Energy

Hello, in this post we're going to be discussing the energy that is contained in food and how the body utilises this energy. We'll take a look at what energy actually is, how we measure it, how the body uses the energy contained in food, and finally Atwater factors and what they are used for. Enjoy!

What is Energy?

Energy is not strictly a nutrient but it is a major component of food. It is "the ability to do work" and allows reproduction, growth, lactation and the maintenance of life processes to occur in the body. Both nutrients and energy are required in feed.

Energy is an important aspect of the feed requirements of animals. If an animal's diet is energy deficient problems may arise. This includes poor growth and production, weight loss, decreased efficiency of nutrient utilisation, impaired reproductive efficiency and even a reduced ability to exercise. If an animal receives too much energy in it's diet similar problems may occur.

Units of Energy

Two units of energy are commonly used: the joule and the calorie. A calorie is the amount of heat energy required to raise the temperature of 1 gram of water from 14.5° C to 15.5° C. 1 calorie equals 4.184 joules and 1 joule = 0.239 calories.  

Partitioning of Energy 

The partition of energy is a system that determines the amount of energy available to the animal after digestion, metabolism, maintenance of homeostasis, and production requirements have occurred. This website has a good flow diagram which illustrates this process. (Figure 2.5, its about 1/2 down the page.)  

In Australia we use Digestible Energy (DE), Metabolisable Energy (ME), and Net Energy (NE) to describe energy in feed. DE is used in diets for horses, poultry and pigs. ME is used for ruminants, dogs, cats and humans. NE can also be used for cattle and pigs. 

Gross Energy

 Gross energy (GE) is the total energy content of a feed. It is measured by complete combustion of the sample in a bomb calorimeter. GE doesn't have a biological value because the utilisation of energy in an animal is never 100% efficient.

Digestible Energy

This is the amount of energy that is absorbed by the gastrointestinal tract. It gives us an idea of the amount of energy the animal has available to use. However, it only partially accounts for the energy lost during the utilisation of the nutrients. DE is the difference between the gross energy and the faecal energy (the energy that isn't digested or absorbed and is lost in faeces.) Ie:

DE = GE - faecal energy

 Faecal energy includes undigested feed, enteric microbes and their products, excretions into the GIT and cellular debris from the GIT. Now, when it comes to faecal energy (FE), you can get either true FE or apparent FE. Apparent FE doesn't take into consideration the endogenous contributions to faecal energy. True FE does take into consideration the metabolic contributions from the gastrointestinal tract, bacteria and protozoa. 

Metabolisable Energy

Metabolisable Energy (ME) is the amount of chemical energy that is available for use by the cells. It is the difference between the energy available for absorption by the GIT (DE) and the energy lost in urine (UE) and the gaseous products of digestion (GDP). Ie:

ME = DE - (UE + GDP) 

It can be quite difficult to measure how much energy is lost in urine and the gaseous products of digestion and so conversion formulae are often used. For example, in ruminants, ME = 0.81 x DE. In pigs ME = 0.96 x DE.

Net Energy

Net energy (NE) represents the true amount of energy available for maintenance, work and production. "It's what's left over at the end". It is calculated using the formula:

NE = ME - heat increment

 
The heat increment is the heat produced from digestion and absorption and comes mainly from the viscera. There are four sources of heat increment:

1. Heat of digestion: the heat given off by the chemical reactions in the digestive tract
2. Heat of fermentation: this is given off by chemical reactions in the bacteria of the digestive tract
3. Heat of waste product formation: this is given off in the making of waste products, especially urea and uric acid.
4. Heat of nutrient metabolism: this is the total heal from all other sources. 

Net Energy can be divided into the energy required for maintenance and the energy required for production. 

Maintenance

Maintenance energy is the amount of energy required by the animal for the continuation of simple life without change in body composition or weight. There are three components to maintenance energy: Basal Metabolic Rate (BMR), muscular work, and thermoregulation.  

BMR is the energy used to keep cells alive and to maintain organ function. In order to measure BMR, the animal must be awake and at rest, it must be in a thermoneutral environment, and no nutrients must be absorbed during measurement. BMR is affected by several factors including:

• Body Size: larger animals require a higher energy input for maintenance but smaller animals use more energy per kilogram of body weight. This is due to an increased surface area to volume ratio which makes thermoregulation more expensive in terms of energy requirements. 
• Sex: males tend to have higher BMRs than females. 
• Age: younger animals have higher BMRs than older ones.
• Previous level of nutrition: animals that have been fasted will have a lower BMR as their bodies' have adapted to a lower energy intake.
• Species: BMRs differ between species.
• Climate: if the climate does not match the animals thermoneutral zone, the animal needs to use more energy to increase or decrease its body temperature affecting BMR.

 Some of the maintenance energy is used for muscular work. This includes normal activity levels for an animal that leads a simple life (ie. not a racehorse or working dog). The intensity and duration of work will affect the amount of energy required. 

The regulation of temperature also requires energy which falls under the maintenance category. If an animal is too hot or too cold it will spend energy trying to heat or cool itself. 

Production

If the energy in an animal's food exceeds the maintenance requirement, the left-over energy is known as retained energy. This energy is used by the body to produce milk, muscle, to support pregnancy, wool etc. However, the retained energy is never the same as the true amount of energy in excess after the maintenance energy has been supplied. This is because some energy is used up in the process of producing the meat, wool, milk etc. 

Atwater Factors

Atwater factors are used to predict the energy content of foods from the proximate analysis. Atwater factors work well for highly digestible foods (such as sugars and starches) but they may not be applicable for diets high in fibre. Modified Atwater factors are used for dogs and cats and take in to consideration the digestibility of the carbohydrates, fats and protein in pet food.


That's all for this topic, if you have any questions please feel free to ask :)