Corn: Digestive curse or culinary delight?

Just do a quick Google search on the terms corn and dog food, cat food, or pet food and you’ll likely come up with a number of “interesting” statements, mostly to the negative.

One website insists that “corn products are difficult for dogs to digest,” another states that “corn, soy and wheat are three of the biggest culprits as far as food allergies in dogs go,” or the one which states that some cat foods use “fillers such as corn meal” or another which suggests that “the nutrients in potatoes and corn are far less available than those in rice.”

These sites, and many others like them, are probably well intentioned; however, they propagate some ugly myths about an ingredient that is a staple in many pet diets and a valued tool in the ingredient toolbox when creating nutritionally balanced diets for companion animals.

So, what is it that gives corn such a bad rap?

Sheer popularity could be part of the problem. Corn is pretty common. Consider this: The U.S. produces nearly 9 billion bushels of corn annually for 38% of the total world production. Corn is everywhere. Due to tremendous technological advancements in crop science and agronomy, corn yield has risen steadily – to an average of well over 140 bushels per acre. This has led to lower costs for corn, ranging from around $85 to $125 per metric ton depending upon the year, transportation costs and locale. Then there is the “target market.” Most of the “dent corn” is fed to livestock, valued merely as an economic input and definitely not considered a delicacy for the family. To be successful in the livestock market, it has to be inexpensive and have a high productive value. Unfortunately, this relegates corn to a sort of “plain-Jane” status – a real commodity. This status is a far cry from the desperate need for differentiation found on store shelves. It’s a crowded world. If corn is part of the competitor’s story, then corn becomes one of the “bad guys” despite whether it is true. And thus the marketing battles begin.

The agronomic and processing practices that have led to improved yields and greater uses for corn have also affected its nutritional and chemical composition. The number of different permutations is surprising, e.g. high lysine corn, waxy maize, high amylose corn, low phytate corn, blue corn, hominy, corn grits, masa, flint maize, sweet corn, popcorn, Bt corn, Star-link, corn dent grain, yellow No. 2, etc. With the need for differentiation, one might anticipate some of these showing up in pet foods. However, for the most part, Yellow No. 2 is the only corn grain seen. Part of the challenge might be the AAFCO official names which mostly allow for particle size designations: (48.4) ground corn, (48.5) cracked corn, (48.8) corn flour (AAFCO, 2004). Not much help to the marketing folks.

Is it really an inferior ingredient?

The protein quality of corn is slightly lower than that of rice due to its deficiencies in the essential amino acids lysine, methionine and tryptophan. In dogs fed corn-based diets, fecal scores were reported to be slightly better than rice-based diets (Murray et al., 1999; Twomey et al., 2002), but dry matter digestibility was slightly lower than rice-based diets (Walker et al., 1994; Murray et al., 1999; Twomey et al., 2002; Carciofi et al., 2004). However, starch digestibility among all these studies exceeded 99%. Not exactly what one would call “difficult to digest.”

Relative to the other cereal grains, corn has a high proportion of “rapidly digestible starch,” almost no “slowly digestible starch,” and a large proportion of “resistant starch” (Bednar et al., 2001). This may explain why corn has a slightly lower digestibility in the small intestine, but an overall high starch digestibility. Trivial maybe, but these subtle differences can have a substantial impact on circulating glucose and insulin. For example, corn diets produced much lower postprandial glucose and insulin responses when compared with rice in both the dog and cat (Sunvold and Bouchard, 1998; Bouchard and Sunvold, 2000). The results are not always consistent as blood glucose and insulin responses for dogs fed corn based-diets was reported to be rapid and similar to rice (Carciofi et al., 2004). Why might this occur? Recently it has been shown that the intensity of the extrusion process (Dust et al., 2004) and temperature (Murray et al., 2001) can affect the proportion and digestibility of these various starch fractions.

Contrary to claims on the web, there are very few case reports of food hypersensitivity or food intolerance to corn reported in the literature (Hand et al., 2000). Yellow No. 2 corn derives its name from the pigments zeaxanthin and β-carotene, carotenoids that add color to foods and support to nervous and immune systems. Corn is also a rich source of the essential fatty acid linoleic acid (C18:2n-6).

On the food-function front, corn expands nicely during extrusion and is a preferred carbohydrate source for unique food shapes. Corn (meal) has even found favor in some “value” brands of canned pet foods to help provide the gel necessary to develop loaf texture and binding. Cooked, extruded and processed corn is well liked by dogs and cats in complete pet foods, often being preferred in palatability tests over other grains.

Although some issues can arise with corn, such as mycotoxins, hypersensitivities and geographic cost constraints, generally it is a high quality grain with many applications suitable for use in pet foods. One should take care not to disparage a single ingredient as “bad” simply to promote the benefits of another without a sound basis. In this case, corn as an ingredient in pet food diets, deserves better consideration.

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