Abstract
Background: Risk of nutrient deficiency in dogs during caloric restriction is not currently known, while obesity is a growing concern.
Objectives: To determine nutrients that might require further evaluation for the risk of deficiency during caloric restriction.
Animals and methods: Five commercially available canine diets, representing a range of caloric density (2900–4240 kcal/kg metabolizable energy), were assessed for potential nutrient inadequacy if fed to a hypothetical overweight dog. Caloric density and typical nutrient analysis for protein, amino acids, fat, fatty acids, minerals, and vitamins were obtained from the manufacturer. Nutrient intake was calculated using ideal body weight for caloric intakes including 87, 79, 70, 61 and 52 kcal/kg0.75 and compared with National Research Council recommended nutrient allowances (NRC-RA) for ideal weight.
Results: No diets were less than NRC-RA when compared to NRC-RA (/1000 kcal). The five evaluated diets varied in terms of which nutrients were less than NRC-RA and the degree of restriction required for this to occur. All diets had at least one essential nutrient less than NRC-RA at 79 kcal/kg0.75/day and multiple nutrients less than NRC-RA at 70 kcal/kg0.75/day. Choline and selenium were the nutrients most often affected by caloric restriction but others were less than the NRC-RA with caloric restriction.
Conclusions: Further research is needed to determine actual nutrient requirements in overweight dogs, and whether clinical nutrient deficiencies actually arise in vivo.
Clinical importance: Weight loss plans for overweight dogs (particularly those with very low-energy requirements) should include consideration for nutrient adequacy.
1. Introduction
Obesity is one of the most common health problems of dogs, with between 34% and 59% of the canine population in the United States and Europe currently being overweight or obese (Lund et al. Citation2006; Courcier et al. Citation2010). The condition is associated with numerous diseases, including pancreatitis (Hess et al. Citation1999), osteoarthritis (Kealy et al. Citation2002), and respiratory disease (Bach et al. Citation2007). In addition, obesity may also contribute to anesthetic complications (Clutton Citation1988) and a shorter lifespan (Kealy et al. Citation2002).
With growing awareness of the consequences of obesity and its related diseases, weight management plans are becoming more common in veterinary medicine. There are different approaches to weight management method and diet (Yaissle et al. Citation2004; Saker and Remillard Citation2005). For diet, both the degree of energy restriction and the composition of an optimal weight loss diet are topics of current interest. Starting points for caloric restriction vary (Laflamme and Kuhlman Citation1995; Burkholder and Bauer Citation1998; Kealy et al. Citation2002; Blanchard et al. Citation2004). If current intake can be obtained or estimated, then a 20% restriction from that has been suggested (Burkholder and Bauer Citation1998). However, when clinicians have trouble estimating current intake, the starting point varies from resting energy requirement (RER; 70 kcal/kg0.75) for current weight to RER for ideal weight to a percentage of the maintenance energy requirement (MER) for current or ideal weight. Using RER for current weight rarely provides adequate restriction to insure weight loss (Blanchard et al. Citation2004), and estimating ideal weight is complicated due to its subjectivity, particularly in extremely obese animals. The aim of the initial energy allocation is to insure that the rate of weight loss is steady, but the exact rate varies considerably in obese pet dogs (Laflamme and Kuhlman Citation1997; Butterwick and Hawthorne Citation1998), with a median rate of 0.85% per week (range of 0.35–1.56%) in one recent study (German et al. Citation2007).
In addition to varying levels of caloric restriction, the optimal choice of weight loss diet is debatable; macronutrient formulations as well as the fiber content and select micronutrients have been studied (Laflamme and Kuhlman Citation1995; Borne et al. Citation1996; Jewell et al. Citation2000; Weber et al. Citation2007). Results of several studies have suggested a higher protein : calorie ratio than in maintenance diets is necessary for the preservation of lean body mass during weight loss. Loss of lean mass may occur in diets that have typical protein concentrations since protein intake might become deficient when significant calorie restriction is required (Blanchard et al. Citation2004; Weber et al. Citation2007; Yamka et al. Citation2007). Though ensuring adequate nutrient : calorie ratio is a concern during severe caloric restriction, optimal nutrient : calorie ratio and concentrations of various nutrients of concern have not yet been determined. The formulation of weight loss diets becomes even more crucial in dogs that require severe caloric restriction (German et al. Citation2007), whether during weight loss or during weight maintenance in dogs that have low-energy requirements. In these animals, severe caloric restriction could result in nutrient deficiencies if the nutrient : calorie ratio of the diet is insufficient.
As there is high variability in the degree of caloric restriction required to achieve successful weight loss or to maintain optimal weight in dogs with low-energy requirements, diet selection and the degree of caloric restriction may be crucial factors in determining whether animals are at risk for nutritional deficiencies. The authors are not aware of any studies evaluating which nutrients may have the potential to be deficient during caloric restriction when using commercial diets. Therefore, with the use of a hypothetical overweight dog, the purpose of this study was to determine nutrients that should be further evaluated in vivo for the risk of deficiency during caloric restriction in commercially available canine diets with varying caloric density.
2. Materials and methods
2.1. Diets
Five commercially available canine diets from a single manufacturer (two therapeutic weight loss diets and three over-the-counter (OTC) diets) representing a range of caloric densities (2900–4240 kcal/kg metabolizable energy (ME)) were selected (). OTC diets were selected even though they were not formulated or marketed for weight loss. These diets were specifically included to evaluate whether they were at higher risk for nutrient deficiency compared to purpose-formulated diets, since anecdotally some owners feed less of these foods to overweight dogs to induce weight loss, unaware of potential for nutrient deficiency. All diets were produced by the same company, but included different brands and represented different markets (grocery, pet store, veterinary). No diets were less than National Research Council recommended allowances (NRC-RA) when compared to NRC-RA (/1000 kcal) when fed at the energy intake for which they were formulated ().
Table 1. Number of nutrients less than the NRC-RA and nutrient profiles of five canine commercial dry diets.
2.2. Study design
Information on both caloric density and typical nutrient analysis for protein, amino acids, fat, fatty acids, minerals, and vitamins was obtained from the manufacturer and used to calculate daily nutrient intake. Daily nutrient intake was calculated for a hypothetical overweight dog with an ideal body weight of 15 kg that weighs 20 kg (i.e., 33% over ideal body weight) at different degrees of daily caloric restriction which may be required for weight loss, including 87, 79, 70, 61, and 52 kcal/kg0.75. This calculation was performed for all nutrients listed in the NRC profiles, except for eicosapentaenoic acid and docosahexaenoic acid (i.e., 36 in total) (National Research Council Citation2006). These amounts were then compared with the NRC-RA for an adult dog of 15 kg on a metabolic body weight basis (/kg0.75) (National Research Council Citation2006). NRC-RA are provided on a metabolic body weight basis (/kg0.75), an energy basis (/1000 kcal ME), or a dry matter basis (/kg dry matter) (National Research Council Citation2006). The metabolic body weight basis was selected for comparison because both NRC-RA and calculated nutrient intake can be expressed per kg0.75 so that a direct comparison was possible. In addition, 2006 NRC requirements state that “For dogs with an unusually low-energy intake (below the suggested requirement), the nutrient concentrations (Amt/1000 kcal) may not be adequate. These animals should be fed the nutrients shown in the column Amt/kgBW0.75” (National Research Council Citation2006). Ideal weight (15 kg) was used in determining NRC-RA, as well as in calculating caloric restriction. As an example, 79 kcal/kg0.75/day for an overweight dog, with an ideal weight of 15 kg (i.e., 33% overweight), would be 602 kcal/day. The NRC-RA of choline is 56 mg/kg0.75. If this dog were fed a diet containing 790 mg choline/1000 kcals and ate 79 kcal/kg0.75/day, it would receive 476 mg/day, more than NRC-RA (427 mg/day). However, if this dog were not losing weight and caloric intake was then reduced to 70 kcal/kg0.75/day (or 534 kcal/day), it would then receive 422 mg/day, less than NRC-RA (427 mg/day). Therefore, depending on the nutrient profile of the individual diet and the degree of calorie restriction required, intake of certain nutrients may not be sufficient to meet NRC recommended daily allowances on a metabolic body weight basis. As nutrient requirements of overweight animals have not been studied, ideal weight (15 kg) was assumed for all comparisons and calculations in this study.
The nutrient intake for the hypothetical dog at each level of caloric restriction was calculated for each of the five diets using ideal weight. The total number of nutrients less than NRC-RA for each diet were recorded at every degree of restriction.
3. Results
The five evaluated diets varied widely in terms of the nutrients that were less than NRC-RA and the degree of restriction required for them to be less than those allowances. At 87 kcal/kg0.75, calculated daily intake for all five diets was less than NRC-RA for at least one nutrient (). At 70 kcal/kg0.75/day, multiple nutrients were less than NRC-RA (median = 5 or 14% of all 36 nutrients, with a range of 6–22% for the five diets). At 52 kcal/kg0.75/day, a median 10 or 28% of all 36 nutrients were less than the NRC-RA (range of 14–61%). The diet with highest caloric density did not consistently have the most nutrients less than NRC-RA at each level of restriction compared to diets with lower caloric density. However, the same amount or fewer nutrients were less than NRC-RA in the therapeutic weight loss diets than OTC diets at every level of restriction ().
The number of nutrients with a calculated intake less than the NRC-RA for a 15 kg dog at different levels of caloric restriction also varied by diet (). Selenium and choline had a calculated daily intake less than NRC-RA in most diets with minimal-moderate restriction (87 and 70 kcal/kg0.75, respectively). At 87 kcal/kg0.75, the median daily selenium intake was 64% of the recommended value (range of 36–76%), with all diets less than NRC-RA. At 87 kcal/kg0.75, the median daily choline intake was 110% of the recommended value (range of 66–134%), with calculated daily intake less than NRC-RA in two of the five diets studied. At 70 kcal/kg0.75, median daily choline intake was 88% (53–108%) of NRC-RA. Other nutrients that were less than NRC-RA at 87 kcal/kg0.75 included magnesium, riboflavin, niacin, methionine/cysteine, tryptophan, chloride, and vitamin D.
Table 2. Median and range of nutrient intake in five commercial dog diets compared to the NRC-RA for a hypothetical dog with an ideal weight of 15 kg that weighs 20 kg (i.e., 33% overweight) at varying daily caloric restriction that might be required to achieve weight loss.
4. Discussion
This study assessed a small sample of diets (representing a wide range of caloric density), in order to determine the nutrients that should be evaluated further in vivo for the risk of deficiency during a weight management program. The nutrients at potential risk of deficiency varied widely but multiple nutrients were less than NRC-RA with energy restriction. All diets had nutrient concentrations greater than NRC-RA (on an energy basis [/1000 kcal]). All diets had at least one essential nutrient less than NRC-RA at 87 kcal/kg0.75/day, and multiple nutrients less than NRC-RA at 70 kcal/kg0.75/day. Although nutrients less than NRC-RA varied among diets, choline and selenium were the nutrients most frequently affected by caloric restriction. However, magnesium, riboflavin, niacin, methionine/cysteine, tryptophan, chloride, and vitamin D were also less than NRC-RA with minimal caloric restriction (i.e., 87 kcal/kg0.75/day). Although the current study is only theoretical, these results do raise the possibility that nutrient deficiencies could develop during caloric restriction; however, further studies in vivo are required to confirm these potential findings.
The range of caloric intake used for the calculations in the current study ranged from 52 to 87 kcal/kg0.75/day using ideal weight, to reflect the restriction that might be required for weight loss in clinical practice. For example, many pet food companies’ feeding directions use 70 kcal/kg0.75/day based on current weight to estimate initial feeding recommendations in an obese animal (Linder and Freeman Citation2010). In a recent study (German et al. Citation2007), the energy intake needed to achieve weight loss ranged from 53 to 86 kcal/kg of target weight0.75/day (median, 63 kcal/kg target weight0.75/day), which was partially the basis for the degrees of restriction evaluated. The goal was to use a range of caloric restriction in this current study that would be comparable to real life situations. Further, it is noteworthy that median energy intake during post-weight loss MER was recently estimated to 65 kcal/kg0.75/day (range 52–104 kcal/kg0.75/day) (German et al. Citation2011), suggesting those dogs with low MERs could have similar nutrient intakes during weight maintenance as to those required for weight loss.
Although many nutrients were less than NRC-RA, depending upon the individual diet and the degree of restriction, the nutrients that were most likely to be low were selenium and choline, both of which are considered essential nutrients by the NRC. Selenium is a mineral involved in antioxidant pathways and thyroid function, as well as a part of the immune system (National Research Council Citation2006). Deficiency in the dog can lead to anorexia, depression, dyspnea, and coma (National Research Council Citation2006). Choline is a vitamin-like substance that has multiple roles in the body, including neurotransmision, hepatic lipid transport and storage, coagulation, and as a methyl donor as a part of phosphatidylcholine (National Research Council Citation2006). Signs of deficiency include weight loss, hepatic lipidosis, anorexia, vomiting, neurological signs, disorders, and coagulopathies (National Research Council Citation2006; Ziesel and Da Costa Citation2009). Though selenium and choline both can have negative consequences if deficient in a diet, excessive levels of dietary selenium can also have adverse effects. As a result, and in contrast to NRC guidelines, regulatory organizations in both the United States [Association of American Feed Control Officials (AAFCO)] and Europe [European Pet Food Industry Federation (FEDIAF)] have set maximum levels for supplementation. Given the narrow safety range of selenium, this is a nutrient for which dietary formulation must be very carefully considered and its optimal concentration is not as easily determined as other nutrients. Given that both of these nutrients were commonly less than NRC-RA, the possibility of deficiency for both selenium and choline is a concern; however, this has not been described within studies of weight loss in dogs, and also not been recognized clinically by any of the authors. Likewise, endogenous production of choline or dietary intake of other nutrient may alter or lessen the dietary requirements, which may make quantification of true requirements more difficult. Nonetheless, having multiple nutrients less than NRC-RA in the same pathways (e.g. methionine, folate and cobalamin), as was calculated for some of these diets, could increase the potential for adverse effects. Given the detrimental effects of deficiency, further studies to evaluate biomarkers for choline and selenium deficiency, as well as all nutrients found to be less than NRC-RA, are warranted to assess true nutrient requirements and confirm these potential findings. Although clinical signs are anecdotally uncommon even with long-term use, assessment of subclinical deficiencies or suboptimal nutrient intake of these nutrients should be considered to ensure optimal nutrition rather than simply avoiding clinical signs or adverse effects. Further work is required to determine the optimal nutrient profile for a food designed for calorically restricted animals (whether for weight loss or due to low-energy requirements).
For all nutrients, the caloric density of the diet was not consistently related to the number of nutrients that were less than NRC-RA at every level of restriction. However, fewer total nutrients were less than NRC-RA in the therapeutic diets compared with the OTC diets. Therefore, specially formulated diets with an increased essential nutrient : calorie ratio may be beneficial in animals during weight loss or in animals that have low MERs, though this finding must be confirmed by further in vivo research.
Given the hypothetical nature of this study, many limitations must be discussed. One important limitation of this study is the small number of canine-only diets selected from one company. Though the nutrients at risk varied between the diets selected, a company may have similar trends in all their diet nutrient profiles and thus limit how applicable the data is to diets from other companies. While this study was meant to be representative of clinical practice, further studies assessing both canine and feline diets from various companies would help to establish common trends across a wider range of diets.
The largest limitation of this study is the need for comparison to one of the current guidelines instead of specific data on nutrient requirements of obese dogs and dogs undergoing caloric restriction. While AAFCO, FEDIAF, and NRC all have regulatory guidelines for minimums and maximums of nutrients in pet food, both FEDIAF and AAFCO only provide guidelines based on a caloric or dry matter basis of the diet being used. AAFCO and FEDIAF are based on the assumption that a dog or cat is receiving its appropriate MER with a specific range of caloric densities for the diet fed. NRC was chosen for comparison in this study because it is able to provide minimum requirements, adequate intake, and recommended allowances based on the body weight of an individual animal, not the amount of calories provided per day. Though NRC provides recommended allowances, many times there is insufficient research to accurately assess true nutrient requirements and extrapolations must be made. Since NRC recommendations are based on a 15 kg dog, that is the ideal weight of the hypothetical dog in this study. For many nutrients, studies have not been done in vivo for all life stages and alternative methods are used to estimate requirements. This may differ for each nutrient and includes using growth requirements or studies in other species to estimate requirements in adult dogs. Also, NRC-RA assume a 1000 kcal/day caloric intake and do not account for dogs with low-energy requirements. More importantly, each nutrient is used for a different purpose within the body, so some nutrients, particularly those used for metabolism (i.e., B vitamins), may be required in relation to energy intake and not based on the weight of an animal. This may mean that although riboflavin and niacin were found to be lower than NRC-RA at minimal caloric restriction, the dog has lower needs due to a lower energy intake and is not truly deficient. This further highlights the need for an in vivo study to assess true nutrient requirements and confirm these potential findings.
Recommended allowances were chosen for comparison in this study because not all nutrients have adequate and minimal values known, although it should be noted that where applicable, NRC-RA have been calculated by the addition of a 25% bioavailability factor (National Research Council Citation2006). It is also important to note, however, that providing amounts lower than recommended allowances may not be lower than the absolute minimum for that nutrient. Both the addition of a bioavailability factor and the use of recommended allowances instead of a minimum requirement further limit the findings of this study and require confirmation in vivo research. While all diets in the study met NRC-RA on a caloric basis (/1000 kcal) when fed at MER, comparing to NRC-RA on a metabolic body weight basis allowed for an initial assessment of nutrient intake with caloric restriction. While NRC-RA are important guidelines, individual variation may either increase or decrease the risk of clinical deficiency, as will the degree of caloric restriction required, other nutrient concentrations of the diet, and the duration of time eating a specific diet. Individual nutrients also vary on how likely a deficiency is to cause clinical signs or detrimental effects.
Another limitation of the study is the fact that nutrient requirements in obese pets are not known. In obesity, approximately 25% of the excess weight is lean tissue and 75% is adipose tissue (Webster et al. Citation1984; Wells et al. Citation2006). Therefore, while it can be argued that the 25% of extra lean tissue will require essential nutrients, it is unknown if the 75% additional weight from adipose tissue increases a dog's nutrient requirement. Further, since energy restriction leads to a reduction in MER (Laflamme and Kuhlman Citation1995), it is feasible that nutrient requirements also decrease, which is likely the case in some nutrients with a primary function in metabolism. Therefore, future studies are necessary to determine the nutrient needs of obese animals in vivo, and whether recommendations of intake should change on this basis.
Overall, the hypothetical nature of this study is an important limitating factor, as noted previously. The main conclusion that can be made is that while the optimal nutrient : calorie ratio is not known, thoughtful reflection on appropriate caloric restriction and diet selection during weight loss should be performed on every individual. Even with all the limitations of this study, some nutrients of potential concern have been identified that would warrant further study. In vivo testing is required to confirm these potential risks, determine nutrient status, and define optimal nutrient requirements in obese dogs that are energy restricted.
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