Evaluation of calcium, phosphorus, and selected trace mineral status in commercially available dry foods formulated for dogs

Jason W. Gagné Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853

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Joseph J. Wakshlag Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853

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Sharon A. Center Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853

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Michael A. Rutzke Robert W. Holley Center for Agriculture and Health, Agricultural Research Service, USDA, Ithaca, NY 14853

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Raymond P. Glahn Robert W. Holley Center for Agriculture and Health, Agricultural Research Service, USDA, Ithaca, NY 14853

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Abstract

Objective—To evaluate concentrations of calcium, phosphorus, zinc, iron, copper, manganese, and selenium in several commercially available dry dog foods and compare these with current Association of American Feed Control Officials (AAFCO) recommendations for maintenance of healthy dogs.

Design—Descriptive study.

Sample—45 over-the-counter dry foods formulated for maintenance of healthy dogs (ie, maintenance foods) and 5 therapeutic dry foods formulated for dogs with hepatic or renal disease.

Procedures—Mineral concentrations were measured via inductively coupled plasma mass spectrometry or inductively coupled plasma atomic emission spectroscopy and compared with AAFCO-recommended minimum and maximum values.

Results—Most (39/45) maintenance foods were in compliance with AAFCO recommendations for all mineral concentrations evaluated. Calcium concentration was > 7. 1 g/1,000 kcal of metabolizable energy (ME) in 4 of 45 maintenance foods, and phosphorus concentration was > 4.6 g/1,000 kcal ME in 3 of these; 2 maintenance foods contained < 34 mg of zinc/1,000 kcal ME. These values were not within AAFCO-recommended ranges. Calcium-to-phosphorus ratio in foods formulated for dogs with renal disease was above, and copper concentration in foods formulated for dogs with hepatic disease was below, recommended ranges for healthy dogs.

Conclusions and Clinical Relevance—Calcium concentrations exceeded recommended limits in some maintenance foods labeled for all life stages, underscoring the need to feed diets appropriately formulated for specific life stages, particularly for large- and giant-breed puppies. Studies investigating the bioavailability of minerals are necessary before firm recommendations can be made.

Abstract

Objective—To evaluate concentrations of calcium, phosphorus, zinc, iron, copper, manganese, and selenium in several commercially available dry dog foods and compare these with current Association of American Feed Control Officials (AAFCO) recommendations for maintenance of healthy dogs.

Design—Descriptive study.

Sample—45 over-the-counter dry foods formulated for maintenance of healthy dogs (ie, maintenance foods) and 5 therapeutic dry foods formulated for dogs with hepatic or renal disease.

Procedures—Mineral concentrations were measured via inductively coupled plasma mass spectrometry or inductively coupled plasma atomic emission spectroscopy and compared with AAFCO-recommended minimum and maximum values.

Results—Most (39/45) maintenance foods were in compliance with AAFCO recommendations for all mineral concentrations evaluated. Calcium concentration was > 7. 1 g/1,000 kcal of metabolizable energy (ME) in 4 of 45 maintenance foods, and phosphorus concentration was > 4.6 g/1,000 kcal ME in 3 of these; 2 maintenance foods contained < 34 mg of zinc/1,000 kcal ME. These values were not within AAFCO-recommended ranges. Calcium-to-phosphorus ratio in foods formulated for dogs with renal disease was above, and copper concentration in foods formulated for dogs with hepatic disease was below, recommended ranges for healthy dogs.

Conclusions and Clinical Relevance—Calcium concentrations exceeded recommended limits in some maintenance foods labeled for all life stages, underscoring the need to feed diets appropriately formulated for specific life stages, particularly for large- and giant-breed puppies. Studies investigating the bioavailability of minerals are necessary before firm recommendations can be made.

Prior to the 20th century, pet dogs were fed table foods without regard for balanced nutrient intake. Since the introduction of commercially available pet foods in the early 1940s, industry sales of dog and cat food have grown to approximately $16 billion annually.1 Regulation of the nutritional content of these foods began in the mid-1970s with designation of the first NRC, which made nutritional profile recommendations for canine and feline diets. Today, most owners feed commercial pet foods with confidence that regulatory agencies mandate safe macro- and micronutrient and mineral content. However, recent widespread media recognition of commercial pet food recalls has engendered consumer concern regarding both the quality and safety of pet foods.

Governance of pet food production, safety, marketing, and sales in the United States relies on multiple organizations. The US FDA has jurisdiction over all complete and balanced pet foods in interstate commerce, whereas state agricultural departments regulate pet foods distributed within a state. The USDA is responsible for inspection of imported pet foods and for ensuring ingredients intended for use in animal foods are not used in human foods.2 The NRC publishes timely updates to guide nutrient recommendations for canine and feline diets, and the AAFCO functions in an advisory capacity at the state level to help ensure that marketed pet foods are nutritionally complete. Major considerations when setting standards for AAFCO Pet Food Nutrient Profiles include NRC-recommended daily allowances, processing and form of feed, bioavailability information, and the variation of ingredients in pet foods that might affect bioavailability.

Attention to dietary mineral intake in commercial foods for pet dogs was heightened in the 1990s when excessive calcium and energy intakes were implicated in developmental orthopedic disorders of large- and giant-breed dogs.3–8 This discovery resulted in a modification of recommended calcium and energy allowances for commercially available foods formulated for growing large- and giant-breed dogs. However, resulting recommendations made by the NRC and AAFCO committees had a nearly 1.5-fold disparity in the range of recommended calcium concentrations (NRC values, 2.0 to 4.5 g/1,000 kcal ME9; AAFCO values, 2.9 to 7.1 g/1,000 kcal ME10). Additionally, in the 1980s, an acquired canine parakeratotic dermatopathy linked with zinc deficiency was attributed to zinc chelation by high dietary concentrations of phytates.11 The AAFCO-recommended dietary zinc requirements were subsequently increased from 14 to 34 mg/1,000 kcal ME.10

Minimum and safe maximum limits for other trace minerals in pet foods may be even more poorly elucidated. Mammals do not have a physiologic pathway for elimination of excess iron, and thus, iron homeostasis in the body is highly regulated at the level of iron absorption.12,13 However, most animals are less affected by chronic dietary iron overload, compared with humans, and most reports involving animals have been limited to case reports12,14–22 involving soil and water contamination. Because there have been no studies confirming dietary iron concentrations associated with toxicosis in dogs, no maximum concentration has been recommended by the NRC.9 Recently, dietary copper was suggested as a potential cause of a copper-associated hepatopathy prevalent in Labrador Retrievers.23,24 However, copper-associated hepatopathy has been long recognized as a disease that occurs sporadically in dogs of several pure and mixed breeds.25–30 Safe upper limits of dietary copper concentrations differ among species, seemingly because of unique differences in copper transporters and chaperones that regulate copper homeostasis. Since the late 1930s, hepatic copper concentrations considered normal in dogs expressed on a dry-weight basis have increased 10-fold.31 This corresponds with a growing reliance on commercially prepared pet foods.32 It is reasonable to speculate that some dogs with necroinflammatory liver disease may accumulate pathologically high hepatic concentrations of copper as well as iron (as determined on the basis of specific staining for these metals and their discrete tissue quantification)33 and that diet may be germane to the pathogenesis of these liver injuries.

Considering the expanding knowledge of macro- and micronutrient requirements, emerging changes in the formulations of pet foods, and current regulatory recommendations, we were interested in examining concentrations of selected major and trace minerals in several commercially available, over-the-counter dry foods formulated for maintenance of healthy dogs and in therapeutic dry foods formulated for dogs with hepatic or renal disease. We focused on minerals implicated in health and disease and sought to determine the range of mineral concentrations present and to assess compliance with current AAFCO recommendations.

Materials and Methods

Analyzed foods—Samples of 45 dry foods formulated for maintenance of healthy dogsa–ss commonly sold over the counter (ie, maintenance foods) were obtained in Ithaca, NY. Seven of these were routinely dispensed from the Cornell University Veterinary Medical Center, and the remaining samples were obtained from a local pet food retailer (n = 18), a local grocery store (17), or 2 large discount retailers (3). Additionally, 5 therapeutic dry foods formulated for dogs with hepatic or renal diseasett–xx were obtained from the Cornell University Veterinary Medical Center. All samples were stored at room temperature and analyzed within a week after purchase.

Sample preparation—A portion of each product was individually placed in a grinderyy and pulverized. To prevent cross contamination, the grinder was cleaned and vacuumed between samples. Approximately 0.5 g of each food was weighed with a scaleyy accurate to 0.1 g and placed in a low thermal expansion borosilicate glass tube. Samples were prepared in triplicate.

ICP-AES and ICP-MS—Four milliliters of a 60:40 mixture of a double-distilled 70% HNO3-HClO4 combination and 0.25 mL of yttrium (40 mg/L) were added as an internal standard to each pulverized sample for ICP-AES. Acid-treated samples were incubated overnight at 20°C, then heated to 120°C in an aluminum heating block for 2 hours for sample clarification. For samples that failed to clarify, 0.25 mL of concentrated nitric acid was added for further digestion until the temperature reached 195°C. After cooling to room temperature (approx 20°C [68°F]), 20 mL of deionized water was added, the samples were vortexed, and the solutions were transferred into tubes for analysis via ICP-AES and ICP-MS.

Concentrations of calcium, phosphorus, iron, zinc, copper, and manganese were determined by means of ICP-AES.aaa In this instrument, a 150-mm lensbbb was inserted in the base of the ceramic purge tube to reduce matrix interferences.ccc–eee Data output from ICP-AES was expressed as mg/L on the basis of three 10-second integrations. Element concentrations were drift corrected and normalized with the yttrium internal standard. To express these data in micrograms per gram, the following equation was used:

article image

where M is the metal of interest in ppm, Y is the yttrium (internal standard) value, and W is the weight of the sample in grams.

The mean of triplicate analyses (μg/g) was divided by the energy density (kcal/kg) of food (on the basis of information obtained from the manufacturer) and then multiplied by 1,000 to express each mineral concentration in mg/1,000 kcal, except for calcium and phosphorus, which were expressed in g/1,000 kcal.

Concentrations of 1 selenium isotope (78Se) were determined via ICP-MS.fff The original sample introduction system was replaced with a double cyclonic spray chamber with a 1-μL nebulizerggg to reduce metal oxide formation in the plasma. A germanium isotope (72Ge) was used as an internal standard and added with a mixing tee just prior to the sample entering the nebulizer. Hydrogen (5 mL/min) was used as a reaction gas to remove polyatomic interferences of Ar-Ar+ (Argon-Argon dimers) at a mass of 78 Da. Data output from ICP-MS was expressed in milligrams per kilogram and converted to nanograms per gram by use of the following equation:

article image

where Z is calculated as 1 mg Se standard per mL/Se counts of the standard, and W is calculated as 20 mL/sample weight in grams.

The mean of triplicate analyses was divided by energy density (kcal/kg) of food to express selenium concentration in mg/1,000 kcal.

Normalization of mineral concentration to food caloric content—All manufacturers were contacted to verify energy content of ME in marketed food on a kilogram basis. Measured mineral concentrations (mg mineral/kg food) were normalized against calculated food energy density from company data and expressed as mg/1,000 kcal ME to allow comparisons between products and estimation of expected mineral intake on the basis of manufacturer feeding recommendations. Mineral concentrations in each food were subjectively compared with AAFCO-recommended values for healthy dogs (Appendix).10

Statistical analysis—Data were examined for Gaussian distribution via Shapiro-Wilk and quantile plots.hhh Due to a lack of parametric distribution, box-and-whisker plots were used for descriptive statistics.

Interassay percentage CVs for ICP-AES and ICP-MS were calculated by measurement of minerals of interest in each of the 45 maintenance foods and in the 5 therapeutic foods at 3 times. The CV for each sample was determined, and the mean CV for each mineral was reported. Intra-assay CVs were evaluated for each mineral by examining a single sample 10 times within the same experiment. A CV of < 10% was considered an acceptable variation.

Results

Interassay and intra-assay percentage CVs for ICP-AES and ICP-MS were summarized (Table 1). Interassay CVs for mineral analytes ranged from 3.9% to 6.6%. Intra-assay CVs ranged from 0.2% to 1.1%.

Table 1—

Interassay and intra-assay CV for mineral content of 45 dry foods formulated for maintenance of healthy dogs and 5 therapeutic dry foods formulated for dogs with hepatic or renal disease.

CV (%)
MineralInterassayIntra-assay
Calcium4.900.46
Phosphorus4.010.48
Zinc5.520.60
Iron5.520.17
Copper3.870.66
Manganese6.570.71
Selenium4.551.10

Selenium concentrations were measured by means of ICP-MS; concentrations of all other minerals were measured via ICP-AES.

Median and range mineral concentrations normalized to 1,000-kcal energy density in maintenance foods (n = 45) and therapeutic foods (5) for dogs were subjectively compared with AAFCO recommendations. Results for maintenance foods were summarized (Figure 1).

Figure 1—
Figure 1—

Box-and-whisker plots of mineral concentrations found in 45 dry foods formulated for maintenance of healthy dogs. Selenium concentrations were measured by means of ICP-MS; concentrations of all other minerals were measured via ICP-AES. Boxes represent the 25th and 75th percentiles, and whiskers represent the 2.5th and 97.5th percentiles. Solid and dotted lines within boxes represent the median and mean values, respectively. Dashed lines above and below boxes indicate the maximum and minimum AAFCO-recommended concentrations10 for each mineral. White circles indicate outliers (< 2.5% or > 97.5%).

Citation: Journal of the American Veterinary Medical Association 243, 5; 10.2460/javma.243.5.658

No foods contained calcium or phosphorus concentrations below the AAFCO-recommended minimum values for maintenance of healthy adult dogs (Appendix). However, 4 foods exceeded the recommended maximum concentration of calcium (7.1 g/1,000 kcal ME) at 7.2,i 7.7,m 7.9,o and 7.4 g/1,000 kcal ME,y and 3 exceeded the recommended maximum concentration of phosphorus (4.6 g/1,000 kcal ME) at 4.8,i 4.9,m and 4.7 g/1,000 kcal ME.o All 45 maintenance foods and 3 therapeutic foodstt,vv,xx had calcium-to-phosphorus concentration ratios within the recommended range of 1:1 to 2:1. Two therapeutic foods formulated for dogs with renal disease had calcium-to-phosphorus concentration ratios of 2.9:1uu and 2.2:1,ww considered appropriate for nutritional intervention of azotemic hyperphosphatemia.

The median concentration of zinc in maintenance foods was 53.0 mg/1,000 kcal ME (range, 33.0 to 82.2 mg/1,000 kcal ME). Two maintenance foods had zinc concentrations slightly below the AAFCO-recommended minimum of 34 mg of zinc/1,000 kcal ME (33.0b and 33.3 mg/1,000 kcal MEg), but no food contained zinc concentrations exceeding the recommended maximum value. Two therapeutic foods for dogs with hepatic disease that were labeled as being fortified with zinc contained comparable zinc concentrations (56.3vv and 60.7 mg/1,000 kcal MExx).

The median concentration of iron in maintenance foods was 62.0 mg/1,000 kcal ME (range, 25.5 to 278.4 mg/1,000 kcal ME); all values were within the AAFCO-recommended range. One foodrr that contained 574.4 mg of iron/1,000 kcal ME was censored from results because it contained iron oxide, a nonabsorbable form of iron used as a coloring agent.10 Two maintenance foods contained iron concentrations > 2 times the median iron concentration for maintenance foods (278.4l and 146.6kk mg/1,000 kcal ME). The 2 therapeutic foods for dogs with hepatic disease contained 49vv and 110 mg of iron/1,000 kcal ME,xx respectively.

With the exception of therapeutic foods, no products had copper concentrations below the AAFCO-recommended minimum of 2.1 mg/1,000 kcal ME; none exceeded the recommended maximum value. The median concentration of copper in maintenance foods was 4.4 mg/1,000 kcal ME (range, 2.3 to 9.0 mg/1,000 kcal ME). Copper concentrations in 2 therapeutic foods for dogs with hepatic disease were 1.3vv and 1.7 mg/1,000 kcal ME.xx Considering that these diets were formulated to maintain neutral copper balance in Bedlington Terriers with copper-storage hepatopathy, maintenance food copper concentration was also compared with that standard.31 Copper concentrations in 29 of 45 (64%) maintenance foods were approximately 2 times the median value for these 2 therapeutic foods (1.5 mg/1,000 kcal ME), and copper concentrations in 4 maintenance foodsd,g,mm,rr were ≥ 3 times this value.

No food was found to contain less than the AAFCO-recommended minimum concentration of manganese; there is no AAFCO-recommended maximum concentration for this mineral. Median manganese concentration was 11.7 mg/1,000 kcal ME (range, 2.3 to 26.5 mg/1,000 kcal ME) in maintenance foods and 8.7 mg/1,000 kcal ME (range, 7.1 to 20.8 mg/1,000 kcal ME) in therapeutic foods. With the exception of 1 maintenance food,dd all foods contained > 2 times the recommended minimum manganese concentration.

The median concentration of selenium in maintenance foods was 0.18 mg/1,000 kcal ME (range, 0.09 to 0.49 mg/1,000 kcal ME), and that of therapeutic foods was also 0.18 mg/1,000 kcal ME (range, 0.12 to 0.24 mg/1,000 kcal ME). All of these values were within the AAFCO-recommended range for selenium concentration.

Discussion

To our knowledge, no studies have been performed to analyze major or trace mineral contents of a regional cross section of dry dog foods for compliance with AAFCO recommendations. During the last 20 years, a number of maladies in dogs have been attributed to mineral excess or deficiency, suggesting that dog foods should be evaluated for absolute concentration as well as bioavailability of these components. The goal of the present study was to quantitatively analyze, in various foods, mineral content/1,000 kcal of company-reported ME and to subjectively compare concentrations of calcium, phosphorus, and several trace minerals with AAFCO-recommended minimum and maximum values for healthy dogs. For most (49/50) foods tested, company-reported ME was calculated, and it should be recognized that without digestibility studies, there is a modest amount of inherent error in this calculation methodology (for the 1 remaining food, ME was calculated on the basis of digestibility information, and the value may have been more accurate). Therefore, true caloric content is not known and may actually be higher than reported in many cases,34 which would make inclusion of extra mineral advantageous for most companies to ensure adequate intake by dogs. However, for the purposes of our study, we used the only known information for all foods, which was calculated ME from guaranteed analysis, because this is the standard set forth by AAFCO to help ensure adequacy of nutrient profiles.

Calcium is a macromineral required for numerous intracellular and extracellular functions as well as for skeletal support. Although most foods analyzed were in compliance with AAFCO recommendations, 3 foods labeled for maintenance of adult dogs and 1 food labeled for all life stages contained mildly high calcium concentrations. Ramifications of excessive calcium supplementation in adult dog health are unknown. However, some breeders feed products formulated for maintenance of adult dogs to growing large- and giant-breed puppies with the intent of restricting energy and calcium intake. Uptake of dietary calcium ranges from 25% to 90%, depending on food intake and age of the dog.35 Puppies 6 to 27 weeks of age may passively absorb up to 53% of ingested calcium.36 However, calcitriol concentration, gastrointestinal transit time, gastrointestinal mucosal permeability and mass, and reproductive status affect calcium absorption as well.37 Chronic intake of a high-calcium food has been linked to increased risk and severity of canine osteochondrosis,5,7 with results of several studies3–8 clearly demonstrating a link between increased calcium intake and developmental orthopedic disease in large- and giant-breed dogs. Currently, the AAFCO recommends minimum and maximum calcium concentrations of 2.9 and 7.1 g/1,000 kcal, respectively, for growing dogs.10 Many manufacturers of puppy, adult, and all-life-stage foods comply with those standards, adding more than the minimum to ensure adequacy. The most recent NRC recommendations for calcium intake for growing dogs range from 2.0 to 4.5 calcium g/1,000 kcal ME.9 Our findings suggest that some foods may not meet this criteria. Furthermore, 12 of these foods were labeled for all life stages even though they did not comply with NRC-recommended safe upper limits. Feeding large- and giant-breed dogs high-calcium foods like these could be detrimental for orthopedic development. This brings to light the potential that even foods formulated for puppies can contain calcium concentrations well above the NRC-recommended maximum values.

Analysis of therapeutic foods formulated for dogs with hepatic or renal disease revealed calcium and phosphorus concentrations below the AAFCO-recommended minimum concentration for growing dogs in 4 of 5 products. Thus, these foods should only be used with veterinary guidance and do not meet standard recommendations for growing puppies. All maintenance foods had calcium-to-phosphorus ratios within the recommended range (1:1 to 2:1).10 As expected, 2 therapeutic foods formulated for dogs with renal diseaseuu,ww had calcium-to-phosphorus ratios outside of this range (2.9:1 and 2.2:1) and would be inappropriate for support of proper bone formation and remodeling in an adolescent dog.

Zinc is a transition metal that exists in multiple oxidation states and plays an essential role in intermediary metabolism as a cofactor for macronutrient metabolism, epidermal integrity, and cell replication.9 Genetic predisposition for zinc-responsive dermatosis is reported in northern breeds (eg, Alaskan Malamutes and Siberian Huskies), Bull Terriers, Great Danes, and Labrador Retrievers. In a previous study,11 a diet associated with zinc-responsive dermatopathy (eg, erythroderma, alopecia, thickened and fissured footpads, and hyperkeratotic plaques at mucocutaneous junctions)38 failed to meet the NRC recommendation for minimum zinc concentration. However, rather than a simple dietary zinc insufficiency, availability of zinc may be reduced by high dietary concentrations of calcium, phosphorus, magnesium, or phytates, which adversely influence zinc absorption.39,40 Foods containing cereal ingredients often contain a high phytate concentration that can compromise zinc availability.11 In the present study, 2 maintenance foodsb,g had zinc concentrations slightly below the current AAFCO recommendation of 34 mg of zinc/1,000 kcal ME (33.0 and 33.3 mg/1,000 kcal ME); however, when one considers the interassay CV, these products may in fact be adequate. More importantly, without bioavailability data, the relevance of borderline low zinc concentrations remains unclear.

An increase in hepatic copper concentration reported in dogs since the late 1930s coincides with introduction of commercially available pet foods and recommendations made by governing bodies regarding dietary mineral supplements.31,41,42 One of the authors (SAC) has observed an apparent increase in prevalence of copper-associated hepatopathies in purebred large- and small-breed dogs and mixed-breed dogs in recent years. From 1990 through 1997, before replacement of cupric oxide and cuprous oxide with more bioavailable forms including copper sulfate and copper chelates,10 approximately 55% of liver biopsies from dogs with chronic hepatitis contained high measured hepatic copper concentrations (≥ 400 μg/g on a dry-weight basis), whereas during the ensuing era, high copper concentrations were detected in approximately 72% of samples from dogs with chronic hepatitis. Similarly, the same author observed an apparent increase in hepatic copper concentration in biopsy samples lacking histologic features of chronic hepatitis (from 17% in 1990 through 1997 to 22% in 1998 through 2009). These observations, along with a recent study33 of hepatic copper concentrations in Labrador Retrievers with and without chronic hepatitis, initiated our interest in the present study. We speculate that a change to more bioavailable forms of copper in dog foods may correspond to the apparently increased prevalence of high hepatic copper concentrations in dogs. Naturally occurring differences in copper transporters might place certain breeds at risk (eg, Labrador Retrievers and West Highland White Terriers have a propensity for hepatic copper accumulation). Data from the present study confirmed that 2 therapeutic foods commonly prescribed for dogs with hepatic insufficiency or copper-associated hepatopathy contain < 50% of the median copper concentration contained in maintenance foods commonly fed to dogs. The clinical experience of one of the authors (SAC) with use of these foods for an entire lifetime in dogs with copper-associated hepatopathy and dogs with portosystemic vascular anomalies suggests that these concentrations of copper are safe and effective for control of hepatic copper accumulation.

For > 30 years, our group has recognized that excessive hepatic macrophage (Kupffer cell) iron accumulation (hemosiderosis) accompanies many necroinflammatory and degenerative liver disorders in dogs. This includes a subset of dogs with portosystemic vascular malformations that accumulate hemosiderinladen macrophages in multifocal lipogranulomas (SAC; unpublished observations). These subjective observations are similar to findings in humans with chronic liver disorders.12,43–47 Regulation of the content and distribution of iron in the body is complex because the process depends on 4 major cell types (duodenal enterocytes, erythroid progenitors, reticuloendothelial macrophages, and hepatocytes) as well as numerous membrane pumps and hormones (erythropoietin and hepcidin).45 Furthermore, the transportation and storage of iron is widely influenced by inflammatory cytokines, oxygen tension (eg, enterocytes and hepatocytes), intracellular iron concentrations, and systemic iron needs.45 Discussion of the complexities of systemic iron management is beyond the scope of this manuscript, but it is important to point out that ferrous iron, in the presence of hydrogen peroxide, generates hydroxyl radicals through the Fenton reaction. These liberated free radicals attack membranes and activate stellate cells, transforming them into a myofibrocyte phenotype that deposits fibrillar collagen within the hepatic extracellular matrix, resulting in liver fibrosis.44 Restriction of dietary iron intake has not yet been considered as a therapeutic nutritional intervention for dogs with liver disease. However, at present, it is unclear what allowance would be considered restricted because of the broad safety margins recommended by regulatory agencies.

Manganese functions as a cofactor for enzymes involved in antioxidation and gluconeogenesis.48 However, manganese accumulation in astrocytes of the substantia nigra disturbs dopaminergic neurotransmission, leading to Parkinsonian-like neurologic effects.49–52 This phenomenon contributes to the syndrome of hepatic encephalopathy in patients with hepatic insufficiency or portosystemic shunting.50 Typical nutritional management for dogs with acquired or congenital portosystemic shunting involves feeding a diet with a modified and restricted protein content. Although feeding to individual protein tolerance is essential in averting hepatic encephalopathy, this complex syndrome is often triggered by potentiating physiologic disturbances (eg, dehydration, azotemia, enteric bleeding, or infection).49 Causal factors and mediators additional to ammonia in hepatic encephalopathy remain incompletely understood even after decades of investigation. Neurologic improvement was reported52 in humans with demonstrated brainstem manganese deposition and hepatic encephalopathy after dietary manganese restriction. Manganese accumulation within the lentiform nuclei also has been detected in dogs with hepatic encephalopathy secondary to portosystemic vascular malformations.51 After surgical attenuation of the shunting vessels, subjective improvement (as determined on the basis of MRI) was suggested.53 The wide range of manganese concentrations (up to an approx 10-fold difference) in therapeutic and maintenance foods in the present study suggests that current recommended allowances might be reconsidered in light of the potentiating role of manganese in the syndrome of hepatic encephalopathy, particularly in therapeutic foods.

Selenium in the form of selenocysteine has an essential role in the formation and degradation of triiodothyronine and in the functional class of proteins referred to as selenoproteins. Of the selenoproteins, the most commonly investigated in health and disease is glutathione peroxidase.54–56 Glutathione is one of the most important natural cellular antioxidants and is conserved by the glutathione reduction-oxidation cycle, in which glutathione peroxidase has a pivotal role.9 Nutrients such as selenium that influence cell redox status have protean effects affecting a wide diversity of metabolic pathways as well as risk for neoplastic transformation.57 Human epidemiological studies implicate dietary selenium inadequacy as an important risk factor for development of cancer.58–61 Selenium concentrations in pet foods have been adjusted with increasingly bioavailable forms of selenium with inorganic salts.57 This has resulted in NRC recommendations that approximate a concentration of 0.088 mg/1,000 kcal ME,9 whereas values suggested by the AAFCO range from 0.03 to 0.57 mg/1,000 kcal ME.10 Standardization of these recommendations would require further research to establish optimal dietary intake.

An important limitation of the present study is that, although nutritional sources can be deduced from product labels, the bioavailability in dogs of each ingredient remains unknown. Few studies have been performed to determine bioavailability of trace minerals in dogs fed a complex diet similar to an over-the-counter maintenance food.62 Complexity derives from the presence of chelating agents and mineral antagonism in complex mixed-feed products as well as whether the source of these minerals is from the ingredients or the additional premix added to such foods. For example, inhibition of zinc by phytate-containing ingredients coincidentally may increase the bioavailability of copper by reduction of zinc-copper antagonism.63 Antagonism between dietary minerals is also common whereby heme iron absorption can be hindered by increased calcium intake.64 Conversely, ascorbic acid was shown to effectively increase non– heme iron absorption.65 The amino acid composition of a diet also may influence mineral bioavailability. For example, cysteine, histidine, and lysine form tridentate chelates with iron, thus increasing iron absorption.66–68 Pectins, a class of water-soluble fibers, have the ability to complex with polyvalent cations such as iron, reducing its bioavailability. However, some pectins have also been shown to enhance iron absorption.69,70 Lastly, the inorganic sources of some minerals may play a role in their bioavailability. Generally speaking, oxide and carbonate forms of minerals have less bioavailability, which is why many pet food companies have switched to alternatives such as sulfate, acetate, gluconate, and various amino acid or protein chelates.10

Results of the study reported here revealed that most (39/45) commercially available dry maintenance foods tested were in compliance with AAFCO guidelines for concentrations of calcium, phosphorus, zinc, iron, copper, manganese, and selenium. However, the range of mineral concentrations varied widely among foods. Use of foods formulated for adult dogs and labeled for all life stages may not be ideal for large- and giant-breed puppies during growth stages. Additionally, if the perceived increase in copper-associated hepatopathies among dogs is indicative of a change in prevalence, it is important not only to provide sufficient dietary concentrations of copper, but also to ensure that a safe upper limit for chronic exposure is determined. Further studies investigating the bioavailability of minerals quantified in this study are necessary to verify appropriateness of the minimum and maximum dietary recommendations established for dogs.

ABBREVIATIONS

AAFCO

Association of American Feed Control Officials

AES

Atomic emission spectroscopy

CV

Coefficient of variation

ICP

Inductively coupled plasma

ME

Metabolizable energy

MS

Mass spectrometry

NRC

National Research Council

a.

4Health Chicken and Rice, Tractor Supply Co, Brentwood, Tenn.

b.

Abady Classic Formula for Maintenance & Stress, The Robert Abady Dog Food Co, Poughkeepsie, NY.

c.

Acana Pacifica, Champion Pet Foods LP, Edmonton, AB, Canada.

d.

Aqualuk, Annamaet Petfoods, Sellersville, Pa.

e.

Authority Adult Minichunk Real Chicken Adult, Authority Pet Food Co, Phoenix, Ariz.

f.

Big Red Nuggets dog food, Pro Pet LLC, St Marys, Ohio.

g.

Biljac Adult Select Formula, Kelly Foods Corp, Berlin, Md.

h.

Blue Buffalo Natural Chicken and Brown Rice Adult, Blue Buffalo Co Ltd, Wilton, Conn.

i.

Canidae All Life Stages, Canidae Corp, San Luis Obispo, Calif.

j.

Country Natural for Puppies, Grandma Mae's Country Naturals LLC, New York, NY.

k.

Dad's Economets Lamb and Rice Meal, DAD'S Pet Care, Meadville, Pa.

l.

Dick Van Patten's Natural Balance Limited Ingredient Diets Sweet Potato and Venison, Dick Van Patten's Natural Balance Pet Foods Inc, Pacoima, Calif.

m.

Eagle Pack Original Pork Meal and Chicken Meal Formula, WellPet LLC, Tewksbury, Mass.

n.

Eukanuba Adult Maintenance, Procter & Gamble Pet Care, Cincinnati, Ohio.

o.

Evo Small Bites Red Meat Formula, Natura Pet Products, Fremont, Nev.

p.

Exclusive Chicken and Rice Adult Formula, PMI Nutrition, St Louis, Mo.

q.

Harmony Farms Chicken and Brown Rice Recipe, Sierra Pet Products LLC, Wilton, Conn.

r.

Hill's Science Diet Adult Advanced Fitness Original, Hill's Pet Nutrition Inc, Topeka, Kan.

s.

Iams Lamb Meal and Rice Formula, Procter & Gamble Pet Care, Cincinnati, Ohio.

t.

Iams ProActive Health Smart Puppy Original, Procter & Gamble Pet Care, Cincinnati, Ohio.

u.

Iams Minichunks, Procter & Gamble Pet Care, Cincinnati, Ohio.

v.

Innova Large Breed Adult Dry Dog Food, Natura Pet Products, Fremont, Nev.

w.

Kibbles 'n Bits Homestyle Grilled Beef Steak and Vegetable Flavor, Del Monte Corp, San Francisco, Calif.

x.

Kibbles 'n Bits Original Savory Beef & Chicken Flavor, Del Monte Corp, San Francisco, Calif.

y.

Merrick Turducken, Merrick PetCare Inc, Amarillo, Tex.

z.

Newman's Own Organics Adult Dog Formula, Newman's Own Organics Inc, Aptos, Calif.

aa.

Nutro Natural Choice Lamb and Rice Formula Adult, Nutro Product Inc, Franklin, Tenn.

bb.

Ol' Roy Complete Nutrition, Wal-Mart Stores Inc, Bentonville, Ark.

cc.

Orijen 6 Fish Dog, Pet Foods LP, Edmonton, AB, Canada.

dd.

Pedigree Adult Small Breed, Mars Petcare US, Brentwood, Tenn.

ee.

Pedigree Puppy Complete Nutrition, Mars Petcare US, Brentwood, Tenn.

ff.

Purina Beneful Original Adult, Nestlé Purina Petcare Co, St Louis, Mo.

gg.

Purina One SmartBlend Chicken and Rice Formula, Nestlé Purina Petcare Co, St Louis, Mo.

hh.

Purina ProPlan Performance Formula for All Life Stages, Nestlé Purina Petcare Co, St Louis, Mo.

ii.

Purina Puppy Chow Complete & Balanced for Growing Puppies, Nestlé Purina Petcare Co, St Louis, Mo.

jj.

Purina One SmartBlend Large Breed Puppy Formula, Nestlé Purina Petcare Co, St Louis, Mo.

kk.

Rachel Ray Nutrish with Real Beef and Brown Rice, Ainsworth Pet Nutrition Inc, Meadville, Pa.

ll.

Royal Canin Energy 4800, Royal Canin USA Inc, St Charles, Mo.

mm.

Royal Canin Medium Adult 25, Royal Canin USA Inc, St Charles, Mo.

nn.

Simply Nourish Adult Dog Food Chicken and Brown Rice Recipe, Simply Nourish Pet Food Co, Phoenix, Ariz.

oo.

Solid Gold Hund-n-Flocken, Solid Gold Health Products for Pets Inc, El Cajon, Calif.

pp.

Taste of the Wild High Prairie Canine Formula, Taste of the Wild Pet Food, Meta, Mo.

qq.

Timberwolf Dakota Bison Canid Formula, Iron Pyramid LLC, Windmere, Fla.

rr.

Wegmans Bruiser, Wegmans Food Markets Inc, Rochester, NY.

ss.

Wellness Super5Mix Complete Health Chicken Recipe, WellPet LLC, Tewksbury, Mass.

tt.

Prescription diet g/d Canine, Hill's Pet Nutrition Inc, Topeka, Kan.

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Prescription diet k/d Canine, Hill's Pet Nutrition Inc, Topeka, Kan.

vv.

Prescription diet l/d Canine, Hill's Pet Nutrition Inc, Topeka, Kan.

ww.

Prescription diet NF Canine, Nestlé Purina Petcare Co, St Louis, Mo.

xx.

Prescription diet LS Canine, Royal Canin USA Inc, St Charles, Mo.

yy.

Cool Grind Blade Grinder, No. 501, Capresso, Closter, NJ.

zz.

Summit Series SI-603, Denver Instrument, Bohemia, NY.

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Thermo Scientific iCAP 6500, Thermo Fisher Corp, Cambridge, England.

bbb.

Optical transfer device for axially viewed ICP spectrometers, Cornell Research Foundation, Ithaca, NY.

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Rutzke MA. An optical interface was developed to reduce the matrix effects observed in an axially viewed ICP-OES (abstr), in Proceedings. Pittcon 1999;038.

ddd.

Rutzke MA. An optical interface that can optically section an axially viewed plasma (abstr), in Proceedings. 24th Annu Conf Fed Anal Chem Spectros Soc 1997;664.

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Rutzke MA. An optical transfer interface system for an axially viewed plasma improves analysis of biological samples. PhD dissertation, Cornell University, Ithaca, NY, 2002.

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Agilent 7500 ce ICP Mass spectrometer, Agilent Technologies, Alpharetta, Ga.

ggg.

Microliter Nebulizer, Microglass, Cedaredge, Colo.

hhh.

SigmaPlot, version 11.0, Systat Software Inc, San Jose, Calif.

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Appendix

Current AAFCO recommendations10 for concentrations of minerals in foods formulated for maintenance of healthy dogs.

Minimum concentration 
MineralGrowth and reproductionAdult maintenanceMaximum concentration
Calcium (g/1,000 kcal ME)2.91.77.1
Phosphorus (g/1,000 kcal ME)2.31.44.6
Zinc (mg/1,000 kcal ME)3434286
Iron (mg/1,000 kcal ME)2323857
Copper (mg/1,000 kcal ME)2.12.171
Manganese (mg/1,000 kcal ME)1.41.4ND
Selenium (mg/1,000 kcal ME)0.030.030.57

ND = Not determined.

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