Epidemiology of Obesity

2. Epidemiology of Obesity

Prevalence and Time Trends

Obesity is an escalating global problem in humans (Kopelman, 2000), and current estimates suggest that almost two thirds of adults in the United States are overweight or obese (Flegal et al., 2002). Prevalence studies of companion animal obesity are more limited; reports from various parts of the world, have estimated the prevalence of obesity in the dog population to be between 22% and 50% (McGreevy et al., 2005, Colliard et al., 2006, Holmes et al., 2007). In cats, information is limited to a handful of studies over a time-frame of over thirty years, using a variety of definitions of overweight/obesity and techniques to estimate body condition (Sloth, 1992; Robertson, 1999; Russell et al., 2000; Harper, 2001; Lund et al., 2005). From this work, estimates of obesity prevalence range from 19 to 52% (Table 1).

One of the most recent studies was from the USA, and utilized 1995 records of the National Companion Animal Study (Lund et al., 2005). The results suggested that approximately 35% of adult cats were classed as either overweight or obese (overweight 28.7%; obese 6.4%). However, prevalence of overweight and obesity varies amongst age groups with middle age cats (between 5 and 11 years of age) particularly at risk (overall prevalence 41%; overweight 33.3%, obese 7.7%). Of particular concern was the finding that a clinical diagnosis of obesity was only recorded in 2.2% of cats (despite the BCS findings), suggesting that veterinarians do not consider the condition to be of clinical significance.

Whatever the true figure for feline obesity, it is clear that the condition is one of the most important medical diseases seen by veterinarians, especially for middle-aged adults. Furthermore, studies have reported that owners tend to under-estimate the body condition of their cats, compared with the estimates of their veterinarians (Kienzle & Bergler, 2006); and these individuals may not be presented for assessment.

Risk Factors for Feline Obesity

Obesity prevalence is influenced by numerous factors. Individual factors that have been identified include gender and neuter status, age, and breed; environmental factors include accommodation, presence of dogs in the household, and the feeding of certain types of diet; additionally, some factors may be the combination of both individual and environmental influences e.g. inactivity. Other studies have implicated owner factors and feeding behavior as risk factors for overweight and obesity in cats (Kienzle & Bergler, 2006).

Figure 1. Prevalence of feline obesity according to age. (From Scarlett, 1994 and Robertson, 1999; study on 2671 cats).

Age

Middle age is a particular risk factor for overweight and obesity in cats (Figure 1). One study identified that body condition was significantly higher in cats <13 years of age compared with those >13 years (Russell et al., 2000). In another North American study, the prevalence for overweight and obesity was greatest in cats between 5 and 11 years of age (Lund et al., 2005). Such data are critical to veterinarians since they help to identify the population most at risk, and would suggest that prevention strategies, if instigated early (e.g. ~2 years of age) might have the greatest impact on satisfactory management of the condition.

Neuter Status and Gender

Neutering is the major cause of obesity in cats with numerous studies confirming the link (Scarlett et al., 1998; Robertson, 1999; Allan et al., 2000; Russell et al., 2000; Lund et al., 2005; Martin et al., 2001; 2006a). Metabolic consequences of neutering will be developed in the pathophysiology section.

Gender itself is also a predisposing factor in some but not all studies, with males over-represented in recent work (Lund et al., 2005). The reasons for such a gender association have not been fully elucidated, not least given that one study has shown that fasting metabolic rate declines in female but not male cats that were neutered (Fettman et al., 1997).

Endocrine Abnormalities

Compared with dogs, obesity in cats is less likely to result from endocrine abnormalities such as hypothyroidism and hyperadrenocorticism. However, the use of progesterones for contraceptive management has been associated with the development of obesity.

In cats, obesity is most of the time associated with increased plasma concentration of prolactin, leptin and insulin-like growth factor (IGF)-1 (Martin et al., 2006a). The hormonal profile is thus completely different from that of the obese dog (Martin et al., 2006b). All these hormones have a direct role in the onset of insulin-resistance (Melloul et al., 2002).

Breed

A handful of studies have examined the influence of breed on the prevalence of feline obesity. Two studies identified that "crossbred" or mixed-breed cats were approximately twice as likely to be overweight as purebred cats (Scarlett et al., 1994; Robertson, 1999). Lund et al. (2005) also found mixed breed cats (domestic shorthair, domestic medium hair, domestic longhair) to be at greater risk; Manx cats were also predisposed.

Environment

Environmental factors reported to influence the prevalence of obesity include the type of accommodation, the number of cats and or the presence of dogs in the household (Scarlett et al., 1994; Robertson, 1999; Allan et al., 2000). With regard to accommodation, both indoor dwelling and living in an apartment have been shown to predispose in some (Scarlett et al., 1994; Robertson, 1999) but not all (Russell et al., 2000) studies, probably because this type of environment does not respect the normal ethogram of the cat. It is likely that inability to maximally exercise and boredom may play a part.

One study demonstrated that the presence of dogs in the household significantly reduced the odds of developing obesity (Allan et al., 2000), possibly due to the behavioral traits of the cats or dogs.

Finally, the type of owner that own exclusively cats might differ from those who own both cats and dogs; in this respect, people who own both might be less inclined to dote upon their cats and less likely to provide premium foods for their pets (see below).

Activity

Activity is one of the factors influenced both by individual and environmental variables. It is possible that the principle influence of type of accommodation relates to ability to exercise outdoors. Numerous studies have identified inactivity as a major risk factor for both overweight and obesity (Scarlett et al., 1994; Allan et al., 2000), although not all studies have confirmed this finding (Russell et al., 2000).

A cat may eat less when it lives with a dog (© Labat):

- It might be intimidated by the presence of a dog, reducing its drive to eat
- A dog may drive the cat away from the food bowl
- The cat can be stimulated by the dog to play. Its physical activity is thus overall more important than if it only lives alone.

Dietary Factors

Some studies have suggested that feeding premium pet foods (Scarlett et al., 1994) conveys an increased risk compared with the risk whilst feeding a grocery store diet. The increased palatability may overcome normal appetite control leading to overeating, but in the 1990"s, the main reason proposed for such an association was that premium food tended to have a higher fat, and hence energy, content than grocery products; today, many moderate fat-diets (10 - 14% fat on dry matter basis [DMB]) are available.

Kienzle and Bergler (2006) conducted a study of owner attitudes and compared cats that were overweight with those that were normal weight or thin. The owners of overweight cats tended to offer food on a free-choice basis, but there was no difference in the type of food fed.

Many veterinarians typically feed cats with high-fat diets specifically formulated to prevent FLUTD. These high-fat calorically dense diets are a frequent cause of obesity.

Owner Factors and Behavior

Some studies have indicated a number of owner factors in the development of obesity (Kienzle & Bergler, 2006) and it is interesting to make comparisons with dogs. For instance, the owners of obese cats tend to "humanize" their cat more, and cats have a potential role as a substitute for human companionship. Over-humanization was also associated with overweight in a dog study, but a close human-dog relationship was not (Kienzle et al., 1998). The owners of overweight cats spend less time playing with their pet and tend to use food as a reward rather than extra play. Further, the owners of overweight cats watch their cats during eating more often than owners of cats in normal body condition; this is similar to findings for dog owners. The owners of both overweight cats and dogs have less of an interest in preventive health than those of pets in ideal body condition. Unlike the owners of overweight dogs who tend to have a lower income, there are no demographic differences amongst owners of overweight and normal weight cats. Finally, the percentage of female owners is higher in overweight than in normal weight cats.

For cats, possible factors involved in the development of obesity include anxiety, depression, failure to establish a normal feeding behavior, and failure to develop control of satiety. (©C. Chataignier (Européen)).

Further, many owners misread signals about the behavior of their cat with regard to eating. It is important to remember that:

• Cats in the wild are designed to be trickle feeders and would typically consume numerous small meals. Despite this, many owners choose to feed their cats in 2 - 3 large meals per day.
• In contrast to humans and dogs, cats do not have any inherent need for social interaction during feeding times. When the cat initiates contact, owners often assume that they are hungry and are asking for food when they are not. Nevertheless, if food is provided at such times, the cat soon learns that initiating contact results in a food reward. If larger amounts or energy-dense foods are offered, it has the potential of leading to excessive food intake and obesity;
• Play is necessary throughout life (Figure 2). Dog owners are usually conditioned to provide regular exercise through both walks and play; in contrast, most cat owners do not engage in play sessions with their pets.

Figure 2. A cat’s life: ethogram*.