Medical Significance of Obesity

3. Medical Significance of Obesity

It has long been known that dietary restriction can increase longevity in a number of species including dogs (McCay et al., 1935; Kealy et al., 1992, 1997, 2000, 2002; Lane et al., 1998; Larson et al., 2003; Lawler et al., 2005). It is probable that a similar association is present for cats, although data to support such a supposition are lacking.

Whilst it is generally accepted that overweight and obesity increases the risk of suffering from a number of associated diseases in cats (Table 2) limited scientific peer-reviewed data are available to support these associations. There have been two large-scale studies assessing disease associations in overweight and obese cats. In a study by Donoghue and Scarlett (1998), the major associations recognized were diabetes mellitus, dermatoses, lameness and diarrhea (Figure 3).These authors also noticed that overweight cats have a shorter lifespan.

A subsequent study (Lund et al., 2005) assessed disease associations in a population of 8159 cats. For overweight and obesity, the major disease associations included oral cavity disease, urinary tract disease, diabetes mellitus, hepatic lipidosis, dermatopathy and neoplasia.

Figure3. Influence of feline obesity on the incidence of skin diseases, diabetes mellitus and locomotive disorders. (From Scarlett & Donoghue, 1998).  

Association between Excessive Weight, Insulin Resistance and Diabetes Mellitus

Insulin secreted by pancreatic β cells controls the uptake and use of glucose in peripheral tissues (see chapter 5).

Cats most often suffer from diabetes mellitus that resembles" type 2" diabetes mellitus in man (DM2), and, therefore, obesity is a major risk factor in this species (Nelson et al., 1990). Of all disease associations, diabetes mellitus is by far and away the most well-known. Indeed, epidemiological studies have confirmed an increased risk of diabetes mellitus in obese cats; in the study by Lund et al (2005), an odds ratio of 2.2 was reported for obese cats versus those in ideal body condition. This supports the work of older studies which have also reported an association (Panciera et al., 1990; Scarlett & Donoghue, 1998). Finally, it has been proven that diabetic cats have significantly reduced sensitivity to insulin than cats without DM2 (Feldhahn et al., 1999).

Dermatoses

Both the Scarlett and Donoghue (1998) and the Lund et al. (2005) studies suggested a link between obesity and dermatoses. Diseases represented included feline acne, alopecia, various forms of dermatitis, scale formation, and dermatophytosis. Diffuse scale is commonly observed, most likely due to reduced ability to groom efficiently.

Similar, one of the authors has observed numerous obese cats with fecal soiling; an association with grooming is suggested by the fact that such problems commonly resolve or improve after weight reduction. A single case report has also been published of perivulvular dermatitis associated with obesity; whilst the authors reported that episioplasty was required for resolution of this problem (Ranen & Zur, 2005); unfortunately, there was no mention of an attempt at weight management in this case. Finally, extreme obesity can lead to physical inactivity and to the development of pressure sores (Figure 4).

Figure 4. 9 year old neutered male Siamese cat with gross obesity (body weight 12.95 kg, condition score 5/5). The obesity had led to inactivity, inability to groom and pressure sores on the ventral abdomen. (© A. German).

Orthopedic Diseases

Similar to dogs, obesity may be a risk factor for orthopedic disease in cats, with one study suggesting that obese cats were five times more likely to limp than cats of normal body condition (Scarlett & Donoghue, 1998). However, not all reports have confirmed this association (Lund et al., 2005). Orthopedic pain may be a reason why obese cats are less likely to groom and hence suffer from dermatoses (see later).

One of the major hurdles with confirming such an association is the fact that the prevalence of orthopedic disease is likely to be under-recognized in this species, compared with dogs. This is likely to be due to differences in behavior between cats and dogs; dogs are commonly taken for walks on a regular basis, such that it will rapidly become evident to the owner if their pet is stiff or lame. In contrast, cats tend to be self-sufficient such that, if orthopedic disease is present, cats rest themselves and it may not be readily evident to the owner that there is a problem. A study examined the prevalence of feline osteoarthritis (OA), by examining radiographs taken to examine other regions (e.g., thorax) (Godfrey, 2005). In this study, there was radiographic evidence of OA in 22% of radiographs from adult cats. These results are particularly concerning in light of the fact that the population examined were not necessarily suspected to have orthopedic disease in the first place. In fact, a recent prospective study of OA in cats has identified the most prominent signs are reduction in the ability to jump and decreased height of jumping (Clarke and Bennett, 2006). This study also demonstrated that the elbow and coxofemoral joints are most commonly affected (Figure 5a and Figure 5b). In the experience of one of the authors, many cats limp at the time of presentation, and mobility improves markedly after weight loss. Thus, like dogs, weight reduction should be pursued in obese cats who limp.

Figure 5a. Hip dysplasia in a cat. Elbow and coxofemoral joints are commonly affected by osteoarthrosis in obese cats. (© Dr Eithne Comerford, University of Liverpool).

Figure 5b. Elbow osteoarthritis in a cat. Elbow and coxofemoral joints are commonly affected by osteoarthrosis in obese cats. (© Dr Eithne Comerford, University of Liverpool).

Gastrointestinal Disease

An association between gastrointestinal disease and feline obesity has been previously reported; Scarlett and Donoghue (1998) reported that obese cats were more likely to suffer from diarrhea than those in normal body condition. Lund et al (2005) reported gastrointestinal diseases included anal sac disease, inflammatory bowel disease, colitis, megacolon and constipation in overweight or obese cats. However, even if the link between constipation and body weight has been studied in human medicine (De Carvalho et al., 2006), the reasons for such a potential association are not clear and would require further study in cats. Very high fiber diets are suspected to increase the risk of constipation in cats.

Hepatic Lipidosis

The association between feline obesity and hepatic lipidosis is well-known. More information on hepatic lipidosis is presented in chapter 4. Concerns over inducing hepatic lipidosis are often cited as a reason why veterinarians are reluctant to instigate weight management in obese cats. However, it is not clear how real this concern actually is. In this respect, even marked dietary energy restriction (e.g., 25% (Biourge et al., 1994) or 45% (Watson et al., 1995) of maintenance energy requirements) did not lead to the development of hepatic lipidosis. Thus, it would appear that for hepatic lipidosis to develop, complete fasting for five to six weeks may be required (Biourge et al., 1993). Clinical hepatic lipidosis is probably associated with other inciting factors e.g., concurrent illness.

Neoplasia

A link between obesity and cancer has been widely reported and, if this link is entirely causal, one in seven cancer deaths in both men and women in the USA, might be the direct result of being overweight or obese (Calle & Thun, 2004). Similarly, studies in cats have reported an association with neoplasia (Lund et al., 2005); reported tumors included adenocarcinoma, basal cell carcinoma, fibrosarcoma, lipoma, lymphoma, mammary tumor, mast cell tumor and squamous cell carcinoma.

Whilst a global association with neoplasia may be present, the risk of developing specific neoplasia would require additional prospective studies. An association between mammary carcinoma and obesity has been reported in some (Sonnenschein et al., 1991), but not all (Perez Alenza et al., 2000a, 2000b), canine reports. Overweight dogs have also been reported to have an increased risk of developing transitional cell carcinoma of the bladder (Glickman et al., 1989), but such a risk has not been reported for cats.

Urinary Tract Diseases

The Lund et al., study (2005) identified that cats which were overweight were more likely to suffer from urinary tract diseases. Diseases reported included acute cystitis, urolithiasis, idiopathic feline lower urinary tract disease, urinary obstruction, and urinary tract infection. As with neoplasia, additional prospective studies are required to determine the exact risk for urinary tract. Of most note is the association with feline lower urinary tract diseases e.g., idiopathic FLUTD and urolithiasis. It is important to remember that obese cats are also most likely to live indoors, which is known as a risk factor for FLUTD.

The association between FLUTD and obesity may be a consequence of orthopedic problems: the painful cat may be reluctant to move and position itself for urination. The reduced frequency of urination can be a cause of lower urinary tract disease.

An association between obesity and diseases of the feline kidney is less clear and, currently, there have been no studies to demonstrate such a link in client-owned cats. However, circumstantial evidence for such a link exists given that there is evidence from dogs that the onset of obesity is associated with histologic changes in the kidney; reported changes include an increase in Bowman’s space (as a result of expansion of Bowman’s capsule), increased mesangial matrix, thickening of glomerular and tubular basement membranes, and increased number of dividing cells per glomerulus (Henegar et al., 2001). Functional changes were noted in the same study, including increases in plasma renin concentrations, insulin concentrations, mean arterial pressure, and plasma renal flow. As a consequence, the authors speculated that these changes, if prolonged, could predispose to more severe glomerular and renal injury.

Oral Cavity Disease

Obesity was shown to be a risk factor for oral cavity disease in a large scale study of cats in North America (Lund et al., 2005), with an odds ratio of 1.4. However, the reasons for such an association are not clear and, to the authors’ knowledge, have not been reported in other species. Further work would be required to determine why obesity, per se, is a predisposing factor.

Cardiorespiratory Issues

In many species, increased body weight can result in effects on cardiac rhythm, increased left ventricular volume, blood pressure and plasma volume. The effect of obesity on hypertension is controversial since studies have suggested that the effect is only minor (Bodey et al., 1996; Montoya et al., 2006). Whilst it could be hypothesised that similar effects may be seen in cats, there are no published scientific studies reporting such a link.

Risks During Anesthesia and Clinical Procedures

Overall, obesity makes clinical evaluation more difficult. Techniques that are more problematic in obese patients include physical examination, thoracic auscultation, palpation and aspiration of peripheral lymph nodes, abdominal palpation, blood sampling, cystocentesis, and diagnostic imaging (especially ultrasonography). Anesthetic risk is reportedly increased in obese dogs, most likely given recognized problems with estimation of anesthetic dose, catheter placement, and prolonged operating time (Clutton, 1988; Van Goethem et al., 2003). Although no published data exist, it is likely that similar problems exist in cats. Finally, decreased heat tolerance and stamina have also been reported in obese animals (Burkholder & Toll, 2000).