Dietary Carbohydrates

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 :)