Nutrient Utilisation

Hello :) In this post we'll compare and contrast growth and digestibility trials. I'll also explain the difference between apparent and true digestibility, why people use markers in food, the advantages of determining ileal digestibility as well as the factors that affect digestibility.  

Growth Trials

Growth trials - when the growth of an animal is measured after it has been on different feeds - is the simplest way to compare different diets or nutrient sources that we give to animals.Feeding different diets ad libitum allows us to determine rate of gain, voluntary feed intake and efficiency of gain.

Growth trials are good because they are relatively cheap and large amounts of data can be collected under realistic conditions. The measurements are also obtained non-invasively and the results represent the biological response to the diet.  

However, growth trials are really only useful for meat-producing animals as they have limited usefulness in animals in which weight change is not the desired outcome (eg. we may want wool to be produced instead). In addition, there is variability between individuals in the biological response to the diets. There are also challenges in obtaining accurate weights and the data often doesn't reflect changes in body composition.

Digestibility Trials

Digestibility trials aim to determine the digestibility of a food. That is, to work out how much of the energy in the food is actually used by the animal and how much is excreted. Digestibility represents the proportion of food that is not excreted in faeces and urine and is assumed to be absorbed by the body. To determine digestibility, animals are fed a diet of known composition for a period of time and all the faeces voided are collected and analysed. 

The results of digestibility trials are only useful if the following conditions are met:

• The same kind of animals are used
• Same gender
• Same age and bodyweight
• Same energy level of dry matter intake
• Same feeding level
• Same diet preparation or processing
• Same environmental conditions. 

 Apparent Digestibility

Digestibility is estimated by the difference between the nutrients ingested and excreted in faeces as a proportion of the amount of food ingested. This can be expressed as:

Apparent Digestibility (%) = (nutrientDIET  - nutrientFAECES) / nutrientDIET   x  100

Apparent digestibility accounts for both undigested/unabsorbed feed residues and the components of faeces that have an endogenous origin. 

True Digestibility

True digestibility refers to the proportion of dietary intake that is absorbed from the gastrointestinal tract excluding contributions from endogenous sources. This is expressed as:

True Digestibility (%) = [nutrientDIET - (nutrientFAECES - nutrientENDOGENOUS)] / nutrientDIET

Markers

Markers can be used in digestibility trials so that not all the faeces has to be collected and all intake recorded to measure digestibility. This is particularly useful when animals are in group housing.  

A good marker must be completely recoverable - it mustn't be digested or metabolised. It should also be safe, palatable and measurable and should be able to mix evenly in the feed. People can use internal markers (present in the food) like lignin or external markers (added to the food) like titanium dioxide or chromic oxide.

Illeal Digestibility

Illeal digestibility is the digestibility measured at the end of the small intestine. This is useful to know because the value will contain a significantly smaller contribution from bacteria. There are various ways to determine this:

• Slaughter method: an indigestible marker is fed to the animal. The animal is then euthanased and the samples of digesta are removed from the intestine. The samples are then freeze-dried and analysed.
• Simple T-cannula: A T-shaped cannula is surgically placed in the distal ileum to collect samples of digesta. An indigestible marker is also fed and the samples are freeze dried and analysed.
• PVTC method: similar to the T-cannula method except that the caecum is removed and replaced with a silicon cannula.
• Re-entrant cannulas: another surgical technique where the digesta is diverted outside the pig and then returned to the ilem or caecum. This is a high-cost technique which is difficult to perform and as such is rarely used. 
• Ileorectostomy: The ileum is joined to the rectum so that the large intestine and caecum are by-passed. Normal electrolyte balance is disturbed and maintenance of the animals is difficult.

Factors Affecting Digestibility

Several factors affect how much food can be used by the body:

• Food composition: the more lignin present in the cell wall of the plants fed to the animal results in lower digestibility. A deficiency of sulphur and nitrogen or an excess of lipids may result in a decrease in microbial activity and thus a decrease in digestibility. High ash content as well as the presence of tannins and phytate may also reduce digestibility.  
• Level of feeding: increased intake causes the food to spend less time in the rumen which results in lower digestibility.
• Particle size: increased particle size increases digestibility to an extent. This is because the surface area available for the microbes is increased. However, if the food particles are too small, the feed passes through the rumen too quickly and digestibility decreases.
• Individual variation: the digestibility may vary slightly in different animals within the same species.  

That's all we need to know for this topic, see you next time :)

Oedema

Hi :) In this post we'll learn about oedema. I'll explain what oedema is and how it occurs and finish off with some good examples of oedema. Enjoy :)

What is Oedema?

Oedema is the accumulation of excess extravascular fluids in tissue spaces or body cavities. Oedema can occur locally or there may be a general tendency for oedema to occur. Oedema may be a transudate or an exudate. Transudates are lower in proteins and cells and are non-inflammatory. An oedema may be termed an exudate if the spaces between the endothelial cells of the blood vessels have increased. This is usually a result of inflammation and results in fluid that is high in proteins and cells. Other differences include:

• Exudates will contain proteins that are normally found in blood plasma while transudates contain only albumin. 
• Exudates contain fibrin which causes clots, transudates have no fibrin.
• Exudates have a high specific gravity while transudates have a low specific gravity (this is because there are less proteins).

How Does Oedema Occur?

Oedema occurs because of changes in Starling Forces (see this post for more info). An oedema may occur when:

• The hydrostatic pressure of the blood increases. This may occur with active or passive hyperaemia.
• The oncotic pressure of the blood decreases. The most important protein in this situation is albumin. Low levels of albumin may be a result of malabsorption of amino acids in the gut, decreases synthesis of plasma proteins by the liver, or excessive loss of albumin through the kidney or gut.
• The hydrostatic pressure of the lymph fluid increases. This may occur due to a blockage of the lymph vessels (eg. a tumour).
• The oncotic pressure of the interstitial fluid increases. This occurs most commonly when there is an increased ion intake as well as renal insufficiency. 
• Increased endothelial permeability due to the neurogenic action of chemical mediators. 

Oedema usually forms in places of low tissue tension in different parts of the body. Grossly, it appears as swollen areas of tissue. Oedema may sometimes be described as 'pitting'. This is when a depression is formed in the tissue after it has been touched. Histologically, oedema can be difficult to spot but some signs to look out for include: a clear space around collagen bundles and enlarged lymphatics.  

There is a naming system used to describe where oedema is in the body and this is best explained with some examples. A transudate that forms in the pleural cavity is called 'hydrothorax', in the pericardium is called 'hydropericardium'. 'Ascites' is fluid accumulation in the peritoneal cavity while 'anasarca' refers to generalised subcutaneous oedema. 

Examples of Oedema

I'll include some examples of oedema and how they form here because it seems like this would be a good question to ask in an exam!

Ascites

Ascites (the accumulation of fluid in the abdomen) may occur as a result of right-sided heart failure. The process is described below:

• Right-Sided Heart Failure causes passive hyperaemia in the right side of the circulation (the systemic circulation).
• This causes congestion of the liver, especially in the periacinar regions.
• This causes the hydrostatic pressure in the sinusoids to increase. 
• Fluid is forced into the Space of Disse lining the sinusoids. 
• Fluid drains into the lymphatics around the portal triads and under the capsule
• Fluid leaks out through the capsule
• Ascites forms. 

Pulmonary Oedema 

Another important example is pulmonary oedema see in left-sided heart failure. 

1.   Left-Sided Heart Failure causes passive hyperaemia in the left side of circulation (the pulmonary circulation).
2. This causes congestion of the lungs
3. Increased hydrostatic pressure in the alveolar walls
4. Fluid is forced into alveoli
5. Pulmonary oedema forms
6. Over time, red blood cells and proteins leak via diapedis
7. Increased numbers of macrophages are attracted to the area to phagocytose the debris
8. Macrophages phagocytose haemosiderin. 

That's all for this post :) Please feel free to leave any suggestions, questions or comments in the space below.