Energy Protein Interrelationships

Protein-energy interactions are essential for controlling nutritional digestibility and intake and the efficiency with which absorbed nutrients are utilised for synthesis. A change in protein intake can affect performance by altering the total dietary pattern. This is due, in large part, to variations in digestibility and the accompanying consumption of nutritional elements. Energy protein interrelationships play an essential role within the rumen and on the ruminant body. Due to the necessary demand for nitrogen (N) by rumen bacteria in rumen fermentation, the ratio of energy to protein nutrients available for absorption becomes constant, regardless of intake. This happens because the amino acids (AA) absorbed in the gut are proportionate to the energy fermented in the rumen.

What are proteins?

Proteins are organic substances composed of several building blocks (basic units) known as amino acids linked together by peptide bonds. For example, a dipeptide comprises one peptide bond and two amino acids, whereas a tripeptide comprises two peptide bonds and three amine groups. A polypeptide is a peptide that has more than ten amino acids. Proteins are massive polypeptide chains. The sequence of individual amino acids in a protein’s polypeptide chain determines its shape initially. This is also known as the protein’s primary structure. Protein is found in multiple forms in the body, like:

• Muscle, hair, hooves, skin
• Proteins in the blood (e.g., albumin, globulin) 
• Proteins found in tissues (e.g., collagen, keratin) 
• Hormones and enzymes 
• Antibodies of the immune system and other peptide growth factors

What is energy?

Energy is the ability to do tasks. This refers to the bodily activities necessary to keep the animal alive as well as production functions such as growth, lactation, and reproduction. 

Weight scales can be used to quantify feed quantity and animal protein or mineral needs. On the other hand, energy cannot be quantified in absolute terms, whereas feed energy content or cattle energy requirements are assessed in relation to a recognised norm.

Variables to measure energy

Gross energy (GE): It is the amount of heat created when a particular feedstock is combusted. 

Digestible energy (DE): It is the feed’s GE less the energy wasted in faeces. 

Total digestible nutrients (TDN): TDN is similar to DE, but it additionally takes into consideration the digestible protein content of the meal because protein has an energy value. 

Metabolisable energy (ME): It is calculated by subtracting DE from the energy lost in urine and gases generated during digestion. 

Net energy (NE): NE is the ME minus the energy lost as heat during the digestive process.

Heat production and net energy measuring methods

Measuring heat production is necessary to estimate the net energy. We can measure heat production in two ways: calorimetry and the comparative slaughter approach.

• Calorimetry: Direct calorimetry is a method of directly measuring heat generation. Indirect calorimetry examines gas exchange as it corresponds to heat generation from organic compound oxidation. 
• Comparative slaughter approach: It is a better estimate than calorimetry, but it takes a long time and is expensive, and the animals can only be used once.

Measures of estimating protein

• Net protein utilisation: This is the fraction of the animal’s nitrogen intake that is retained. 

1. The net protein utilisation value is the product of biological value and digestibility.
2. It is a measure of how much protein is accessible for animal metabolism.

• Protein efficiency ratio (PER): It is the most straightforward technique of determining protein quality. It measures a developing animal’s weight increase in relation to its protein consumption.

Protein and energy requirements for maintenance 

When there is no gain or loss of nutrients from the body, it is called maintenance. The energy demand for care is the bare minimum required to maintain the animal’s energy balance. The best way to assess energy demand is to measure energy expenditure.

Estimating protein maintenance requirements are more complex than counting energy maintenance requirements. Because protein deposition in adult animals is restricted, an excess of protein leads to the deamination of protein and the use of the resultant N-free substances as a source of energy. 

How is a protein related to energy?

When amino acids are supplied more than capacity, they are deaminated, incurring the energy cost of generating and excreting the amino group as urea. The carbon skeleton is then oxidised, and a portion of the carbon skeleton may be retained as fat. At high crude protein intakes, one would predict a mean energy use efficiency reduction. Absorbed amino acids might be utilised for tissue protein, enzyme, and hormone production, as well as deamination or transamination, and the carbon skeleton could be used as an energy source.

If an animal is suffering from an energy deficiency, a lack of protein in its diet exacerbates the illness. Therefore, when consumption of overall energy (carbohydrates and fats) should be increased, protein supplementation is frequently proposed incorrectly.

Conclusion

It has been hypothesised that a reduction in nitrogen balance is associated with decreased energy consumption. This is further explained by satisfying energy demands takes precedence over protein. As a result, when the metabolic requirement for protein is sufficient or excessive, proteins are utilised as an energy source. The energy protein interrelationship is based on the fact that protein is transferred to energy only when it exceeds the metabolic demand or when sufficient calorie intake. In energy scarcity, protein may also be utilised as an energy source.