Casey L. Bradley¹, Ph.D.; Tsung Cheng Tsai², Ph.D.; Richard Mudarra²; Thomas Shau Shieh¹, Pharma D.

¹ Vitech Bio-chem Corp.; ²University of Arkansas

In modern day nutritional programs for livestock, protein is formulated in respect to bioavailable amino acids and ratios of the essential amino acids to lysine; while at the same time understanding the animal has a defined need for nitrogen or crude protein, beyond these essential amino acids. Historically protein nutrition has evolved down this path primarily due to analytical capabilities of the time.

To meet the nutritional needs of protein and/or amino acids, formulas contain protein-rich feedstuffs, synthetic amino acids and exogenous enzymes to promote digestibility of the feedstuffs. For younger animals, highly digestible protein sources are utilized due to the immature digestive system. Many of these specialty proteins naturally contain peptides, a group of smaller molecular proteins. Peptides are defined as short chains of two to 50 amino acids as oligopeptide, whereas protein is considered any branched chains of amino acids, in a tertiary structure.

As for now, diet formulation for both younger and older animals still remains focused on digestibility of amino acids and ratios, while there is mounting evidence that a balanced approach of peptide to amino acid to protein considerations should be considered, which could enhance digestibility of the diets, and potentially the efficiency or growth of the animal. This review will highlight the evolution of our understanding of protein nutrition.

The evolution of protein nutrition

Protein requirements were recognized in the 1700s by Albrecht von Haller, regarding the need of animal flesh in diets and in 1838 Gerardus Johannes Mulder introduced the term "Proteine." Before the 1950s, protein absorption was "hypothesized" that protein is completely digested into free amino acids and then absorbed or diffused as such. In 1951, Mathews and Smyth found that the absorption of L-amino acids was preferred by energy dependent amino acid transport carriers. Further work found that this "active absorption" was saturable and competitive among different amino acids. Continued research indicated that peptides were absorbed through different mechanisms than free amino acids, which was also found to be more rapid. Advancing analytical technologies also allowed for the observation that digestion in the gastrointestinal tract resulted in mainly small peptides then followed by free amino acids, which is different than earlier studies. Through this continued research, nutritionists can move beyond the "classical hypothesis" of only free amino acids are absorbed and apply the current knowledge of the peptide to amino acid model to meet the animal's protein requirements in diet formulation.

How are peptides absorbed differently than free amino acids?

The peptide absorption mechanisms are 1) cation dependent, energy required peptide transporters, PepT1 and PepT2; 2) endomembrane peptidase that hydrolyze di-, tri-peptides and transport the free amino acids into cell; 3) pinocytosis that envelope the larger peptide molecule and absorbs it intact into the blood stream; and 4) passive diffusion, which as much as 10% to 19% of oligopeptide molecules are absorbed in this manner. Whereas majority of free amino acids will compete with other free amino acids for specific transporters.

Interestingly, it has been demonstrated that feeding a peptide mixture with free amino acids can increase the overall absorption of amino acids. Peptides appear to reduce the competition for specific amino acid transporters, while increasing the speed and overall amount of free amino acids that are absorbed. These effects have been demonstrated in swine, poultry and ruminants.

Classification of peptides

Peptide molecules can be divided into two groups, nutritional and bioactive peptides, or previously known as functional peptides. Nutritional peptides are molecules that are digested further into amino acids. These amino acids are being used for protein synthesis or as an energy source. Bioactive peptide molecules possess biological or physiological functions. These molecules can exert their biological function locally or throughout the body.

Locally active bioactive peptides include antimicrobial peptide in the gastrointestinal tract; exorphine peptide can act on gastrointestinal tract to affect motility, gastric emptying time, satiety, etc.; phosphopeptide can chelate with minerals to enhance their absorption. Other peptides exert their function in the body to lower blood pressure, act as a growth hormone (insulin-like growth factor, glucagon-like growth factor, epidermal growth factor, etc.), counteracting endotoxin effects, modulating immune system. Research is ongoing to further discover biological and physiological functions of peptides in the body, especially for pharmaceutical applications.

Peptide-rich feedstuffs

A wide range of plant and animal protein sources have been used to supply protein in animal feeds, such as soybean meal, canola meal, fish meal and other animal byproducts. A group of proteins that are rich in peptides are spray-dried plasma, spray-dried eggs, dried whey protein and porcine intestinal mucosa. These products are commonly used in livestock feeds for neonates and aqua species, with different levels of inclusion. Not only are these ingredients rich in peptides, but are highly available protein sources, most likely due to their naturally higher percentage of peptides to amino acids.

An evolution of peptide-rich feedstuffs, for both nutritional and health benefits, are classified as bioactive peptides and can be considered as pronutrients. There is a plethora of research involving bioactive peptides for humans and the database is growing for animals as well. Research has shown that plant or animal protein can generate various bioactive peptides by enzyme hydrolysis and other processing techniques, similar to the digestive action of the gastrointestinal tract. Utilizing different types of feedstuffs, one can generate several types and/or amounts of bioactive peptides to be utilized in formulation. Therefore, their use and inclusion rates can vary dependent on the mode of action and animal species or subgroup. Some commonly processed feedstuffs to produce bioactive peptides include milk products, egg products, fish products and soybean meal.

There has also been an evolution of methodologies to generate these nutritional or bioactive peptides. These methods include acid or basic hydrolysis, enzymatic hydrolysis, fermentation, bioengineering and chemical synthesis. For the animal feed industry, the most cost-effective methods are enzymatic hydrolysis, fermentation or bioengineering. Acid or basic hydrolysis has many undesirable byproducts that may limit its use without further refining. Chemical synthesis is still cost-prohibitive. Many fermentation byproducts also contain peptides and can be used in feed.

In summary, there is mounting evidence to suggest a different approach to understanding bioavailability or digestibility of protein sources, which include both peptides and free amino acids. The inclusion of peptides has both nutritional and physiological capabilities that could benefit the sustainability of animal production. But further understanding on these modes of actions are needed to optimize the use and acceptance of peptides in animal nutrition.