How Can You Compare Digestibility Claims and Comparisons?
Understanding dog food labels allows you to compare one brand with another. There are a host of terms on the bags and in manufacturers’ advertising and brochures. If you know what they mean, you can evaluate any dog food for its nutrient value and cost.
Many manufacturers use terms which allude to the digestibility of their food. We’ll explain how digestion works, what the terms mean, and why they’re important.
Digestion is the process where nutrients, proteins, fats, and carbohydrates are broken down by enzymes into smaller units which the body can absorb. Digestive enzymes are organic catalysts which are produced by cells in the body. These enzymes facilitate the biochemical reactions of digestion at normal body temperature.
Digestibility refers to the food consumed and the quantity of the food absorbed in the digestive process. The more nutrients which are available (proteins, fats, carbohydrates, vitamins, minerals), the more will be digested, and the higher the digestibility figure.
Digestion data measures the dis-appearance of a nutrient as it is absorbed through the gastrointestinal tract. To determine the digestibility of a nutrient, a digestion trial is conducted. The food is analyzed to determine its nutrient content.
The dog is fed a “marker substance” which is inert and non-toxic. Then the dog is fed a specific amount of the food, followed by the marker substance again. The feces are collected and analyzed for the undigested residues of the food consumed. The marker substance shows up before and after, so the right feces are collected for the test food. (Urine is not collected or analyzed in a digestibility trial.)
Many animals must be used when determining the digestibility of food. The final figures are averaged from all the animals used in the trial. This is to minimize the variability of results from individual to individual.
When discussing the term “digestibility,” nutritionists are always talking about apparent digestibility. The word “apparent” refers to the fact that we cannot directly determine digestibility – we must deduce it from what goes in and what comes out.
Digestibility equals the amount of food consumed minus the amount of undigested or unabsorbed food in the feces.
Therefore, digestibility refers to the relationship between the food consumed and the quantity of that food which is absorbed by the body. The difference between the two amounts is referred to as the apparent coefficient of digestibility.
This coefficient is expressed as a percentage of the food or nutrient that is absorbed. For example, if 10 grams of protein are consumed, and 9.5 grams are absorbed, then the digestibility of this protein would be 95%.
The apparent coefficient of digestibility is used for the calculation of digestible crude protein, digestible crude fat, and digestible nitrogen-free extract (carbohydrates). These figures are then used to determine total digestible nutrients or TDN. TDN is the sum of the digestible crude protein, digestible crude fiber, digestible NFE (carbohydrates) and 2.25 times the digestible crude fat. This is divided by the feed consumed, and multiplied by 100 to give a percentage. TDN is used as a yard-stick for how much energy is in the food, much as calories are used for human food.
Another term used by dog food manufacturers is Total Dry Matter Digestibility (TDMD). This literally represents the difference between the food consumed (minus any moisture content) and the food excreted as feces, or the difference between what goes in and what comes out. It is difficult to compare TDMDs of different foods, as companies vary in the techniques used to determine it. TDMD, in actuality, merely tells you how efficiently dogs process the food. It doesn’t tell you anything about the energy value of the food.
While some dog food manufacturers use the TDN figure, most are now using more accurate measures of energy. These are kilocalories per pound, Digestible Energy (DE), and Metabolizable Energy (ME).
Energy is the largest essential nutrient needed by the dog. Energy is required for all of the processes involved with growth, lactation, reproduction, and for physical performance.
Energy is expressed in terms of calories – the amount of energy (heat) required to raise the temperature of water 1 degree Centigrade. One thousand calories equals 1 kilocalorie (kcal).
It is common to express the caloric content of dog food in terms of kilocalories per pound or kilocalories per cup of dog food.
Remember, kcal is just a figure. It doesn’t tell you anything about the ability of the dog to utilize the food for energy.
A more precise method of describing the caloric content of dog food is DE or digestible energy. You’ll find this figure in some brochures, but it’s not permitted to be put on the dog food label. (You can always call the manufacturer. Most, but not all, will tell you the DE.)
Before you can determine the digestible energy of the food, you must determine the total caloric content of the food (gross energy). Gross energy is analytically deter-mined in a laboratory using an instrument known as a bomb calorimeter. Gross energy is the calories (heat) resulting from the complete combustion of the food.
Of course, not all of the gross energy of a food is available to the dog. A certain percentage will not be absorbed, and will be passed through and excreted in the feces. When this energy is accounted for, the amount of energy actually absorbed by the animal will be the digestible energy or DE.
Like digestibility, DE is also ex-pressed as a percentage. DE is the gross food energy less the gross fecal energy, the actual amount of energy utilized by the dog.
We’ve said already that not all of the digestible energy can be used by the dog. Some is passed through as feces. However, other energy is passed through in the urine.
Metabolizable energy (ME) takes into account the amount of energy lost in both the feces and the urine. ME gives a more accurate figure regarding the amount of energy which is used by the dog.
For example, proteins are broken down into a nitrogen substance called urea. Urea cannot be used by the dog, and is eliminated in urine. The amount of energy lost in the urine is about 1.25 kcal per gram of protein.
In order to determine the ME, the digestibility of the diet must first be found through feeding trials.
Again, ME is gross food energy mi-nus gross fecal and urinary energy.
Today, dog foods are analyzed by three methods: actual feeding trials, highly sophisticated chemical procedures, or nutritional calculations. The guaranteed analysis gives us the foundation for evaluating the dog food. What do the terms on the dog food package mean?
Commercially prepared dog foods are required by state laws to be labeled. The laws are coordinated between all of the states by the Association of American Feed Control Officials. Although state laws vary slightly, most of them require that the labels show the minimum crude protein and the minimum fat. The label must also show a maximum of crude fiber and moisture.
What do these terms mean? These figures are your assurance that the food contains the minimum stated amount of the higher cost ingredients – protein and fat. It also insures that there aren’t more of the lower-cost, less nutritionally valuable items in the diet – crude fiber and ash.
The ash portion represents the inorganic constituents of the dog food. It may be things like mineral elements. Samples of the dog food are ignited at temperatures in excess of 600 degrees C. (1,112 F.) The residue that is left after the burning process is called ash.
The crude fiber content of the dog food is a good indicator of the digestibility and bulk of the food. Fiber is extremely undigestible to dogs because they lack the enzymes necessary to break it down.
The percentage of crude fiber is deter-mined by taking a sample of the dog food. This is boiled in a dilute acid, then in an a dilute alkali. This simulates the digestive activities and the gastric secretions of the animal. The residue of this “digestion” process is weighed and ashed (burned). The difference between the initial residue weight and the ash weight indicates the amount of fiber present in the food.
The crude fiber portion actually con-sists of cellulose, hemicellulose, and lignin. Approximately 95% of the crude fiber portion is cellulose, which is not digestible.
Just because there is a fiber content doesn’t mean the food is bad for the dog. If the fiber content is too high, though, the food will pass through the animal’s system too quickly – like eating one of Mom’s famous bran muffins. When all of the food passes quickly through the system, digestion is less efficient. The food doesn’t have enough time to be exposed to all of the digestive enzymes, and the animal doesn’t get all of the nutritional benefits.
Dogs who are fed just once a day have a similar problem. They eat large quantities of food at a single feeding. Since dogs have little storage space for food, the exposure to digestive enzymes is also inefficient. To have your dog utilize his food most efficiently, feed him twice a day.
Crude Protein and Fat
Crude protein is the first item in the guaranteed analysis. Protein, on the average, contains 16% nitrogen. Scientists use this little fact as a guide for determining the protein level of the food. If they know the nitrogen content, they will know the protein level.
To determine the nitrogen content of a food, a process is used called the Kjeldahl Determination. In this process, a sample of the food is destroyed with sulfuric acid. The nitrogen which remains is then converted into an ammonia compound, which is distilled and titrated (compared) against a standard. This gives a numerical value which is plugged into a formula to deter-mine the nitrogen content, and ultimately, the protein content.
The process of determining the protein content is not very accurate. In fact, the analysis process is rather crude, hence the name “crude protein.” Now you know why it is called crude protein!
Crude Fat is material that is extracted from moisture-free food by using ether. It consists largely of fats and oils. Small amounts of waxes, resins and coloring matter are also contained in it.
All of this information lets you look at dog food labels and make some judgements about the food. We’ll explain these areas in more detail in Part 2 of this book.