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* Escuela de Graduados and Instituto de Producción Animal, Facultad de Ciencias Agrarias, Universidad Austral de Chile, Valdivia, Chile; ** Institute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand; and Animal Nutrition Group, Department of Animal Sciences, Wageningen University and Research Centre, Wageningen, The Netherlands
4 To whom correspondence should be addressed. E-mail: shodgkin{at}uach.cl.
KEY WORDS: • lysine • reactive lysine • dogs • pet food • heat damage
The protein quality of a food is the product of its amino acid content and the nutritional availability of these amino acids. Heat processing has been shown to have a negative impact on the nutritional value of the amino acids in animal feeds (1,2). In foods that are heat processed or stored for long periods of time, the free 
-amino group of lysine can react with the carbonyl group of other compounds such as reducing sugars. The complex that is formed (Maillard complex) may be partially absorbed but cannot be utilized by the animal (3). Moreover, a proportion of the complexes formed can revert back to lysine during conventional amino acid analysis, which does not occur in the digestive tract of the animal. This results in an overestimation of the amount of lysine that is available to the animal if it is determined using conventional methods. A method in which the lysine content with a free -amino group in foods is measured [such as the O-methylisourea (OMIU)-reactive lysine assay] provides a superior estimate of the lysine available to the animal (4).
The production of commercial pet foods can involve heat treatments including sterilization, extrusion, cooking, and baking. During these heat treatments and the subsequent storage of the food, Maillard complexes may form, thereby reducing the availability of lysine for the animal. In addition, most pet food manufacturers use by-products of the meat, fish, and milling industries as primary ingredients, which, because of the processing required in their manufacture, can include lysine with a blocked -amino group. Although some pet foods are formulated using large proportions of non-heat-treated ingredients such as meat, lamb, rice, and corn, the manufacture of pet foods still promotes a reaction between the -amino group of lysine and other compounds. Although pet foods are extensively heat treated, little research has been conducted into the effects of the various heat processes on the nutritional value of pet foods.
The objectives of this study were to determine the total and reactive lysine content of commercial dog foods and to determine whether a relation exists between lysine availability and product price.
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MATERIALS AND METHODS |
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 TOP MATERIALS AND METHODS RESULTSDISCUSSIONLITERATURE CITED |
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The prices of the foods were obtained from local commercial outlets and were converted to US dollars to allow comparison. Comparisons between the results obtained for crude protein, total lysine content, reactive lysine content, and percentage of bound lysine between the maintenance and growing diets were conducted using Student's t tests with the program SAS (version 8e, SAS Institute). Correlations were determined between the prices of the foods and their OMIU-reactive lysine content and prices of the food versus the ratio of OMIU-reactive to total lysine content, using the correlation procedure of SAS (Pearson and Spearman coefficients, SAS Institute). A probability value of <5% was considered significant.
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RESULTS |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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DISCUSSION |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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A significant amount of total lysine present in the dog foods, as measured using conventional amino acid analysis, was bound with the percentage of the total lysine that was bound (and thus would be unavailable to the dog) showing a very high variation, ranging from –5 to 56% in the maintenance diets and from 2 to 53% in the growth diets. To meet the minimum lysine requirements of adult and growing dogs, a food must contain a minimum available lysine content of 0.48 and 0.59%, respectively, for diets containing 3,500 kcal ME/kg [calculated from values published in AAFCO (9), taking into consideration the correction for bioavailability]. All of the maintenance diets and all except one of the growth diets did contain sufficient available lysine to satisfy the nutritional requirements of their target dogs.
In all cases, the prices of the growth diets were greater than the prices for the maintenance diets, which is to be expected because of the higher concentration of protein present in these diets. Ingredients with a relatively high protein content (particularly ingredients from animal sources) tend to have higher prices than ingredients incorporated for their energy content (e.g., cereal grains). Canine diets are normally composed of 25–40% animal by-products [dry matter basis (10)]. An alternative to animal proteins is the use of soybeans, which are considered to be a good source of vegetable protein. Soybeans are generally incorporated into canine foods as soybean meal, which has a relatively low price (11). Soybean meal is manufactured using extrusion, to inactivate its antinutritional factors, but this process will also decrease the digestibility of the amino acids in this ingredient (11) and would be expected to result in a decrease in the amount of reactive lysine present. The sources of protein used in the formulation of canine diets will affect the final price of the food, especially in the case of growth diets, which contain greater concentrations of protein in comparison to maintenance diets to satisfy the higher requirements for amino acids in growing dogs.
It was expected that diets formulated using ingredients that had been processed before their incorporation would contain more bound lysine than diets elaborated with primary ingredients such as meat, without prior processing. This difference in protein source was expected to be reflected in the commercial prices of the canine diets, with the diets containing by-products as the main protein sources having a lower price than those that contain primary ingredients without prior processing. Because there was no relation between the proportion of the lysine that was bound and the price for the canine diets evaluated, this suggests that the main process causing the lysine to become bound is the heat processing of the food (extrusion or pelleting). There was a very notable variation between dog foods of different brands in the proportion of the total lysine that was bound, with a range between 0 and 56% of the total lysine being bound and thus nutritionally unavailable to the dog. This suggests that the production systems, including conditions for extrusion or pelleting, differ between manufacturers, and it is important to determine the conditions necessary to minimize the nutritional "loss" of essential amino acids such as lysine during processing.
Damage to lysine in canine diets, most likely by factors such as the use of ingredients containing damaged lysine (e.g., corn, wheat), extrusion, and/or storage, appears to be significant and varies greatly between brands. Although the total lysine content may be sufficient to meet lysine requirements for growth and maintenance, a large proportion of the lysine in the diets appears to have been damaged and can be expected to be unavailable to dogs.
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FOOTNOTES |
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2 Author disclosure: no relationships to disclose.
3 Financial support was provided by the Centre for Feline Nutrition, Massey University, Palmerston North, New Zealand.
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LITERATURE CITED |
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TOPMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1. Hendriks WH, Emmens MMA, Trass B, Pluske JR. Heat processing changes the protein quality of canned cat food as measured with a rat bioassay. J Anim Sci. 1999;77:669–76.
2. Mavromichalis I, Baker DH. Effects of pelleting and storage of complex nursery pig diet on lysine bioavailability. J Anim Sci. 2000;78:341–47.
3. Hurrell RF, Carpenter KJ. The estimation of available lysine in feedstuffs after Maillard reactions. In: Eriksson L, editor. Progress in food and nutritional science. Maillard reactions in food. Vol. 5. Oxford: Pergamon Press. 1981. p. 159–76.
4. Moughan PJ, Rutherfurd SM. A new method for determining digestible reactive lysine in foods. J Agric Food Chem. 1996;44:2202–9.
5. Gajda M, Flickinger EA, Grieshop CM, Bauer LL, Merchen NR, Fahey GC Jr. Corn hybrid affects in vitro and in vivo measures of nutrient digestibility in dogs. J Anim Sci. 2005;83:160–71.
6. Rutherfurd SM, Moughan PJ, Van Osch L. Digestible reactive lysine in processed feedstuffs: Application of a new bioassay. J Agric Food Chem. 1997;45:1189–94.
7. Rutherfurd SM, Moughan PJ. Application of a new method for determining digestible reactive lysine to variably heated protein sources. J Agric Food Chem. 1997;45:1582–6.
8. Rodhouse SL, Herkelman KL, Veum TL. Effect of extrusion on the ileal and fecal digestibilities of lysine, nitrogen, and energy in diets for young pigs. J Anim Sci. 1992;70:827–35.[Abstract]
9. Association of American Feed Control Officials Dog and Cat Food Nutrient Profiles (AAFCO). Official Publication. Atlanta, GA. 2002.
10. Murray SM, Patil AR, Fahey GC, Jr., Merchen NR, Hughes DM. Raw and rendered animal by-products as ingredients in dog diets. J Anim Sci. 1997;75:2497–505.
11. Yamka RM, Hetzler BM, Harmon DL. Evaluation of low-oligosaccharide, low-phytate whole soybeans and soybean meal in canine foods. J Anim Sci. 2005;83:393–99.
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