Ms. Holloway is a veterinary sales consultant for Nestlé Purina PetCare.
Pet food myths abound among consumers, especially myths concerning the nutritional value and digestibility of corn as an ingredient in pet food. Corn is commonly thought to be a low-quality food ingredient (i.e., a filler) that has minimal nutritional value. This article addresses common nutrition myths regarding corn in pet food and explains why this diverse grain is valuable as an excellent source of protein, carbohydrate, and essential fatty acids.
Myth 1: The primary ingredient in many dry commercial pet foods is not protein, but cereal. Corn and wheat are the most commonly used grains, but as with meat sources, the nutritious parts of the grain are generally present only in trace amounts.
Protein quality is based on the bioavailability (digestibility) of protein, the content of essential amino acids, and the amino acid requirement of the animal consuming the protein.1,2 Based on these criteria, corn has a high protein value. The digestibility of corn is discussed in myth 2 below. Cells of the body use amino acids from multiple sources, including food proteins, single amino acids, and amino acids synthesized in the body.
There are two groups of amino acids:
• Essential amino acids cannot be synthesized by the body in sufficient quantities and, therefore, must be supplied in the diet.
• Nonessential amino acids are synthesized by the body if sufficient “building material” is available, which is usually the case if a pet receives a balanced diet.
The amino acid requirement for dogs and cats includes arginine, methionine, histidine, phenylalanine, isoleucine, threonine, leucine, tryptophan, lysine, and valine. In addition, cats need taurine.
Corn has been determined to have a protein content of 16.1%, with respectable levels of essential amino acids. According to Kemmerer and Acosta,3 corn is “approximately optimum in histidine, leucine, phenylalanine, and valine.” In comparison, broccoli has a 29.4% protein content, cauliflower has 24.3%, and carrots have 6%. The authors state that “carrot protein is deficient in all the essential amino acids.”3
Cells cannot distinguish between amino acids obtained from plant or animal protein. Combining animal and plant proteins in the diet can provide a complete source of essential amino acids. Therefore, protein sources that have a low biologic value alone can be combined with other protein sources, resulting in a high biologic value.4,5 This process is called protein complementation. Quality pet foods complement highly digestible animal protein with a natural plant protein source, such as corn, to deliver all the essential amino acids that pets need4,5 (FIGURE 1).
Myth 2: Plant-based proteins (e.g., soy, corn, by-products) are difficult to digest.
This statement can be valid if grains are not processed properly. However, plant protein sources that are properly processed can provide highly digestible, high-quality dietary protein for dogs and cats. For example, using a wet milling process for corn maintains its protein quality. In this process, the corn kernel is separated into starch, fiber, and protein. The protein component becomes corn gluten meal—a source of highly digestible protein for dogs and cats.
In a balanced diet, the digestibility of nutrients is as important as appropriate nutrient levels; therefore, the processing of ingredients can be the rate-limiting factor in the total bioavailability of a food.1,2 TABLE 1 indicates the superior digestibility of corn gluten meal compared with meat products. Therefore, corn is an excellent source of digestible energy.
Myth 3: Corn causes many allergy problems.
Corn has been thought to be highly antigenic and to cause food allergies. However, in a study6 that examined the frequency with which specific food ingredients cause a reappearance of pruritus during a food elimination trial, corn had the lowest frequency among the ingredients evaluated1,2 (FIGURE 2).
Myth 4: The corn gluten meal and soybean meal added to pet foods have little nutritional value because they are leftovers from processing grain for human use.
Corn gluten is the protein portion of corn. Corn gluten meal (the dried form) provides protein that is complementary to many meat sources of protein. The digestibility of corn gluten meal is as high as that of many meat meals (TABLE 1).1,2 Corn gluten meal has an amino acid profile that is quite different from those of meat-based protein sources. Corn gluten meal is especially high in the amino acids cystine and methionine, which are particularly important for skin and haircoat health.
In addition to providing energy and protein, corn provides other essential nutrients, including vitamins (B complex, E, and A), minerals, and insoluble fiber, which are important to digestive and immune health. Corn also provides fatty acids (e.g., linoleic acid) and antioxidants (e.g., β carotene, lutein, zeaxanthin), which protect the entire body from oxidative cellular damage and support eye health.
A particular benefit of using corn in pet food is that it is the only commonly used grain that contains linoleic acid—an omega-6 essential fatty acid required for healthy skin and hair in dogs and cats.
Antioxidants help to reduce cell damage. Vitamin E is a major fat-soluble antioxidant, and corn oil, a common ingredient in pet food, is a major source of vitamin E. In a human study conducted by the Institute of Nutritional Science at the University of Vienna, Austria, a diet containing corn oil was found to reduce DNA damage more effectively than a diet containing an olive/sunflower oil combination.7
It is commonly thought that processed ingredients have a lower nutritional value than fresh ingredients. However, a study conducted by Cornell University found that cooking corn at 150°C (302°F) for 50 minutes increased its antioxidant levels by as much as 53%.8
Energy requirements can be met with carbohydrate, fat, or protein; therefore, providing a portion of the energy from a carbohydrate such as corn permits the overall diet formula to be lower in fat and/or protein. Corn is a very useful ingredient in pet food because it is a high-quality source of carbohydrate and protein. Cornmeal, a major source of carbohydrate in pet food, contains approximately 75% carbohydrate6 (FIGURE 3).
Many pet food myths circulate among consumers every day. Do not be misled by myths about corn in pet diets. Good nutrition involves not only the list of ingredients but also the right balance of nutrients. Corn has proven to be a very useful ingredient because of its high digestibility, low allergenic tendency, and excellent nutrient content, including antioxidants, protein, carbohydrate, and essential fatty acids. When properly processed and provided in a balanced manner, corn is healthy for pets.
1. Murray SM, Patil AR, Fahey GC Jr, et al. Raw and rendered animal by-products as ingredients in dog diets. J Nutr 1998;128(12 suppl):2812S-2815S.
2. University of Alberta. Department of Agricultural, Food and Nutritional Science Web site. http://www.ales.ualberta.ca/afns/.
3. Kemmerer AR, Acosta R. The essential amino acid content of several vegetables. J Nutr 1949;38(4):527-533.
4. Hand MS, Thatcher CD, Remillard RL, et al, eds. Small Animal Clinical Nutrition. 4th ed. Topeka, KS: Mark Morris Institute; 2000:141.
5. Oklahoma State University. OSU Offering Weekly Feed Commodity Bulletin on Web. November 1997. http://www.livestockweekly.com/papers/97/11/13/index.html.
6. Jeffers JG. Results of dietary provocation in dogs with food hypersensitivity. Vet Dermatol 1994;5(3):127-144.
7. Elmadfa I, Park E. Impact of diets with corn oil or olive/sunflower oils on DNA damage in healthy young men. Eur J Nutr 1999;38(3):286-292
8. Dewanto V, Wu X, Liu RH. Processed sweet corn has higher antioxidant activity. J Agric Food Chem 2002;50(17):4959-4964.