The past decade has seen a revolution in our understanding of adipose tissue. The functions of fat have traditionally been understood as energy storage, thermal insulation, and structural support for some organs. It is now known that adipose tissue is metabolically active and constitutes the largest endocrine organ in the body with unlimited growth potential at any stage of life.
The past decade has seen a revolution in our understanding of adipose tissue. The functions of fat have traditionally been understood as energy storage, thermal insulation, and structural support for some organs. It is now known that adipose tissue is metabolically active and constitutes the largest endocrine organ in the body with unlimited growth potential at any stage of life. Recognizing that adipose tissue is not inert has helped us understand the complex relationship between obesity and some of the diseases associated with obesity in humans (i.e., heart disease, diabetes and chronic degenerative joint disease).
The relationship of obesity to other types of diseases, such as type 2 diabetes mellitus (T2DM), is not easily understood. The link is a group of proteins, collectively called adipokines, which are secreted by adipose tissue and adipose-resident macrophages and fibrocytes. Adipokines exert their effects in the central nervous system and peripherally, in tissues such as skeletal muscle and the liver. Leptin, adiponectin, resistin, visfatin, retinol-binding protein and tumor necrosis factor-alpha (TNFα) are some of the main adipokines of interest. Enzymes such as lipoprotein lipase are also abundantly produced and released from adipose tissue. Finally, many pro-inflammatory cytokines and acute-phase proteins originate in adipocytes.
Of the adiopkines, leptin has received the most attention. In 1995, leptin was identified as the fat cell-specific secretory factor that mediates the hormonal axis between fat and the brain. Leptin concentrations increase with increased body fat in all species studied including dogs and cats. Adequate energy stores are signaled by leptin and permit reproduction and normal immune function. Leptin also functions to reduce appetite. Despite high hopes that leptin would be the long-sought "lipostat", it is now known that leptin resistance develops with increasing obesity. The ability of low leptin levels to stimulate appetite is greater than the ability of high leptin levels to suppress appetite. Leptin, however, may provide a link between osteoarthritis and obesity. In humans, increased leptin in synovial fluid has been seen in patients with either rheumatoid arthritis or osteoarthritis.
The number of pets that are overweight or obese has reached epidemic proportions in the USA and other industrialized countries. There are various reports as to how common obesity is, but it has been shown that just over 35% of adult cats in the USA were overweight or obese. In addition, 45% of the cats aged 10-11 were considered overweight or obese. Studies investigating the prevalence of overweight/obesity in dogs have varied from 24% to 34%.
Obesity prevention in pets needs increased emphasis with focus on wellness plans through owner education. Significant health benefits to maintaining a normal to lean body weight have been shown in dogs and other species. The veterinary visit for spaying/neutering is an important, but often neglected, opportunity to reassess diet type and feeding management and make appropriate awareness of obesity issues to clients.
• Dogs - Specific breeds are more likely to become overweight. These include but are not limited to Shetland Sheepdogs, Golden Retrievers, Dachshunds, Cocker Spaniels, Labrador Retrievers, Dalmatians, Rottweilers, and Mixed Breeds
• Cats -Mixed breed (DSH, DLH, DMH) and Manx cats were found more likely to be obese than most purebred cats.
2. Gender/neuter status-
• Dogs - Neutered female dogs are about twice as likely to be overweight than are intact female dogs. Similar trends have been seen in castrated male dogs.
• Cats - Male cats are predisposed to being overweight. Neutering further increases the risk of obesity by decreasing the metabolic rate by at least 25%. Removal of estrogens may also increase food consumption independent of the decreased metabolic rate. This may also be accompanied by an increased appetite following surgery.
3. Age- Risk increases with increasing age in both dogs and cats
4. Activity- Reduced activity increases risk for weight gain in both dogs and cats.
5. Food and feeding- Highly palatable foods, free choice feeding and excessive treats. In particular, feeding high fat foods is associated with obesity.
6. Other associations – In cats, other factors such as apartment dwelling, presumably due to decreased exercise opportunities - this is "softer" data, but seems to be a commonly observed association.
Studies investigating overweight dogs and cats have identified many of the same health problems observed in humans. In cats, T2DM, neoplasia, dental disease, dermatologic diseases, and lower urinary tract problems have been associated with obesity. In dogs, overweight or obesity has been linked with diabetes, pancreatitis, cruciate ligament rupture, hypothyroidism, hyperadrenocorticism, lower urinary tract disease, oral disease, neoplasia dyslipidemia, osteoarthritis, hypertension, altered kidney function. In addition, although harder to measure, obesity exacerbates existing musculoskeletal problems, respiratory distress from upper airway obstruction, and pregnancy complications, and is associated with delayed wound healing, increased anesthetic/surgical risk, and probably reduced life expectancy. Obesity also makes tasks like collecting blood samples and placing intravenous catheters much more difficult.
Preventing or treating obesity may delay and or prevent many of these obesity-related diseases. A successful weight loss program requires a reduction in caloric intake (owner compliance) and an increase in physical activity. Weight loss studies in dogs have found positive associations on biomarkers associated with obesity-related diseases. Weight loss in dogs has been associated with a reduction in triglycerides, cholesterol, thyroxine and leptin. In addition, weight loss in dogs lead to an increase in insulin sensitivity and lowering of adipokines linked with insulin resistance (such as tumor necrosis factor alpha and insulin like growth factor-1).
As indicated earlier, the relationship of obesity to other diseases is complicated and only recently is being recognized as a key factor affecting overall health. New research tools such as genomics have enabled scientists to shed some light on the underlying mechanisms which link obesity with other diseases. By performing microarray analysis on lean and obese adipose tissue and lymphocyte samples, scientists have begun to understand the processes behind obesity and, importantly, how obesity links to other diseases.
When comparing lean vs obese adipose tissue. The gene expression of obese adipocytes showed a down regulation of PPAR-gamma, uncoupling protein-2, carnitine O-palmitoyltransferase 1 A and acyl-CoA synthetase. When functioning properly these genes are important in the beta-oxidation of fatty acids. The down-regulation of these genes may explain why obese animals are fat storing instead of fat burning. In addition, the obese adipose tissue also had down-regulation of genes associated with glucose metabolism. Down-regulation of pyruvate dehydrogenase kinase-4 and glucose-6-phosphatase may be a potential link between diabetes and obesity.
Many of the same pathways altered in the obese adipose tissue have also found to be altered in lymphocytes from obese dogs. Microarray analysis of lymphocytes revealed that overweight dogs had decreased carbohydrate metabolism, interleukin signaling, PPAR signaling, IGF-1 signaling, insulin receptor signaling, amino acid metabolism, branch chain amino acid degradation and lipid metabolism compared to results of similar analyses in lean dogs These observations of both adipocytes and lymphocytes may explain why obese animals become insulin resistant and have increased circulating glucose, insulin, IGF-1 and inflammation.
Recent studies Dr. Yamka at Hill's Pet Products have investigated the effects of weight loss on the gene expression profiles of obese dogs. In one study, dogs (> 35% % body fat by DEXA) were fed a dry low-fat, fiber-enhanced therapeutic food (33.2% crude protein, 8.7% crude fat and 26.7% total dietary fiber on dry matter basis) for a period of 4 months. On average, dogs lost 2.8 ± 0.8 kg body fat (41.2% of initial fat mass) in 4 months. The nutrigenomic effect of the food can be seen in the shift from an obese to a lean gene expression profile. Of the genes identified, there was a down regulation of genes associated with fat accumulation (i.e leptin and IGF-1) once the dogs lost weight. This data suggest that obese dogs fed the weight loss food had a shift in metabolism to a lean genomic profile. However, weight loss alone does not alter gene expression. Changes in gene expression occur as a result of foods with specific nutrient profiles and weight loss working together.
The effect of weight loss on the gene expression profiles of obese cats has also been examined. In one study, obese cats (> 30% % body fat by DEXA) were fed Hill's Prescription Diet® feline r/d® dry for a period of 4 months. On average, cats lost 0.61 ± 0.13 kg body fat (30.7% of initial fat mass) in 4 months. The nutrigenomic effect of the food was associated with the down regulation of genes associated with inflammation, obesity and T2DM. These data suggest that weight loss can correct the systemic effects obesity.
In summary, obesity is not just a weight issue. The biochemical changes occurring with obesity result in an increased susceptibility to other diseases. Genomics may provide valuable insights into the underlying mechanisms which link obesity with other diseases. The biochemical changes which occur in the adipocyte influence the adipokines released which ultimately will affect the body systemically. These changes can be quantitated by studying the genomics of lymphocytes and adipose tissue, and by measuring circulating adipokines. Weight loss appears to reverse many of the changes that occur with obesity.
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