An evidence-based review of the use of nutraceuticals and dietary supplementation for the management of obese and overweight pets

Philip Roudebush Scientific Affairs, Hill's Pet Nutrition Inc, PO Box 148, Topeka, KS 66601

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 DVM, DACVIM
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William D. Schoenherr Pet Nutrition Center, Hill's Pet Nutrition Inc, PO Box 1658, Topeka, KS 66601

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 PhD
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Sean J. Delaney Davis Veterinary Medical Consulting PC, 707 Fourth St, Ste 307, Davis, CA 95616.

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 DVM, MS, DACVN

Successful treatment and prevention of overweight and obese cats and dogs require a multidimensional approach to ensure causes or exacerbating factors are identified and eliminated, professional examination and care are provided on a regular basis, and a comprehensive management program is planned and implemented. Over the years, many therapeutic and preventive interventions have been developed or advocated for obese animals, but evidence of effectiveness is often lacking or highly variable. Accordingly, the primary objective of the information reported here was to identify and critically appraise the evidence supporting various aspects of managing obese and overweight pet animals.

Obesity

Obesity in humans has been defined by the federal government as a body mass index (calculated on the basis of height and body weight) of ≥ 30.1 This represents people who are at least 24% to 26% over expected body weight because of adiposity. Although body mass index is not used in domestic animals, the same over-weight percentages are used to define obesity in body condition scoring systems used for cats and dogs. Body condition scoring systems use both visual and tactile cues when assigning a numeric value to a patient's degree of adiposity. Body condition score correlates with more complex measures of body composition, such as results for dual-energy x-ray absorptiometry.2,3 Because body condition scoring can be readily performed by any veterinarian or veterinary technician and has good repeatability, it is becoming widely integrated in clinical practice and is used for identifying and staging obese patients.3

To assist members of the veterinary health-care team when scoring patients and educating clients, pet food companies have created posters and handouts that illustrate the appearance of patients with various body condition scores. These systems are based on a 5-, 7-, or 9-point system. Each point on the 5-point scale represents an increment of 20% to 30% of body weight (ie, 1 = extremely thin; 3 = ideal body weight; and 5 = obese). Each subsequent point on the 7-point scale represents an increment of 15% to 22.5% of body weight (ie, A = extremely thin; D = ideal body weight; and G = severely overweight). Each subsequent point on the 9-point scale represents an increment of 10% to 15% of body weight (ie, 1 = emaciated; 5 = ideal body weight; and 9 = grossly obese). Therefore, patients with a score of ≥ 4 on a 5-point scale, F or G on a 7-point scale, or 7 on a 9-point scale may be obese by use of the definition for humans (ie, at least 24% to 26% overweight).

Prevalence, Risk Factors, and Goals of Treatment

Prevalence data published in 2005 for 8,159 cats examined at private US veterinary practices indicated that 28.7% of adult cats were overweight and 6.4% were obese.4 A higher proportion (44%) of cats between the ages of 5 and 11 years were considered overweight or obese.4 Data published in 2006 for 21,754 dogs examined at private US veterinary practices revealed that 29.0% of adult dogs were overweight and 5.1% were obese.5 A higher proportion (42%) of dogs between the ages of 5 and 11 years were considered overweight or obese.5 Data obtained more recently from a referral hospital revealed that 23.5% of adult dogs were obese and 16.5% were overweight.a The 2005 study4 found that overweight cats were more likely to be neutered, male, and fed a premium or therapeutic food and to have concurrent oral or urinary tract disease. Obese cats were likely to be fed a premium or therapeutic food; to have concurrent diabetes mellitus, oral disease, dermatopathy, or neoplasia; and to be a domestic, mixed, or Manx breed. Overweight dogs were more likely to be older, of certain breeds, neutered, and fed semimoist foods as their main diet.5 In addition, overweight dogs were also more likely to have hyperadrenocorticism, a ruptured cruciate ligament, hypothyroidism, a urinary tract infection, bladder disease, or oral disease. Obese dogs were more likely to be older, of certain breeds, and neutered.5 Obese dogs were also likely to be fed other foods (meat or other food products, commercial treats, or table scraps), homemade foods, or canned foods as their main diet.

The main objective of treatment for obese animals is weight reduction. Typically, weight loss is targeted at a rate of 1% to 2% of body weight/wk to minimize hunger, prevent loss of lean body mass, and help reduce the likelihood of rebound weight gain.6 The desired endpoint may be an ideal body weight or may be a reduction in clinical signs associated with a concurrent condition (eg, improvement in mobility following weight loss in an obese dog with arthritis).

Concepts for Evidence-Based Clinical Nutrition

Evidence-based medicine represents a major, although largely untested, intellectual advance when making clinical decisions and determining patient care.7,8 Evidence-based medicine has been defined as the integration of the best research evidence with clinical expertise and patient values.9 Best research evidence means clinically relevant research, especially from patient-centered clinical studies. Clinical expertise refers to the ability to use clinical skills and past experience to rapidly identify each patient's unique health state, establish a diagnosis, and determine the risks and benefits of potential interventions for that specific patient. For veterinary medicine, the concept of patient values must be extended to include the unique preferences, concerns, and expectations of each owner, as well as those of their animals (ie, the patients). These values and attitudes may be especially important when dealing with overweight pets. Another dimension of evidence-based clinical decisions is availability of resources. When a drug, food, or other therapeutic intervention is not readily available to a veterinarian or animal owner, then it is unlikely to be used. It is further believed that integration of all these elements (clinically relevant research, clinical expertise, owner-patient preferences, and availability of resources) will result in clinicians, animal owners, and patients forming a diagnostic and therapeutic alliance that optimizes clinical outcomes and quality of life.9,10

Veterinarians making decisions about treatments should consider the quality of data supporting a recommendation to use (or not use) a specific treatment when prioritizing therapeutic recommendations. A classification scheme has been proposed for veterinary clinical nutrition and may be useful for establishing rules or dimensions of evidence for recommendations regarding weight management in pets.11 Evidence-based medicine guidelines for veterinary clinical nutrition have been outlined11 (Table 1). Evidence in grades I and II has the highest quality for application in the clinical environment, whereas evidence in grade IV has the lowest quality. This scoring system recognizes that the quality of the evidence supporting a recommendation is an important consideration when making therapeutic decisions.

Table 1—

Guidelines for quality of evidence that can be used for veterinary clinical nutrition. (Adapted from Roudebush P, Allen TA, Dodd CE, et al. Application of evidence-based medicine to veterinary clinical nutrition. J Am Vet Med Assoc 2004; 224: 1766–1771. Reprinted with permission.)

Evidence gradeEvidence guidelines
IEvidence obtained from at least 1 properly randomized, controlled clinical study that used the nutritional product in the target species with animals that had developed the disease naturally. Data published in peer-reviewed journals are preferred.
IIEvidence obtained from randomized, controlled clinical studies conducted in a laboratory setting that used the nutritional product in the target species with animals that had developed the disease naturally. Data published in peer-reviewed journals are preferred.
IIIEvidence obtained from 1 or more of the following:
IVEvidence obtained from 1 or more of the following:

The evidence-based medicine grading system was used to systematically evaluate evidence for use of interventions in obese cats and dogs. However, recommendation of any of the specific strategies described here should be assessed on the basis of its evidence grade and also on the basis of the clinical expertise of the attending veterinarian, pet owner's preferences, and availability of resources. In addition, the impact from providing unnecessary or unproven treatments on the pet-owner relationship should be considered. Treatments that the pet or owner find undesirable may have the impact of impairing the relationship between the pet and owner.

Pyruvate supplementation—Pyruvate is the anionic form of the 3-carbon organic acid, pyruvic acid. Pyruvate is a key intermediate in the glycolytic and pyruvate dehydrogenase pathways, which are involved in biological energy production. Pyruvate is not an essential nutrient because it can be synthesized in cells of the body. Research suggests that pyruvate supplementation may play a role in weight-management strategies, although the mechanism by which supraphysiologic amounts of pyruvate cause fat loss is unclear.

Rats that consumed food supplemented with pyruvate and dihydroxyacetone had an increased utilization of fat as an energy source, as well as an increase in resting metabolic rate.12 In addition, rats that received pyruvate and dihydroxyacetone had increased concentrations of thyroxine, lower concentrations of plasma insulin, and lower rates of lipid synthesis in adipose tissue. Clinical studies13–15 in overweight people revealed that dietary supplementation with pyruvate enhances weight loss and results in greater reduction in body fat when used in conjunction with a low-calorie diet. However, relatively large quantities of pyruvate were necessary for the effect, the amount of weight loss was minimal, and pyruvate supplementation did not appear to meet the expectations of human dieters.13,16

A studyb was conducted to evaluate the effect of dietary pyruvate supplementation on weight and fat loss in dogs. Overweight adult dogs were randomly assigned to receive a low-fat food or the same low-fat food supplemented with pyruvate at 0.6%. The concentration of pyruvate was determined on the basis of the amount recommended for human consumption (ie, 2 g/d). All dogs were fed the same number of calories daily to promote weight loss. At the end of 16 weeks, dogs had lost 16% to 17% of body weight, and there were no significant differences between the groups for food intake, weight loss, or changes in body composition. Investigators concluded that dietary supplementation with pyruvate at 0.6% does not enhance weight loss or fat loss in overweight dogs during caloric restriction.

In summary, although dietary supplementation with pyruvate has been reported to be of some benefit for overweight rodents and humans, an unpublished grade II studyb in dogs found that pyruvate supplementation of food failed to enhance weight or fat loss in overweight dogs consuming a low-fat food. Therefore, current evidence does not support the use of pyruvate supplementation in foods designed for weight loss in dogs. Data on the use of dietary pyruvate supplementation in overweight cats are currently lacking.

Omega-3 fatty acid supplementation—Mitochondrial uncoupling proteins are proton transporters, which dissipate the electrochemical gradient across the inner mitochondrial membrane and thereby uncouple oxidative phosphorylation to result in heat production.17 Mitochondrial uncoupling proteins have been implicated in the regulation of energy expenditure and could be a target molecule for prevention of obesity and treatment of obese animals.18 Gene expression of uncoupling proteins may be regulated by a nuclear receptor via polyunsaturated fatty acids as effective ligands. Thus, fish oil or other sources of omega-3 fatty acids may be useful in weight-management strategies.

In 1 study,c investigators evaluated feeding a food rich in fish oil (omega-3 fatty acid content of 4.1%) and a similar food containing tallow (omega-3 fatty acid content of 0.2%) to obese male Beagles. The amount of energy intake was restricted similarly in both groups. Body weight decreased in both groups during a 14-week period, but it decreased significantly more in dogs that consumed the fish-oil food, compared with weight loss in dogs that consumed the food with tallow. Total body fat and serum leptin concentrations also decreased more in dogs consuming the fish-oil food. Uncoupling protein 3 mRNA concentrations in skeletal muscle increased in dogs that consumed the food with fish oil, compared with concentrations in dogs that consumed the food with tallow.

In summary, results of a single unpublished grade II study suggest that increased consumption of dietary omega-3 fatty acids may be beneficial in overweight dogs subjected to caloric restriction. Additional controlled, randomized studies with larger numbers of obese dogs and cats are indicated.

Amylase inhibitors—Plant extracts from beans and wheat inhibit amylase activity, which may help prevent the digestion of complex carbohydrates. Thus, these extracts may be useful for weight control and management in animals with diabetes mellitus. Products that contain these plant extracts are often called starch blockers. Purified white bean and wheat amylase inhibitors sufficiently reduce postprandial amylase concentrations to cause a delay in carbohydrate digestion and absorption in humans and dogs.19,20 However, several randomized clinical trials21–24 in humans have found that starch blockers have failed to be clinically effective; to our knowledge, clinical studies on the use of starch blockers in obese cats or dogs have not been published.

In summary, a pathophysiologic rationale (ie, grade IV evidence) exists for use of compounds to delay carbohydrate absorption in obese dogs. However, because of a lack of results from appropriately designed clinical studies, use of plant extracts that inhibit amylase activity (starch blockers) are not recommended for routine management of obese pets.

DHEA supplementation—Dehydroepiandrosterone is an intermediate in the biosynthesis of androgenic and estrogenic steroids. It exerts several physiologic effects that do not involve sex hormones. Ergosteroids such as DHEA may be beneficial for weight management because they apparently induce thermogenic enzymes in the liver, increase resting heat production, impair fat synthesis, and promote accretion of lean tissue.25–28 Use of DHEA in the treatment of rats increases resting heat production and decreases gains in body weight without affecting food intake.25,29 Use of DHEA in the treatment of obese dogs has also been evaluated. In 1 study,30 two thirds of obese dogs lost body weight without reduction of food intake when receiving DHEA for 3 months. Serum cholesterol concentrations were within the reference range during the same period. The effect of DHEA administration combined with a low-fat, high-fiber therapeutic weight-management food has also been evaluated in obese dogs.31,32 Dogs treated by administration of DHEA had a significant increase in rate of weight loss, compared with the rate of weight loss in dogs administered a control substance. Investigators concluded that DHEA in combination with a low-fat, high-fiber therapeutic food results in a faster rate of weight loss than does use of the food alone.

Unfortunately, DHEA is not useful as a therapeutic agent for controlling weight gain or promoting weight loss because the dosage of DHEA necessary to achieve these desired results may also cause adverse effects from excess production of sex hormones. The 7-hydroxylated metabolites of DHEA (eg, 7-oxo-DHEA) appear to offer several advantages because of their increased activity and because they are not converted to testosterone or estrogens.27,29 Similar to DHEA, 7-oxo-DHEA enhances thermogenesis but appears to be safe and tolerated well by healthy subjects.27–29,33,34 A randomized, controlled clinical trial35 in healthy, overweight adult humans revealed that 7-oxo-DHEA combined with moderate exercise and a reduced-calorie diet significantly reduced body weight and body fat during an 8-week period, compared with results for exercise and a reduced-calorie diet alone. No adverse effects were reported throughout the study period. Products containing 7-hydroxylated metabolites of DHEA are available for use in obese pets, but to our knowledge, no clinical studies to support their effectiveness have been conducted.

In summary, grade II studies in obese dogs suggest that DHEA may be beneficial as part of a weight-management program when used in conjunction with calorie-restricted foods. Unfortunately, DHEA cannot be recommended because of undesirable adverse effects associated with excessive amounts of sex hormones. Newer and safer forms of DHEA are available as weight-loss products, but clinical studies involving the use of these products in obese dogs or cats have not been conducted.

L-carnitine supplementation—Carnitine is a vitamin-like amino acid derivative that plays a central role in many cellular processes, most of which are related to fat metabolism and energy production.36–38 The primary function of carnitine is mitochondrial transport and subsequent β-oxidation of long-chain fatty acids, which results in ATP production. Additional important functions of carnitine include buffering and eliminating membrane-toxic acyl-CoA molecules from mitochondria and helping regulate metabolic processes, such as urea cycle metabolism, gluconeogenesis, and fatty acid synthesis.36

The potential influence of carnitine on body condition has drawn interest from people involved in production agriculture who have used it in an attempt to partition nutrients away from fat accretion and toward muscle deposition. Results for various species during active growth include improvements in nitrogen balance, increases in protein accretion, acceleration of growth rates, reduction in body fat mass, increases in fatty acid metabolism, and improvements in glycemia in animals provided supplemental L-carnitine.37

Use of foods supplemented with L-carnitine has been evaluated in dogs. In a 12-week study,d investigators examined weight loss in obese dogs (> 30% over ideal body weight) fed a dry, low-fat, high-fiber therapeutic food alone or fed the same food supplemented with L-carnitine (300 mg/kg of diet). Energy intake was adjusted to obtain a consistent 2.5% loss of initial body weight each week. Mean food, energy, and protein intake were similar between groups. Dogs lost similar amounts of weight during the study period. However, dogs fed the food supplemented with L-carnitine maintained lean body mass and had a slightly but not significantly greater loss of body weight.d Abnormalities in glucose metabolism in the obese dogs did not appear to be affected by consumption of the diet supplemented with L-carnitine during weight reduction.e In a 19-week study,39,40,f,g investigators examined weight loss in obese dogs (42% to 43% body fat) fed a dry, low-fat, low-fiber food alone or fed the same food supplemented with L-carnitine (50 or 100 mg/kg of diet). During an ad libitum feeding phase that lasted 7 weeks, dogs consuming the food supplemented with L-carnitine lost more body weight, compared with weight loss in dogs consuming only the food.39,40,f Energy intake was then adjusted to result in a weight loss of 10% of body weight from weeks 7 to 19 of the study. Dogs consuming the food supplemented with 50 or 100 mg of L-carnitine/kg of diet typically had a higher lean body mass and lower fat mass, and they lost more body weight than dogs consuming the nonsupplemented control food.39,40,g Similar findings were reported in healthy, obesity-prone dogs fed a low-fat, high-fiber food supplemented with 300 mg of L-carnitine/kg of diet for 6 months.h

Use of foods supplemented with L-carnitine has also been evaluated in cats. Efficacy, safety, and metabolic consequences of L-carnitine supplementation in obese (at least 20% overweight), neutered, pet cats were evaluated during an 18-week period of rapid weight loss.41 In that study, an aqueous solution of L-carnitine (250 mg/cat) or a control solution (water) was administered in conjunction with a moist, high-protein, low-carbohydrate food designed for weight loss in cats. Significant weight loss was achieved by week 18 in each group without adverse effects. Cats receiving L-carnitine lost weight at a significantly faster rate than cats receiving the control solution, despite equivalent degrees of adiposity in each group. In other studies,42,43 investigators have evaluated the influence of supplemental L-carnitine (50 to 150 mg/kg of diet) in foods provided to obese colony-housed cats. Cats consuming low-fat foods supplemented with L-carnitine for 16 weeks had significantly more weight loss, compared with weight loss in similar cats consuming a low-fat control food without L-carnitine. Similar findings were reported for healthy, obesity-prone cats fed a low-fat, moderate-fiber food supplemented with 500 mg of L-carnitine/kg of diet for 6 months.i

Studies44,j have also been conducted to determine whether dietary L-carnitine supplementation will protect obese cats from hepatic lipid accumulation during food restriction. Results of those studies suggest that an abundance of carnitine in the diet may protect cats from hepatic lipid accumulation during food restriction and experimental induction of hepatic lipidosis.

In summary, results of grade I and II studies suggest that L-carnitine supplementation of a variety of foods designed for weight management can provide beneficial effects when the foods are fed in appropriate amounts to obese and obesity-prone cats and dogs. Unfortunately, many of these studies were described only in abstracts, industry publications, and patent applications, which makes it difficult to review experimental methods and data analysis. On the basis of the current evidence, use of L-carnitine supplementation of foods designed for weight management can be recommended, but additional studies are warranted.

CLA supplementation—Conjugated linoleic acid is a name given to a group of isomers of the fatty acid, linoleic acid.13,45,46 The natural source of CLA is microbial isomerization of dietary linoleic acid, and CLA is found primarily in meat and dairy products. Also, CLA can be produced synthetically by hydrogenation of vegetable oils.

Conjugated linoleic acid is touted to have a number of nutritional benefits, including repartitioning of fat to lean tissue during growth, improvement in bone formation during growth, improvement in glucose tolerance, increases of metabolic rate and energy expenditure, alteration of lipoprotein metabolism, increase in fatty acid oxidation, reduction in size of fat cells, and reductions in food and energy intake.13,45–47 The effect of CLA on body weight has been investigated in various animals, including mice, rats, and pigs.13,46 Most studies in animals have found that CLA reduces weight gain and fat tissue mass, although no effect was reported for CLA in 1 study.46 Although CLA feeding reportedly reduces food or energy intake, the reductions appear marginal and cannot fully account for the marked decrease in fat deposition.46 Evidence suggests that mitochondrial uncoupling proteins play an important role in CLA's regulation of energy expenditure.46 Similar to the effects for omega-3 fatty acids, upregulation of the expression of uncoupling protein 2 could contribute to the increase in energy expenditure caused by CLA.

Results of clinical studies46–48 of the effects of CLA on body weight and composition in humans are inconsistent. Most of the clinical studies were conducted in free-living subjects who were not strictly controlled for nutrient and energy intakes. Reductions in body weight were only detected in human patients with type-2 diabetes; in a few studies, a significant but relatively small decrease in body fat was detected. Some studies in humans suggest that CLA may increase lean body mass, but CLA has no significant effects on concentrations of plasma cholesterol or other lipids. Clinical studies in humans indicate that the effect of CLA on body weight and body fat is considerably less than that anticipated for domestic animals, and CLA appears to have no major effect on plasma lipid concentrations.

Dietary supplementation with CLA has been evaluated in clinically normal adult cats and puppies, and overweight adult dogs. In 1 study,k investigators detected no change in body weight, body composition, or daily energy expenditure in normal-weight adult cats fed a CLA-supplemented (0.4%) extruded food for 6 months. In another study,l feeding puppies a food containing hydrolyzed sunflower oil with increased amounts of CLA helped reduce body fat during growth. At 1 year of age, puppies consuming food containing additional CLA had significantly lower body fat (16.5%) than puppies fed food without CLA (19.8% body fat). The actual amount of CLA used in that studyl was not reported. Reducing body fat during growth may promote lean adult animals, but long-term studies have not been reported. Investigators compared effects of feeding a high-carbohydrate food to those of feeding a high-protein, low-carbohydrate food with and without CLA on weight loss in dogs.m After 12 weeks on a weight-reduction program, dogs in the high-protein and high-protein plus 0.56% CLA food groups lost significantly more body weight and body fat, compared with results for dogs in the control food or control food plus CLA groups.

In another study,49 an extruded high-fiber food with and without CLA was fed to obese dogs. In that study, addition of 0.5% CLA did not significantly affect food intake, energy intake, final lean body percentage, change in lean body percentage, or final fat percentage in obese adult dogs fed a high-fiber food for weight loss. Investigators also conducted a studyn in which 3 amounts of CLA (0%, 0.25%, or 0.50%) were added to a moist food fed ad libitum to obese (mean body fat, 38%) adult dogs. In that 90-day study, increasing the amount of CLA in the diet hindered weight gain and change in body fat percentage. Addition of CLA to food consumed by obese mature dogs hindered weight gain and body composition changes when the dogs were allowed to consume an excessive number of calories.

In summary, there is good evidence from studies in a variety of species to suggest that dietary CLA supple-mentation helps repartition fat to lean tissue in growing animals and may be useful in promoting a long-term lean body condition. However, the effects of CLA on body weight and composition of mature animals remain largely unknown. Limited data suggest that dietary CLA supplementation does not significantly alter body composition or daily energy expenditure in healthy adult cats, but it may hinder weight gain and alter body composition in obese mature dogs allowed access to excessive amounts of food. It has also been suggested that CLA does not significantly affect adiposity of obese adult dogs during weight loss. Unfortunately, many of these studies are described only in abstracts, which makes it difficult to review experimental methods and data analysis. On the basis of the available evidence, CLA cannot be routinely recommended for management of overweight and obese adult pets.

Dietary phytoestrogens—Gonadectomy has a strong association with the prevalence of obesity in pets.4,5 Evidence is emerging that dietary phytoestrogens may play a beneficial role in management of obese animals and animals with diabetes mellitus.50 Results of nutritional intervention studies performed in humans and other animals suggest that the ingestion of soy protein associated with isoflavones and flaxseed rich in lignans improves glucose control and insulin resistance. In animals with obesity or diabetes mellitus, soy protein can reduce serum insulin concentrations and insulin resistance. In humans with or without diabetes, soy protein also appears to moderate hyperglycemia and reduce body weight, hyperlipidemia, and hyperinsulinemia; this supports its beneficial effects on obesity and diabetes. However, most of these clinical trials were conducted over a relatively short period and involved a small number of patients. Furthermore, it is not clear whether the beneficial effects of soy protein and flaxseed are attributable to isoflavones and isoflavone metabolites (daidzein, genistein, or equol), lignans (matairesinol or secoisolariciresinol), or some other component. Isoflavones and lignans appear to act through various mechanisms that modulate pancreatic secretion of insulin or through antioxidative actions. They may also act via mechanisms mediated by estrogen receptors. Some of these actions have been identified in in vitro experiments, but the relevance of these results to disease management is not known. The diversity of cellular actions of isoflavones and lignans supports their possible beneficial effects on various chronic diseases.

Gonadectomy in cats is associated with a decrease in resting energy requirement and an increase in food intake, body weight, and body fat mass.51–53 In neutered, overweight male and female cats, administration of estradiol significantly reduces food intake and prevents increases in body weight and body fat mass.53,54 In a controlled study,54 addition of the phytoestrogen genistein to the diet (100 mg of genistein/kg, PO, q 24 h) had no effect on food intake or increases in body weight in neutered cats, but it was associated with a significant increase in lean body mass and less accumulation of body fat. There were no significant differences in responses to genistein between neutered male and female cats. Genistein is a common component of commercial foods formulated for cats and may be found in amounts predicted to have biological effects.55 These findings indicate the importance of gonadal estrogen for the control of food intake in cats and suggest that the provision of an estrogenic compound could help prevent obesity in cats after gonadectomy.

Limited data are available for use of isoflavones for inducing weight loss in obese dogs and preventing weight gain in normal-weight dogs. Unpublished results of a study56 revealed that feeding restricted amounts of a low-calorie food supplemented with isoflavones (10% soy germ meal), 1.5% CLA, and L-carnitine to over-weight dogs resulted in greater loss of body fat and increased lean muscle mass, compared with results after feeding the low-calorie food alone or the low-calorie food with isoflavones. Weight gain in normal-weight dogs was significantly lower in dogs fed a low-calorie food with isoflavones for 12 months, compared with weight gain in dogs fed a low-calorie food alone.

In summary, results of controlled studies in colony-housed cats indicate that provision of an estrogenic compound could help prevent obesity after gonadectomy. Limited, unpublished data support the use of dietary isoflavones for preventing weight gain in obesity-prone dogs. Similar unpublished data indicate that dietary isoflavones may have beneficial effects in management of overweight dogs when used in conjunction with CLA and L-carnitine. Additional investigations are needed to evaluate the long-term effects of dietary isoflavones and phytoestrogens on obese pets and pet cats and dogs with diabetes mellitus.

Diacylglycerol supplementation—Diacylglycerol is a natural component of edible oils that has metabolic characteristics distinct from those of triacylglycerols or triglycerides.57 Diacylglycerol is composed of 2 fatty acids with a backbone that consists of a glycerol or fat molecule instead of 3 fatty acids found in a triglyceride. Most fatty acids in diacylglycerol are located on each end of the glycerol molecule, which appears to provide beneficial effects on fat and glucose metabolism in the body. Beneficial effects of dietary diacylglycerol may include altering fatty acid metabolism in the liver, lowering plasma triglyceride concentrations, decreasing postprandial hyperlipidemia, increasing energy expenditure, altering body composition, and improving weight loss.

Investigators examined the effects of long-term (16 weeks) ingestion of dietary diacylglycerol, compared with effects for ingestion of dietary triacylglyerol oil, on lipid metabolism in healthy nonobese men consuming a low-fat diet.58 Body weight decreased in both groups by the end of the 16-week period. However, decreases in total fat, visceral fat area, subcutaneous fat area, and hepatic fat content were significantly greater in the group receiving dietary diacylglycerol. In another study,59 investigators evaluated the effects of incorporating diacylglycerol oil or triacylglycerol oil into a reduced-energy diet for reducing body weight and fat mass in overweight or obese men and women. By the end of the 24-week study, there was a significantly larger decrease in body weight and fat mass for the diacylglycerol group.

Compared with effects for meals containing the more typical triacylglycerol, meals enriched in diacylglycerol (19% diacylglycerol oil) appear to decrease postprandial hypertriglyceridemia in clinically normal dogs.60 In 1 study,61 overweight sexually intact dogs were fed dry foods (17% fat) coated with 7% diacylglycerol oil or 7% triacylglycerol oil. Although caloric intake was not restricted, diacylglycerol-fed dogs lost 2.3% of body weight within 6 weeks, whereas triacylglycerolfed dogs maintained their obese body weights. In addition, the diacylglycerol group had a reduction in body fat content and serum triglyceride and total cholesterol concentrations. Investigators evaluated the addition of diacylglycerol oil (concentrations or amounts not reported) to a control food in groups of obese dogs fed at a slight caloric restriction (15% restriction as based on baseline caloric intake) to promote weight loss.62 In that unpublished study, inclusion of diacylglycerol oil resulted in slightly more loss of body weight and fat mass by the end of the 12-week feeding period, compared with results for dogs fed the control food without diacylglycerol.

In summary, preliminary studies suggest that foods containing diacylglycerol oil help promote weight loss and reductions in body fat and may be useful as an adjunct to dietary management for obese humans. Inclusion of diacylglycerol appears to alter postprandial lipid responses in clinically normal dogs. Two studies (1 published and 1 unpublished) involving the use of diacylglycerol oil in overweight dogs have been performed, but investigators in those studies did not evaluate effects of diacylglycerol-supplemented foods fed at restricted caloric intakes typically used for weight loss. Additional clinical studies are needed on use of diacylglycerol in dogs fed restricted amounts to induce weight loss. To our knowledge, no data are currently available on use of diacylglycerol in cats.

Chromium supplementation—Chromium is an essential trace element required for carbohydrate and lipid metabolism. Because the role of chromium in glucose and insulin metabolism has been reported repeatedly, it has been hypothesized that dietary chromium supplementation would have an effect on body composition and body weight.13 The role of chromium in the regulation of lean body mass, percentage body fat, and weight reduction is controversial because results of some studies do not support an effect of chromium on body composition, whereas other studies have revealed significant benefits of dietary chromium supplementation.63,64 However, many of the studies that involved dietary chromium supplementation were not conducted in obese subjects; instead, the chromium-supplemented diets were provided to growing animals or nonobese humans participating in resistance training.13,63 A meta-analysis was conducted of 10 double-masked, randomized, controlled studies that compared dietary supplementation with chromium and dietary supplementation with a control product during an intervention period of 6 to 14 weeks in obese humans.16 The results of that study suggest a relatively small increase in weight loss with chromium supplementation, compared with results for the control supplementation, but this change was not considered clinically meaningful. Thus, studies in humans and nonpet animals have not indicated a sustained or specific benefit of dietary chromium supplementation during weight loss programs in obese subjects.

Chromium supplementation has been evaluated in normal-weight and obese cats. Investigators evaluated the effects of dietary chromium supplementation on glucose and insulin metabolism in healthy nonobese cats.65 In that study, a small, dose-dependent improvement in glucose tolerance was detected in healthy nonobese cats fed a diet supplemented with chromium (300 or 600 μg/kg of diet). In another study,66 dietary chromium supplementation (100 μg/d) was found to be safe but did not affect glucose tolerance in obese or nonobese cats. Furthermore, dietary chromium supplementation (300 or 600 μg/kg of diet) during weight loss (all cats lost approx 23% of body weight) in obese ovariohysterectomized cats did not result in significant differences in body condition score, lean body mass, or fat mass.67,o

Chromium supplementation has also been evaluated in normal-weight and obese dogs. Investigators failed to detect clinically significant changes in glucose metabolism in nonobese female Beagles with dietary supplementation with chromium (300 or 600 μg/kg of diet).68 Obese female and male Beagles induced to lose weight via consumption of a high-fiber, low-calorie food supplemented with chromium (200 μg/kg of diet) had no improvements in weight loss or percentage body fat when compared with results for dogs consuming a similar weight-loss food without chromium.d Obese dogs in that study had significant alterations in glucose tolerance variables, compared with values when they were normal-weight dogs, but none of these variables were affected by chromium supplementation.e

In summary, the role of dietary chromium supplementation in regulation of body composition and weight reduction remains controversial, but a limited number of grade II studies in obese cats and dogs have failed to reveal clear benefits of dietary chromium supplementation in weight-management programs. Additional randomized, controlled studies are needed to indicate a benefit of dietary chromium supplementation in obese or overweight cats and dogs before routine use of chromium supplementation of diets or chromium supplementation of weight-management foods can be recommended.

Vitamin A supplementation—Leptin is a hormone secreted by adipose tissue that acts as a major regulator for food intake, energy expenditure, glucose transport, and lipogenesis.69 Serum leptin concentrations are highly correlated with adiposity in rodents, cats, dogs, and humans.70–75 As body fat increases, leptin secretion is stimulated. There also appears to be a strong relationship between circulating plasma leptin concentrations and insulin resistance in cats and dogs.76,77,p

Dietary vitamin A supplementation may be beneficial in obese animals by inhibiting expression of the leptin gene and thus returning the high serum leptin concentrations typical of obese subjects to the reference range or by promoting energy expenditure by increasing gene expression of the aforementioned mitochondrial uncoupling proteins.70,78 Initial studies with vitamin A supplementation were conducted in rats and revealed a decrease in serum leptin concentrations, a decrease in leptin mRNA concentrations, and an increase in gene expression of uncoupling proteins in rats consuming a vitamin A–supplemented food.79,80 These results suggest that dietary supplementation with vitamin A may contribute to an increase in energy expenditure and a decrease in adiposity.

Dietary vitamin A supplementation has been investigated in healthy cats and dogs during weight gain,70,q but to our knowledge, no studies have been conducted in obese animals undergoing weight loss. Cats were allowed ad libitum consumption of a highfat food with a control (7,800 U of vitamin A/kg of diet) or a high (121,500 U of vitamin A/kg of diet) concentration of vitamin A for 18 weeks.70,78,q At the end of the 18-week period, food intake was unchanged between the 2 groups. However, body weight expressed as the percentage change from that at study initiation increased by nearly 4% in the cats fed the control diet but remained the same in the cats fed the diet high in vitamin A. For the groups as a whole, mean adiposity (body fat) was unchanged over the 18 weeks, and there was no difference in mean body fat between groups. Dogs were allowed ad libitum access to a high-fat food with a control (10,100 U of vitamin A/kg of diet) or a high (140,400 U of vitamin A/kg of diet) concentration of vitamin A for 12 weeks.70,78 Food intake did not differ between the 2 groups, and body weight increased 23% to 25% in both groups. In addition, there were no statistical differences in body fat percentages between groups of dogs over the course of the study.

In summary, dietary supplementation with vitamin A appears to influence leptin synthesis and energy metabolism in several mammalian species. Results of preliminary grade II studies indicate that dietary vitamin A supplementation may help blunt increases in body weight in cats and dogs allowed ad libitum consumption of high-fat foods, but studies have not been conducted to clearly identify benefits of dietary vitamin A supplementation as part of weight-loss programs. Randomized, controlled clinical studies are needed to evaluate the role of dietary vitamin A supplementation in weight-loss programs.

Conclusions

The concepts of evidence-based medicine can be readily applied to management of overweight and obese pets (Table 2). Guidelines on quality of evidence, which have been published in the human and veterinary literature, serve as an excellent example of a rigorous application of an evidence-based appraisal system. By use of this system, grade I, II, and III evidence is the most reliable predictor of likely results in clinical practice for weight management of pets. Grade IV evidence provides substantially less robust support for recommendations. On the basis of this grading system, the best evidence exists for use of dietary L-carnitine supplementation in obese or overweight pets, although many of the results for use of L-carnitine are described only in abstracts, industry publications, and patent applications, which makes it difficult to review experimental methods and data analysis. More research-based evidence is needed to support routine use of other nutraceuticals or dietary supplementation, with compounds such as pyruvate, amylase inhibitors, safer forms of DHEA, CLA, diacylglycerol, chromium, and vitamin A, for weight loss in pets.

Table 2—

Summary of the quality of research evidence to support recommendations for the use of nutraceuticals and dietary supplementation in the management (weight loss) of obese pets.

Grade I evidence  
Grade II evidence  
Grade III evidence  
Grade IV evidence  

ABBREVIATIONS

CLA

Conjugated linoleic acid

DHEA

Dehydroepiandrosterone

a.

Weeth LP, Fascetti AJ, Kass PH, et al. Prevalence of cancer in obese dogs (abstr), in Proceedings. Univ Calif Davis Vet Med Teaching Hosp 28th Annu House Officer Semin Day 2006;9.

b.

Zhang P, Jackson JR, Roos M, et al. Evaluation of pyruvate supplementation on body weight and fat loss in overweight dogs (abstr), in Proceedings. Purina Nutr Forum 2003;101.

c.

Ishioka K, Sagawa M, Okumura M, et al. Treatment of obesity in dogs through increasing energy expenditure by mitochondrial uncoupling proteins (abstr). J Vet Intern Med 2004;18:431.

d.

Gross KL, Wedekind KJ, Kirk CA, et al. Effect of dietary carnitine or chromium on weight loss and body composition of obese dogs (abstr). J Anim Sci 1998;76(suppl):175.

e.

Gross KL, Wedekind KJ, Kirk CA, et al. Dietary chromium and carnitine supplementation does not affect glucose tolerance in obese dogs during weight loss (abstr). J Vet Intern Med 2000;14:345.

f.

Sunvold GD, Tetrick MA, Davenport GM, et al. Carnitine supplementation promotes weight loss and decreased adiposity in the canine (abstr), in Proceedings. 23rd World Small Anim Vet Assoc 1998;746.

g.

Sunvold GD, Vickers RJ, Kelley RL, et al. Effect of dietary carnitine during energy restriction in the canine (abstr). FASEB J 1999;13:A268.

h.

Allen TA, Jewell DE, Toll PW. The effect of carnitine supplementation on body composition of obese-prone dogs (abstr). In: Obesity: weight management in cats and dogs. Topeka, Kan: Hill's Pet Nutrition Inc, 1999;27.

i.

Jewell DE, Toll PW. The effect of carnitine supplementation on body composition of obese-prone cats (abstr). In: Obesity: weight management in cats and dogs. Topeka, Kan: Hill's Pet Nutrition Inc, 1999;29.

j.

Armstrong PJ, Hardie EM, Cullen JM, et al. L-carnitine reduces hepatic fat accumulation during rapid weight reduction in cats (abstr), in Proceedings. 10th Am Coll Vet Intern Med Forum 1992;810.

k.

Leray V, Dumon H, Martin L, et al. No effect of conjugated linoleic acid or Garcinia cambogia on body composition and energy expenditure in nonobese cats (abstr), in Proceedings. Waltham Int Nutr Sci Symp 2005;29.

l.

Anonymous. Conjugated linoleic acid (CLA) reduces body fat accretion (abstr). Nestlé Purina Res Report 2005;9:5.

m.

Bierer TL, Bui LM. High protein, low carbohydrate diets and not conjugated linoleic acid promote weight loss in overweight dogs (abstr). FASEB J 2004;18:A874.

n.

Schoenherr W, Jewell DE. Effect of conjugated linoleic acid on body composition of mature obese beagles (abstr). FASEB J 1999;13:A262.

o.

Sunvold GD, Bouchard G. Influence of supplemental dietary chromium on body composition during weight loss in cats (abstr), in Proceedings. Obesity Recent Adv Understanding Treat 1997;10.

p.

Gavet C, Siliart B, Shibata H, et al. Adiponectin and leptin plasma levels: early markers in the time course of obesity-associated insulin resistance (IR) in dogs (abstr). J Vet Intern Med 2004;18:421.

q.

Scarpace PJ, Sunvold GD, Bouchard GF. Vitamin A supplementation in cats abolishes the relationship between adiposity and leptin (abstr). FASEB J 2000;14:A215.

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    Ivy JL, Cortez MY, Chandler RM, et al. Effects of pyruvate on the metabolism and insulin resistance of obese Zucker rats. Am J Clin Nutr 1994;59:331337.

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    Kalman D, Colker CM, Stark R, et al. Effects of pyruvate supplementation on body composition and mood. Curr Ther Res 1998;59:793802.

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    Kalman D, Colker DM, Wilets I, et al. The effects of pyruvate supplementation on body composition in overweight individuals. Nutrition 1999;15:337340.

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