The prevalence of overweight in adult cats visiting an academic medical center in our study was high, with almost every second cat considered overweight when body condition was assessed by a veterinarian or veterinary student. In the second cohort, where owners estimated their pets’ body condition, the prevalence of overweight was lower, with one in five cats considered overweight. The prevalence of overweight found in this study was in the same range as in previous reports [8,9,10,11]. Different scales were used for scoring, a nine-grade for the medical records cohort and a five-grade scale for the questionnaire cohort, but because all cats were grouped into only two groups for the analyses, overweight versus not overweight, the influence of using different scales would in our opinion be slight. The divergence between the cohorts can be explained by the actual differences present between the cohorts. There were differences in breed composition, mean age, number of male versus female cats, and neutering status present between cohorts, but the larger proportion of domestic, neutered and older cats in the questionnaire cohort would implicate a higher prevalence of overweight in this group, which was in contrast to what was found. It has previously been described that there is a tendency of owners to underestimate the BCS of their pet, which probably contributes to the lower prevalence seen in the questionnaire cohort [12, 31, 32]. Courcier et al.  found that owner misperception was more likely when owners rated cats with BCS 1 (very thin) and 4 (overweight) on a five-grade scale, and in longhaired cats. Moreover, it has been shown that owner underestimation of the cat’s BCS is in itself a risk factor for obesity [10, 12, 32]. Because owners tend to underestimate their pet’s BCS, we believe that cats judged as overweight in the questionnaire are in fact truly so, strengthening the associations found with overweight in the study. All cats visiting the University Animal Hospital are supposed to have an assessment of their BCS, but in reality only one in five cats was scored, and there is probably also a selection of which cats are actually selected for scoring, which are limitations of the study. It is unknown whether there is a tendency to more often score obese cats, or thin cats. It is possible that healthy cats undergoing routine prophylactic procedures such as vaccinations, and critically ill cats, are not being scored to the same extent as other cats. How this affects our results is unknown. Moreover, cats visiting an animal hospital might not be representative of the general population.
Several diagnostic code groups were associated with overweight. Scarlett and Donoghue reported associations between obesity and lameness, DM, and non-allergic skin disorders , similar to the findings in our study, and Lund et al. found associations between obesity and urinary tract disorders , also found in our study. Excess weight affects joints mechanically and can lead to OA, but it has also been shown in people that arthrosis is a hormonally mediated disease associated with obesity . The association between DM and overweight in cats is well-known and supported by our findings [1, 4, 7]. Scarlett and Donoghue found a fourfold increased risk of DM in obese cats . Lower urinary tract disease such as urethral obstruction is commonly seen in overweight, neutered, middle-aged male cats , similar to our findings. It is not clear from our cross-sectional study design if overweight predisposes cats to certain diseases, or if being overweight is a consequence of disease. It is possible that if overweight predisposes cats to disease, more overweight cats will be encountered at an animal hospital than in the general population.
We found that cats eating predominantly dry food were more often overweight than cats eating predominantly wet food, in a cohort consisting of mature and mainly neutered cats. An association between dry food and overweight in adult cats has to our knowledge not been reported before, although recent studies on younger cats showed feeding a dry diet to be a risk factor for overweight [14, 15]. Rowe et al.  showed that cats fed dry food as the only or major part of their diet at about 2 years of age, were twice as likely to be obese compared to those fed a wet or a mixed diet. Because dry food is fed to a vast number of cats worldwide, this finding warrants further investigation. Our group has previously shown an association between dry food and an increased risk of DM in cats assessed as normal weight by their owners . Many studies have investigated associations between food type and risk of overweight, but only associations with therapeutic and premium dry diets have been found, a finding likely to be a confounder because weight loss diets are often prescribed to overweight patients [6, 9, 10, 31, 34,35,36]. However, the change from a low-carbohydrate diet in feral cats consuming wild prey to a typical high-carbohydrate diet fed to many cats today, has been suggested to be partially responsible for the recent increase in obesity and DM seen in domestic cats . In 1963, Joshua  stated that a diet change was not necessary for cats diagnosed with DM because they were already on a very low carbohydrate intake anyway, which reflects the difference in how cats were fed then compared to now, with many cats fed a high-carbohydrate diet such as dry food. High-carbohydrate diets have been shown to lead to higher insulin blood concentrations than high-protein and high-fat diets . The exact macronutrient content in food given to cats in our study is unknown, although a typical commercial dry diet generally contains more carbohydrates than a typical wet diet . Dry food is typically more energy-dense than wet food, which contains more water, which also may contribute to our findings . Cats have been shown to decrease their voluntary energy intake when fed a canned diet ad lib compared with a freeze-dried version of the canned diet, indicating that the bulk water might promote weight loss in cats . Because it is not possible to alter one macronutrient without altering another, the difference in BCS between cats fed dry food and cats fed wet food might relate to a protein effect rather than a carbohydrate effect. Studies have shown that increased dietary protein promotes fat loss and reduces loss of lean body mass during weight loss in cats . It has also been shown that high-protein diets can increase energy expenditure, as protein can induce a higher thermic effect than the other macronutrients . Cats are obligate carnivores, whose natural diet consists mainly of protein-rich animal prey . Moreover, cats lack several enzymes involved in carbohydrate metabolism, such as salivary amylase, and have low activities of intestinal amylase and disaccharidases, indicating that they are not adapted to using carbohydrates as a primary energy source, although they can still digest and utilize cooked starch [45, 46].
There was no association between feeding regimen and overweight in our study, in contrast to some previous studies showing conflicting results, with both ad libitum feeding and being fed twice daily as risk factors for obesity [31, 36]. Being defined as a greedy eater, however, was associated with an increased risk of being overweight. Being greedy was an independent risk factor also for DM in a previous study from our group . In people, eating slowly is associated with a lower caloric intake and enhanced satiety , but this has to our knowledge not been evaluated in cats.
Activity level was associated with overweight in our study, with inactive cats at higher risk. On the other hand, we could not detect an association between indoor confinement and access to the outdoors and overweight. Some studies have shown indoor confinement to be a risk factor for obesity [9, 15, 34], whereas others have failed to show such an association [12, 31, 36]. Inactivity has been reported as a risk factor for obesity, but others have reported no associations between activity and obesity [9, 10]. According to the present study, it is the activity in itself that is important, not whether it is performed outdoors or indoors. However, in a previous study from our group, investigating risk factors for DM in cats, the opposite was found, with indoor confinement being a more important risk factor for disease than the activity levels . It should be noted that the measurement of activity level is subjectively made, and performed by the owners. Future studies investigating the effect of activity on body weight will benefit from using objective measurements of the cats’ activity levels.
Male sex was associated with an increased risk of overweight in both cohorts, similar to what has previously been shown [6, 11, 12, 18, 48]. Male cats have been shown to gain weight more easily than female cats . Neutering was a risk factor found only in the medical records cohort, because almost all cats in the questionnaire cohort were neutered, making comparisons between neutering statuses unfeasible. Neutering can increase daily food intake, decrease the metabolic rate, and cause activity levels to drop, thereby predisposing neutered cats to obesity [48,49,50,51,52]. Caloric restriction is generally required after neutering, and a failure to adjust food supply to meet the lower energy requirements can easily lead to obesity in the neutered cat . Both male sex and neutering have been identified as risk factors also for DM [5, 53]. It is not clear whether the neutering itself causes insulin resistance or whether it indirectly influences the risk of DM by increasing the risk of obesity.
Geriatric cats were less likely to be overweight compared with mature cats in both cohorts, in concordance with previous studies [6, 11, 13, 34, 36]. Sarcopenia is a natural age-related change that likely contributes to this finding, as well as the presence of concurrent diseases causing weight loss more commonly seen in older cats, such as chronic kidney disease, hyperthyroidism, and dental problems . In the medical records cohort, the junior and prime age groups showed less overweight compared with mature cats, but in the questionnaire cohort, the lack of cats younger than 5 years excluded comparisons. It is interesting that the age incidence of DM in cats closely follows the age incidence of overweight, again stressing the importance of obesity as a major risk factor for DM .
Birman and Persian cat breeds showed a decreased risk of overweight in our study. We did not identify any particular cat breed at an increased risk of being overweight, but when comparing purebred and domestic cats in the questionnaire cohort, the domestic cats were more often overweight, similar to what has been shown previously [6, 12, 13]. There is also an increased risk for DM in domestic cats compared with purebred cats . The association between breed and overweight likely to some extent reflects the genetic aspect of the condition. Haring et al. have recently shown that a genetic component is responsible for the development of overweight in cats . In the questionnaire cohort, with owners estimating their pet’s body condition, the Norwegian cat breed showed a decreased risk of obesity, differing from previous studies . It also differed from our expectations, because the Norwegian forest cat has been shown to have a breed predisposition to DM, and a propensity for obesity was anticipated . The Norwegian forest cat is according to the breed standard a large cat breed, which might lead owners to underestimate their pet’s body condition. It should be noted that in the medical record cohort, when BCS was assessed by a veterinarian or veterinary student, the Norwegian forest cats did not differ in body condition compared with other pedigree cats.
Limitations of our study are mainly related to the study design, particularly the problems with recall bias, because some of the answers in the questionnaire referred to several years back in time, and also the difficulties for owners to accurately assess their cats’ BCS which can be a significant confounder. The multiple assessors of the BCS is also a drawback of the study. Moreover, the potential selection bias may also affect the estimated associations of overweight with risk factors and diseases. On the other hand, associations between overweight and demographic factors including age, breed and sex, were similar between cohorts, strengthening the results.