Hello, in this post we'll be discussing dietary carbohydrates. We'll take a look at what dietary fibre, Non-Starch Polysaccharides and Resistant Starch are and how the body uses them. We'll finish off by discussing how the carbohydrates in a food can be measured.
Dietary Fibre
Dietary fibre is a complex mixture of carbohydrate polymers that are associated with non-carbohydrate components. It consists mainly of non-starch polysaccharides and lignin with small quantities of insoluble proteins, fatty acids and waxes. Dietary fibre is resistant to digestion by enzymes in the stomach and small intestine and is fermented by microbes in the large intestine and/or rumen of the animal.
Non-Starch Polysaccharides (NSPs)
NSPs come mainly from cell walls and are the building blocks of carbohydrates. There are three main components of NSPs:
- Cellulose
- Non-cellulosic polymers: this includes arabinoxylans, mixed-linked beta-glucans, mannans, galactans, and xyloglucan.
- Pectic Polysaccharides: this includes polygalactouronic acids, substituted with arabinan, galactan and arabinogalactan.
NSPs can be soluble or insoluble and these may have differing physiological effects. Soluble NSPs are soluble in water and tend to form viscous gels. Their effects include:
- Decreased digestibility
- Decreased nutrient absorption
- Increased water-holding capacity
- Increased transit time and thus a delay in gastric emptying
- Increased gastric, biliary and pancreatic secretions
- Increased loss of enterocytes
- Increased growth of pathogenic bacteria
Insoluble NSPs:
- Decrease transit time
- Increase water-holding capacity
- Assist in Faecal Bulking.
Resistant Starch
Resistant starch (RS) is starch that escapes enzymatic digestion in the small intestine and are fermented in the large intestine. They provide a substrate for the growth of 'good bacteria' as well as butyrate (a volatile fatty acid) for the colonocytes. There are four types of resistant starch:
- Starch physically inaccessible to enzymes (eg, in intact cells)
- B-Type starch granules (as is found in uncooked potatoes)
- Retrograded amylose in processed foods
- Chemically modified starch.
Resistant starch is thought to lower blood pressure and the amount of lipids in the blood. It also increases glucose tolerance. This is because gluconeogenesis is required to use the starch as energy, this process is well regulated and so glucose is used at a slower rate. This leads to a lower GI (glycaemic index).
In the gastrointestinal tract, resistant starch lowers the pH of the colon through fermentation (the process forms volatile fatty acids). Faecal bulk is also increased and more good bacteria (Bifidobacteria) grow. It is also thought to reduce the risk of colon cancer.
Measuring Carbohydrates
Crude Fibre, derived from proximate analysis, may be flawed when it is applied to plant material. This is because the process used to determine Crude Fibre requires the sample to be boiled in a weak acid and then in a weak alkali in order to simulate the effects of the digestive tract. It assumes that any carbohydrates that are soluble can be digested by the animal. In this method most of the lignin (a non-digestible component) is dissolved.
There may also be errors in the crude fibre values because of errors in each of the proximate principles determined before the crude protein value was determined.
Because of these limitations, Van Soest analysis is used to analyse carbohydrates in feed.
Van Soest Analysis
Van Soest analysis splits plant carbohydrates into two components: cell contents and the cell wall. Structural carbohydrates are measured as Neutral Detergent Fibre (NDF). The Acid Detergent Fibre (ADF) represents NDF minus the hemicellulose content.
ADF represents the least digestible components of the feed and so diets high in ADF have lower digestibility and energy content.
Overall, Van Soest Analysis provides a good estimate of the energy contribution for herbivores but it is less helpful for describing the physiological effects of the food.
That's it for this post. See you next time :)
Hi :) In this post we'll learn how dietary protein is used by both monogastric and ruminant animals. We'll take a look at what happens to the protein that we feed monogastrics and ruminants as well as what affects dietary protein quality.
Proteins are chains of amino acids which can be essential or non-essential. Essential amino-acids are those that are not produced by the body and so must be obtained by the animal through its diet. Animals can get non-essential amino acids from their diet or they can be synthesised by their body. Proteins and energy are some of the most important components of an animal's diet and the effects of a protein deficiency are quite rapid. Thus, it is important that animals receive enough protein in their diet.
How much is enough? Well, the amount of dietary protein required by an animal depends on its species, physiological state (eg reproduction, lactation, growth etc), level of exercise, and disease status. The amount of protein needed increases with growth, breeding, reproduction, lactation and work.
Fates of Dietary Protein in Monogastric Animals
The protein that is provided in its diet is known as ration protein. Some of the ration protein is lost as faecal nitrogen. Faecal Nitrogen consists of: unabsorbed ration protein, spent enzymes, sloughed cells and intestinal microbes. The remainder of the ration protein is digested and absorbed and is now known as metabolisable protein because the body is able to use it.
Some of the metabolisable protein is used to make digestive proteins which contribute to the ration protein because the enter the digestive tract where they can be digested and absorbed by the body. Some of the metabolisable protein is used to generate energy. The by-products of this process are carbon dioxide, water and urinary nitrogen. The remainder of the metabolisable protein is used to make other proteins to make more tissues and to produce milk, eggs, wool, etc. Some protein is lost through tissue catabolism and takes the form of endogenous urinary nitrogen.
What About Ruminants?
Ruminants use their dietary protein differently to monogastrics in two main ways:
- Rumen microbes ferment the dietary nitrogen.
- Ruminants have a high efficiency of nitrogen recycling through saliva and conversion to absorbable amino acids.
Ruminants receive protein in their diets in the form of Dietary Crude Protein. This protein enters the rumen where it is either degraded or left undegraded. The degraded protein is either degraded slowly or rapidly to amino acids, peptides and ammonia. The ammonia can form urea which may enter the urine or be recycled in the rumen where it is broken down to ammonia. Ammonia, peptides and amino acids are used by the rumen bacteria to make
bacterial and protozoal protein.
The bacterial and protozoal protein as well as the undigested dietary crude protein are broken down into amino acids in the abomasum and small intestine. The amino acids are used to make tissue protein.
Thus, ruminants are able to utilise non-protein nitrogen (eg nitrates, nitrites and other nitrogen from plants) and protein for the synthesis of microbes which they then digest. This microbial protein also provides the full range of essential and non-essential amino acids. How cool is that!
Rumen Degradability
The degradability of a foodstuff is dependent on several factors:
- Rumen Solubility: more soluble proteins are more degradable because microbes are able to use them more rapidly.
- The Amino Acid Sequence in the Polypeptide: this is because the rumen microbes only have enzymes which can cleave particular types of bonds.
- Rumen Residence Time: The longer the protein is in the rumen, the longer there is for it to be broken down.
- Intake of Feed: the higher the intake, the less time the proteins can stay in the rumen.
- Particle Size: smaller food particles increases the surface area of feed available to the microbes and so can be expected to increase rumen digestibility. However, increased particle size also decreases the time spent in the rumen which decreases digestibility. Thus, these two factors need to be balanced.
Factors That Affect the Level of Microbial Protein Synthesis
The amount of protein produced by the microbes in the rumen depends on a few factors:
- Energy supply to the microbes
- Nitrogen supply to the microbes
- Level of feed intake by the animal
- Outflow rate: this is affected by the level of feeding and also lactation.
Protein Requirements of Ruminants at Different Stages of Production
The protein supplied by the microbes is generally enough for the protein requirements for maintenance and slow growth. However this supply is not enough for early growth, late pregnancy, parturition and lactation.
Once the dietary crude protein reaches 14% the ruminal microbes are at their crude protein cap - they are using as much of this dietary protein as they possibly can. When the dietary crude protein exceeds 16% the protein is converted to ammonia. Increasing levels of ammonia can be dangerous because it puts the animal at an increasing risk of ammonia toxicity.
So How Do We Supply Protein Beyond Microbial Capacity?
The solution is to increase the level of dietary protein that can't be degraded by the microbes (Undigestible Protein). This can be done by decreasing the size of the food particles, increasing the amount of food available or treating the food with chemicals or heat.
Protein Quality
There are three factors that affect the quality of the protein given to an animal:
- Amount of protein
- The mix of essential amino acids relative to the target protein: for example, if you want to maximise egg production in a chicken you should feed it a diet that contains a essential amino acids that mimic the essential amino acid mix in eggs.
- The digestibility of the protein.
- This is the percentage of protein digested in the GI tract.
- Apparent CP digestibility (%) = ( (N Intake - Faecal N) / N Intake) x 100
- True CP Digestibility (%) = [(N Intake - (Faecal N - Endogenous Faecal N) ) / N Intake] x 100
The rate-limiting amino acid is the essential amino acid that is in greatest deficit in regards to the target protein. Lysine is usually the rate limiting amino acid in most production systems.
That's all for now, if you have any questions let me know in the comments section :)
