Energy and protein metabolism in pigs

Metabolism is defines as the chemical changes in living cells by which energy is provided for vital processes and activities and new material is assimilate, as the sum of the processes by which a particular substance is handled in the living body and as the sum of the metabolic activities taking place in a particular environment. This text describes the metabolism of energy and proteins in pigs.

Energy metabolism

A pig needs energy for maintenance (necessary bodily functions) and for production (growth, lactation, piglet production). Pigs get energy from all organic compunds which they can digest, absorb and which their metabolic routes can use.  Energy is gained from carbohydrates, proteins, fats and organic acids. Carbohydrates are the most important energy source, but pigs can digest only starch and sugars, no cellulose or lignin.

Levels of energy (c) Evonik.com

Only a part of the energy gained is used for maintenance or production. Loss of energy happens in three stages:

• Gross energy: all the energy gained from feed
• Digestible energy: gross energy - energy in feces
• Metabolizable energy: digestible energy -
  - the energy in urine and metabolic gases
• Net energy: metabolizable energy - energy of heat produced by basic metabolism (heat increment)

Of these levels, net energy (NE) is the actual amount of energy the animal can use for production. It is only approximately 60 % of the gross energy (GE). Most energy is lost in heat (~20 %) and feces (~18 %). Chemically, oxidizing one mole of glucose (2870 kJ) produces 1976 kJ, so only 69 % is used as energy and 30 % lost as heat.

Energy is used primarily for maintenance. If a pig gets less energy than it needs for the maintenance, it utilizes its tissues and loses weight. When the energy gain exceeds the maintenance level (over 10 ME / day) , a pig is able to retain proteins and water into its tissues, building muscles. If plenty of energy is still available, pigs will convert the extra energy as body fat, which is not desirable in meat production. A pig growing 850g a day uses 3,8 MJ to gain 160g of protein for its muscles, 4,1 MJ to gain 105 of fat and 6,7 MJ for metabolism.

Nutrients can be either used as energy or in building body tissues. If fatty acids are used for energy, 66 % of the energy can be utilized. When using fatty acids for creating adipose tissue, the transformation efficiency is 90 %. For glucose the percentages are 68 % (for energy) and 74 % (for fat). From carbohydrates, starch, saccharose and glucose are most effective with 67 % of energy utilized as ATP. 

The amount and type of fibres in the feed affect the digestibility of carbohydrates. Pig feed must have enough sugar and starch to provide the needed energy.The more dietary fibre the feed has, the less energy is metabolizable. If the feed has 50 % of fibre, ~55 % of energy is metabolized. Neutral-detergent fibre (NDF) cannot be used by pigs, so the more NDF a feed has, the less energy pigs can get. High amounts of fibre also increase microbial fermentation in the colon, which increases loss of energy in gases.

Energy contents of nutrients and different feeds.

Protein metabolism 

Pigs need protein for several functions: own muscle growth, milk production and muscle growth for piglets, creation of enzymes and other proteins in the body, etc.

Animals don't actually need proteins but amino acids, which are the building blocks of proteins. Most important amino acids for pigs are lycine and metionine, but there are 9 essential amino acids in all. These amino acids can be mixed directly to the feed either as pure amino acids or as a part of a premix. The composition of amino acids in a feed determines its value as a protein source. When using home-made feeds, the amino acid composition must be determined with tests. It is also important to determine the intestinal digestibility of the amino acids. The need for amino acids can be scientifically determined by measuring production parameters (growth, carcass composition) and metabolic parameters (nitrogen in urine, urine and amino acids in plasma). The optimal composition of amino acids is called the "ideal protein", where every amino acid is present at the same time and at the correct ratio.

Animals can build non-essential amino acids only if they get enough of the essential ones at the same time. Essential amino acids cannot be synthetized at all. Unused or spare amino acids are fermented into ammonia by microbes in the colon, and the ammonia is then transferred into the liver and secreted as nitrogen. Pigs can only use amino acids which are absorbed from the small intestine. Excretion is an energy-consuming process, and increases the nitrogen emissions from pig production.

Comparing common feeds to the ideal protein gives an estimate on which amino acids and how much should be added to the feed. Barley meal lacks lysine and threonine the most,while soybean meal has almost enough of every amino acid, some even in excess. Maize lacks especially lysine and tryptophan, peas methionine and cysteine and fish meal histidine, phenylalanine and tyrosine. Digestibility trials show that adding lysine increases meat production in sows and hogs alike.

Proteins, which pigs get from their feed, are first denaturated to peptides in the stomach by HCl and pepsin. The peptides continue into the intestine, where several pancreatic enzymes break them further into oligopeptides. The intestinal wall secretes dipeptidase and aminopeptidase, which split the oligopeptides into amino acids. Amino acids are absorbed through the intestinal wall against the concentration gradient. Small peptides are also absorbed and hydrolysed. Blood stream carries the peptides and amino acids into different tissues, where they are used for protein synthesis and as energy. 

Different feeds induce a different amount of endogenous secretion: the secretion of mucus, microbe material and other nitrogenous compounds in the feces. Endogenous excretion must be taken into account when doing digestibility and metabolization trials. True digestibility excludes endogenous excretion, while apparent digestibility includes it. The digestibility of amino acids also varies between meals. Lysine is digested best from soybean meal and maize, and least from beet pulp and wheat. 

Soybean, fish and corn meals. (c)http://fomicanimalmeal.en.ec21.com

The need for protein can be calculated using a simple formula:

I = a (M + R/e)

where
I = need of amino acids from feed
a = utilization efficiency of amino acids
M = protein need for maintenance
R = protein need for muscle growth
e = degree of utilization