Protein digestibility kinetics: A key criterion for discriminating between protein sources in pet food (Part 1)

Introduction 

The search for innovative protein sources is on the rise as a result of increasing global food consumption and consumer concerns about sustainability, animal welfare, and strain on agricultural land. In line with the humanization trend, pet owners are likely to select a pet food based on their own dietary preferences. Moreover, the increased awareness of the importance of proper nutrition has led pet owners to become more and more careful when looking at pet food labels. Clean label is amongst the top trends in the pet food market and seems to be driven by the perceived link between health and naturalness. Consequently innovative protein sources are of great interest in the Pet Industry as a way to meet the growing market demand for pet nutrition. 

Proteins: The importance of digestibility and absorption kinetics 

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Proteins are biological macromolecules made up of amino acids. Protein bioavailability is strongly related to its amino acid composition, size, and mass range. Big proteins, proteinassociated molecules and nucleic acids have lower digestibility compared to free amino acids and small peptides, which have been exposed to a multiple nitrogen hydrolysis process that makes them more bioavailable (Figure 1). 

The digestion process is complex. It is driven by enzymatic activities and is pH dependent. In monogastric animals, this process starts in the stomach where protein denaturation occurs due to pepsin activation. In the duodenum, pancreatic and intestinal wall enzymes continue the digestion process, breaking down the proteins. The absorption of amino acids, dipeptides and tripeptides occurs in the jejunum. The large intestine is the site of microbial fermentation, where short chain fatty acids (SCFA) are produced. 

Supplying animals with a protein source that is already processed and rich in free amino acids and small peptides helps ensure better absorption in the gut. 

Protein digestibility of feed materials is an important factor for nutritionists, but it does not reflect the timing of absorption in the gut. The kinetics of protein absorption should be considered in addition to protein digestibility. It allows nutritionists to properly assess the quality of the protein source. Many properties and functionalities of proteins are related to their absorption kinetics. The kinetics of protein absorption helps categorize the protein fraction according to the speed of absorption. Four categories can be defined as flash, fast, slow and resistant along the digestive tract (Figure 2).

Did you know? Indigestible proteins are used as a substrate for undesirable bacteria in the gut, inducing proteolytic fermentation. 
Ultimately, it may lead to diarrhoea due to microbiota dysbiosis (Jung Gao et al., 2019). Therefore, the higher the protein digestibility, the greater the protein source for animals from a digestive standpoint. 

Kinetics of protein absorption: How is it measured?  

Several in vitro methods simulating the digestive processes have been developed to estimate the protein digestibility and the kinetics of absorption. 

For example, the Boisen standard method (Boisen and Fernandez, 1995) is very accurate in measuring in vitro protein digestibility, which is calculated from the difference between nitrogen in the raw material sample and undigested residue measured at different time points of the intestinal pase (Figure 3).

Did you know? In most cases, validation of the methods is based on the relationship between in vitro results and values of apparent digestibility at the fecal or ileal level (Furuya et al., 1979; Metz and van der Meer, 1985; Graham et al., 1989; Cone and van der Poel, 1993). However, endogenous losses of proteins at the ileal level might have a great influence on in vivo digestibility while in vitro digestibility reflects the real digestibility of proteins and amino acids (Boisen and Eggum, 1991). Since in vitro digestibility does not take into account endogenous losses, the apparent ileal digestibility can only be predicted accurately from in vitro measurements, after correcting for the endogenous losses. 
These losses may be influenced by a variety of factors, especially the fiber content in the diet (Taverner and Farrel, 1981; Wünsche et al., 1987). Alternatively, a considerable part of this variation may be explained by the amount of undigested dry matter at ileal level (Boisen, 1991). 

By: Francesca Susca -Global Pet Product Manager- and Lisa Saibi -Product Manager Yeast derivatives & antioxidants

Source: Lallemand Animal Nutrition