitor in soybean meal. However, overheating (browning, or Maillard reaction) makes amino acids unavailable due to a complex reaction with sugars. In addition, other interrelationships such as antagonism, excesses of one amino acid interfering with the metabolism of another amino acid, making them unavailable (e.g., lysine and arginine) can precipitate amino acid imbalance.

Methods for Assessing Protein Quality

Protein quality can be assessed by bioassays using live animals or chemical assay.

Biological Value (BV): This is a measure of the proportion of absorbed protein from a feed that becomes incorporated into the proteins of the animal’s body. It determines how readily the digested protein can be used in protein synthesis in the animal. BV assumes protein is the only source of nitrogen and measures the proportion of nitrogen absorbed by the body that is then excreted (fecal and urine). The remainder must have been incorporated into the proteins of the animal’s body. A ratio of nitrogen incorporated into the body over nitrogen absorbed gives a measure of protein “usability” or BV.

As a value, NPU can range from 0 to 1 (or 100), with a value of 1 (or 100) indicating 100% utilization of dietary nitrogen as protein and a value of 0 an indication that none of the nitrogen supplied was converted to protein. Egg protein has a high value of 1 (or 100)

It turns out that NPU = digestibility × BV. Since NPU considers digestibility, it is a better estimate for protein quality.

Protein Efficiency Ratio (PER): The PER was the first method adopted for routine assessment of the protein quality of food. The PER is based on the weight gain of a test animal divided by its intake of a particular food protein during the test period. PER assumes that all protein is used for growth.

Both BV and PER = 0 when nongrowing animals are used.

Other chemical (in vitro) methods of assessing protein quality include the amino acid score. This is done through laboratory analysis of amino acid profiles using high-pressure liquid chromatography, and the results are compared to a standard (or reference) protein such as egg protein (albumen) and given a score. Although no animals are involved, these scores can give some easy and quick information. However, they do not give any information on palatability, digestibility, or availability.