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A nutrient is a substance used in an organism’s metabolism. The six main groups of nutrients are carbohydrates, fats, proteins, vitamins, minerals and water. Nutrition is the series of processes, by which an animal takes in and assimilates these nutrients for promoting maintenance, growth, reproduction, milk or work.
Proteins are high weight molecules inevitable for all cell functions. Animals rely on a permanent protein supply throughout life for maintenance, growth and reproduction. Amino acids are the building block of protein, each protein consisting of a chain made up of amino acids in a unique sequence.
Proteins are complex organic compounds found in every living cell. Numerous proteins are enzymes that catalyze biochemical reactions or hormones vital to cell metabolism. Some prominent proteins representing enzymes or hormones are pepsin, lactase, insulin and adrenalin. Other proteins like collagen are important for extracellular structure while myosin is an example for a protein associated to mechanical functions in muscle tissue. Yet other proteins are involved in cell signalling, immune response and cell adhesion.
In line with carbohydrates and fats all proteins are composed of carbon, hydrogen and oxygen. Furthermore they all contain nitrogen, frequently sulfur and sometimes phosphorus.
Primarily proteins are built up of individual amino acids. An amino acid is a molecule containing two functional groups (carboxyl- and amino group) and an organic substituent (R group), which accounts for the characteristic properties of the different amino acids. Due to the possible spatial configuration of these groups around a central carbon atom each amino acid has a so called D- and L-form. Thereby the D-amino acid structure is the mirror image of the L-amino acid (chirality), like your left and your right hand. Amino acids found in proteins all belong to the L-form. The protein chain (polymer) is generated by linking the carboxyl group of one and the amino group of the other amino acid to each other.
Thereby a total of 20 proteinogenic or protein building amino acids exist. Depending on the length of the protein chain generated a distinction is drawn between peptides (2-20 amino acids long) and poly peptides (20-100 amino acids long) even though most proteins are longer than 100 amino acids.
The digestion (breakdown) of proteins is necessary in order to allow the passage through the intestinal wall and into the bloodstream. Digestion starts in the stomach where the protein is denatured by the excretion of hydrochloric acid. In addition pepsin a so called protease cleaves the protein chain. The stomach content is then transported to the small intestine where further proteases operate. The proteases in stomach and small intestine differ in their properties to cleave proteins.
Today’s amino acid sources in animal nutrition are feed proteins or supplemental amino acids. Their digestibility is influenced by different factors. Amino acids offered as supplements for animal diets are created via chemical or fermentative production processes. When applied correctly in feed these amino acids have a digestibility of 100 percent. Today the so called SID (standardized ileal digestibility), which considers the basal losses, is used as the best measure of digestibility.
The biological value of a feed protein depends on its amino acid configuration. Optimum animal performance (feed intake, weight gain) is achieved when the protein fed contains an ideal amount and proportion of all essential amino acids. This feed protein is called an “ideal protein”. Feeding diets based on vegetable protein or animal protein show that the performance of the animals is significantly better when animal protein is fed. This means that animal protein is closer to an ideal protein than vegetable protein. But adding a small amount of a certain essential amino acid to the vegetable protein diet improves the biological value of its protein measured by animal performance. Raising the concentration of this amino acid in the diet can further improve performance but only up to a certain level. Then adding more of this amino acid cannot induce further improvement. The “Liebig barrel” illustrates this limitation of protein synthesis due to the lack of an essential amino acid. The shortest stave of the barrel represents the first limiting amino acid (here Met).
It is necessary to fill this gap in order to compose an ideal feed protein. Of course if a first limiting amino acid exists optimum supply of a second (Lys) and third limiting amino acid (Thr) will also contribute to the formation of an ideal protein and “help completely fill the barrel”. This is the so called concept of the first limiting amino acid: “If the rate of protein synthesis is lowered due to an inadequate supply of a limiting amino acid then increasing the limiting amino acid should increase protein synthesis” (Mitchell et al. 1946) and thereby improve animal performance.
In recent years a related feeding concept has been pursued. Conventional diet formulation is based on crude protein estimation with a safety premium in order to avoid amino acid malnutrition. Consequently higher excretion of nitrogen containing compounds is the result because protein supply is not matched as close as possible to the requirements. But increasing feed costs and environmental aspects have caused rethinking. It is possible to lower the crude protein content of a diet and provide the missing amino acids using supplemental amino acids. These diets are called low protein diets. This concept enables the producer to maintain animal performance while feeding a lower cost diet with less environmental burden.
The optimum protein nutrition of farm animals allows minimum nitrogen excretion. All excess or nonutilizable protein in a diet needs to be detoxified via nitrogen containing compounds in urine. Nitrogen excretion in turn is associated to several environmental aspects including: animal welfare, pollution and economical benefits.