Written by Dr. Jill Cline, Ph.D.
Digestibility determines how much nutrition a food provides in a given volume, while palatability affects how appealing a food is to the animal. Both of these characteristics are important criteria when selecting the best food for your dog or cat.
What are digestibility and bioiavailability and how are they measured?
A food’s digestibility is the collective proportion of all nutrients in a food that is available to the dog or cat for absorption from the gut (intestine) into the bloodstream. Nutrient bioavailability is the proportion of the absorbed nutrients that are carried to target tissues and are available for use by the body. Because a highly digestible food provides a higher proportion of absorbed nutrients than a less digestible food, digestibility provides one measure of a food’s nutritional value and quality. In general, as the quality of ingredients in the food increases, so will the food’s digestibility and nutrient bioavailability.
Pet food companies measure the digestibility of their products using several tests, most of which include feeding trials. The food is fed to a group of dogs or cats for a selected period of time and the level of undigested matter excreted in the feces is measured and used to calculate nutrient digestibility. Although all reputable manufacturers conduct digestibility tests on their foods, the Association of American Feed Control Officials has not yet established a standard protocol for digestibility studies and does not allow the inclusion of digestibility claims on pet food labels. Commercial foods vary significantly in digestibility and ingredient quality, and so it can be difficult for pet owners to differentiate between super premium, premium and economy products. For example, the labels of two pet foods may have the same ingredient lists and guaranteed analysis panels, but when fed may have substantially different digestibilities (see below).
Total Protein vs. Digestible Protein
The pet food label provides an estimate of a food’s crude protein content on its guaranteed analysis panel. This measure reflects only the total amount of protein and does not indicate differences in protein digestibility between high and low quality protein sources. For example:
Dog Food A contains 21 percent crude protein and is 86.0 % digestible.
Dog Food B contains 23 percent crude protein and is 76.0 % digestible.
Food A: 21g protein/100g diet x 0.86 = 18.1g protein absorbed
Food B: 23g protein/100g diet x 0.76 = 17.5g protein absorbed
Although the crude protein value reported for Dog Food A is lower than that for Dog Food B, Dog Food A’s higher digestibility results in more protein being available to the dog, in a given volume of food.
What factors affect pet food digestibility?
A number of pet food characteristics influence nutrient digestibility. These include:
• Formula: The food’s formula refers to the type and quantity of different ingredients that are included in the product. Because pet food ingredients vary significantly in digestibility, the overall product formula is influenced by the digestibility and nutrient bioavailability of its various ingredients.
• Ingredient Quality: Overall pet food digestibility is increased by the inclusion of high quality ingredients and decreased when poor quality ingredients are included. For example, a pet food’s digestibility is decreased by the presence of poor quality protein, ash, certain types of dietary fiber, and phytate (a component of plant ingredients that decreases the availability of certain essential minerals in the diet).
• Processing: Proper processing techniques, cooking temperatures, and storage procedures support optimal nutrient digestibility and bioavailability. Conversely, digestibility and nutrient bioavailability can be significantly reduced by improper processing or excessive heat treatment. For example, excessive heat damages protein, resulting in decreased digestibility of the protein and reduced bioavailability of certain amino acids. This means that a smaller proportion of the protein is digested and absorbed. Some of the amino acids that are absorbed are also altered in structure (called a “Maillard Reaction”) leading to a reduced ability of target tissues to use them efficiently.
What is palatability and how is it measured?
Palatability refers to perceptions of a food’s taste, smell, and texture. It is an important food characteristic because, simply put, pets must be willing to eat adequate amounts of the food to meet their calorie and nutrient needs. Unpalatable foods will be rejected, regardless of the quality of their ingredients or balance of essential nutrients. Dogs and cats differ somewhat in the food characteristics that they find desirable. Cats are strongly affected by the aroma of a food and will carefully smell a new food before tasting it. Dogs often prefer foods that are high in fat and include protein from animal sources. For both dogs and cats, the texture, size, and shape of food pieces are important; scientists who study palatability refer to this as “mouth feel”. Finally, in addition to animals’ sensory preferences, scientists who study palatability also consider the pet’s environment and the owner’s reactions to different types and flavors of food.
Similar to digestibility, there are a number of ways that pet food companies assess a food’s palatability. Tests that measure the animal’s preference when initially presented with a new food provide information about the immediate appeal of the food’s smell, appearance and texture. Long-term interest is measured using food preference studies. Each dog or cat is offered a choice of two diets that are presented in identical bowls to the left and right. Surplus food is offered in each bowl and the positions of the bowls are switched daily to account for dogs or cats with right- or left-side preferences. The amount of each food that is consumed at each meal is measured over a period of several days. These tests provide information about a food’s acceptability to dogs and cats over time and its relative palatability when compared with other foods. And, finally, the ultimate test of palatability involves presenting the food to pets in homes, where both the pet’s and the owner’s perceptions of the food are considered.
Selecting a Digestible (and Desirable) Food for Your Pet
Some pet food companies provide digestibility data with product literature or through the company’s customer service web site. However, neither digestibility nor palatability is explicitly reported on pet food labels. Here are a few helpful tips for selecting a food that is both highly digestible and palatable for your dog or cat:
• High quality ingredients that are correctly processed produce highly digestible foods. When digestibility information is available, select a food from a reputable manufacturer that has a dry matter digestibility of 80% or greater. Reject maintenance diets that have digestibilities that are lower than 75%.
• A product that is highly digestible will produce normal stool volumes and well formed and firm feces. In addition, the fecal matter will not contain mucous, blood, or any recognizable components of the food.
• Highly digestible foods result in relatively low defecation frequencies, and bowel movements that are regular and consistent. Foods that are not highly digestible may cause excessive gas (flatulence), loose stools, or diarrhea.
• Your dog or cat should readily consume and enjoy the food, in a quantity that promotes normal growth rate and optimal body weight. There should be no need to entice your pet to eat the food by adding treats, table scraps, or other human foods. An excessive quantity of food should not be needed to maintain your pet’s normal body condition.