In developed countries, obesity is a major problem in pet cats and can be a risk factor for other diseases such as diabetes mellitus, urinary tract diseases, respiratory diseases, and degenerative joint disease. This prospective study, a large-scale longitudinal study of pet cats in the United Kingdom, was designed to identify early-life risk factors for feline overweight or obesity at the approximate age of one year. In Great Britain, the prevalence of overweight or obese body condition in pet cats was estimated to be 6-12.5% and 39-52% in the 1970s and in the 1990s and 2000s, respectively.
When a pet animal is 10-19% above ideal weight, it is considered overweight, and when it is 20% or greater than ideal weight, it is obese. Previous studies, all cross-sectional, wherein reverse causality cannot be ruled out, have identified a wide variety of factors which may increase the risk of cats becoming overweight or obese: male sex, neutered status, mixed breed/non-pedigree, indoor confinement, inactivity, middle age, free-choice feeding, longer leg length, residence in single- or two-cat households, living in rural or semi-rural areas, owner underestimation of the cat's body condition, a close owner-cat relationship. A previous small longitudinal study involving 80 colony cats rather than pet cats found that a higher percentage change in body weight between 3 and 12 months of age as well as male sex, significantly increased a cat's risk for becoming obese in adulthood.
Cats 2 to 4 months of age owned by people aged 18 years and over were eligible for inclusion in the study, called the 'Bristol Cats' study; 2189 animals were enrolled between 2010 and 2013. The owners provided data by responding to questionnaires when their cats reached specific ages (questionnaire 1, 2-4 month old cats; questionnaire 2, 6.5-7 month old cats; questionnaire 3, 12.5-13 month old cats). The questionnaire provided to owners when their cat was in group 3 included a 5-point body condition score (BCS) system, with pictures as well as written descriptions corresponding to each BCS on the scale. Once owners had assigned a BCS to their cat, the scores were consolidated into a dichotomous variable for analysis: overweight/obese (BCS 4-5) and non-overweight (BCS 1-3). The BCS so identified was used as the dependent variable in the analyses. Ultimately, only 788 of the cats were included in the multivariable analysis, as these animals had no missing data on their questionnaires for any of the variables eligible for inclusion in this analysis.
When the diet data reported in the third and final questionnaire for the age cohort of 12.5-13 months of age was analyzed using univariable analysis, there was a significant difference in the probability of being overweight or obese between cats eating a dry diet exclusively compared to those eating mixed, wet, or fresh (raw or cooked meat or fish) diets. Of the 768 cats with no missing data included in the multivariable analysis, 52 (6.8%) were overweight/obese. The relatively low prevalence of overweight/obese body condition in the study cats may be attributable to the use of owner-reported BCSs, as well as the likelihood that owners voluntarily participating in such a study may be more attentive to their cat's BCS than the general cat owner population.
The variables considered potential risk factors for feline overweight/obesity analyzed in the multivariable analysis included breed, spay/neuter status, outdoor access, type of diet fed, frequency of wet meals fed, frequency of dry food fed, frequency of treats fed, and exact age at the time of questionnaire 3 completion. Results of the multivariable analysis demonstrated that no or restricted outdoor access and a dry diet at the time of completion of questionnaire 3 were both independently associated with an increased risk of overweight/obesity at 12.5-13 months of age. A second model using the same variables measured at an earlier age (time of questionnaire 2 completion) was also developed; in this model the relationship is weaker, but the same direction of effects is present in that a dry diet and no outdoor access both independently predict an overweight/obese body condition at 12.5-13 months of age. Other variables evaluated in this study were not found to be significant in terms of predicting an overweight/obese BCS at 12.5-13 months of age.
Based on the results of this study, the first prospective study to utilize data from a large-scale longitudinal study of owned pet cats, these researchers recommend that extra care should be paid to diet by owners of indoor only cats. While outdoor access is not necessarily recommended due to the increased risks to life and limb associated with this lifestyle, indoor environmental enrichment (or possibly a secure outdoor enclosure) should be provided to encourage activity, which increases energy utilization and decreases boredom and increased attention to food. Dry food typically contains approximately 4 times the number of calories as wet food, so overfeeding even a small amount of dry food may lead to a large increase in calorie intake. The package feeding guidelines on many dry foods often suggest feeding a larger portion of food than is appropriate for the age, neuter status, and activity level of the cat. Also, recommended portions of dry food may appear inappropriately small to owners. In other studies cited by the authors, overweight cats were less likely to be fed grocery store dry diets and more likely to be fed prescription diets, which are likely to be both more nutrient-dense and more palatable than grocery store foods. Weighing the portion of dry food rather than using a measuring cup, which was found to be an inaccurate estimate of portion size in another study, may be the best method of insuring the animal is not overfed.
In the current study, owners were not asked if they fed wet or dry food in meals or if the food was made continuously available. The univariable analysis demonstrated that cats fed wet food 2 to 3 times a day were at a reduced risk of developing obesity, but this variable did not remain significant in the multivariable analysis. Nonetheless, a wet food diet may be protective against obesity due to the high water content and low calorie density of this food type. Parallel to studies in human beings, the authors found that the cat's early life environment is important with respect to the risk of obesity development in early adulthood. Owners of kittens and adolescent cats with limited or no outdoor access and/or a principally or entirely dry diet are advised to pay careful attention to the amount of food provided, offer opportunities for exercise, and monitor their pet's BCS.
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