Fat as an Endocrine Organ
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.
Prevalence of Obesity in Dogs and Cats
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.
Risk Factors for Obesity in Dogs and Cats
• 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.