Studies evaluating laboratory changes in different stages of feline obesity are scanty. We observed increased TAC by four different methods, with no alteration in TOC and lipid peroxidation in obese cats. Moreover, changes in CBC such as increased MCV and RDW and in biochemical parameters such as increased triglycerides and HDL cholesterol levels, as well as decreased GGT activity, were also observed in different stages of obesity. In this regard, the current study sheds greater light on the pathophysiological mechanisms of feline obesity.
The average age of obese cats in this study was 6 years. Several authors have linked obesity to increasing age (Burkholder and Toll 1998; Colliard et al. 2009; Courcier et al. 2010). Laflamme (2012), Oh (2011) and Diez and Nguyen (2008) state that cats between 5 and 10 years old are more prone to obesity, and that this risk increases greatly from the age of 10 years onwards. Thus, we show that middle-aged cats start out overweight and become obese.
All the animals of the obese group were neutered and this group was predominantly composed of males. Previous studies have demonstrated that neutered male cats are more prone to obesity (Diez and Nguyen 2008; Cave et al. 2012; Rowe et al. 2015). Male cats are approximately 13 times more likely to develop obesity than females, and if neutered, that chance increases to 15 times (Robertson 1999; Russell et al. 2000; Lund et al. 2005). In dogs, obesity is usually more common among neutered females, as males have a higher resting metabolic rate than females (Burkholder and Toll 1998; German 2006; Diez and Nguyen 2008; Kil and Swanson 2010; Courcier et al. 2010).
Obese cats showed increased MCV and RDW, showing not only an increase in the size of red blood cells but also greater variation in cell size. The lifespan of erythrocytes in feline species is shorter than in other species (Christian 2010; Tasker 2012; Korman et al. 2013; Lalor et al. 2014); hence, we hypothesize that the removal of senescent erythrocytes in obese cats is even faster. This means there is an increase in the replacement of young red blood cells, leading to an increase in MCV and RDW. However, further studies are needed to expand our understanding of the results reported here, since these changes are discrete and are insufficient to exceed the reference range for the species.
As for the WBC count, no changes were observed in the population of blood leukocytes. Previous studies have shown that obesity leads to a pro-inflammatory state with increased inflammatory cytokines in humans (Nemet et al. 2005; Zaldivar et al. 2006), dogs (Radakovich et al. 2017) and cats (Tanner et al. 2007), and changes in WBC, such as increased neutrophils, monocytes and lymphocytes in obese children (Nemet et al. 2005; Zaldivar et al. 2006) and obese dogs due to increased levels of neutrophils and monocytes (Radakovich et al. 2017).
As for lipid metabolism, obese cats showed higher levels of triglycerides, as described in the literature of different species (Vasan 2003; Alberti et al. 2006; Hoenig 2006; Mori et al. 2012). Lipid alterations are relatively common in veterinary medicine, especially in obese animals, often as a result of excessive intake of high calorie diets containing large amounts of carbohydrates and lipids (Barrie et al. 1993; Chikamune et al. 1995; Bailhache et al. 2003; Johnson 2005; Jeusette et al. 2005; Hoenig 2006). However, in the present study, all the owners reported feeding their cats solely with commercial cat food, which suggests that this change in lipid metabolism is not due to increased lipid intake. In addition, total cholesterol levels did not change as a function of obesity levels, corroborating the findings of previous studies that found no change in cholesterol levels in feline obesity (de Freitas et al. 2017; Aguiar et al. 2018). Higher HDL cholesterol levels in obese cats have been previously demonstrated (de Freitas et al. 2017). Unlike humans, cats have predominantly circulating HDL lipoprotein (Bauer 1996). Hoenig et al. (2003) emphasizes that obese cats have high levels of HDL cholesterol, which suggests the presence of cholesteryl ester transfer protein deficiency.
Obese cats showed reduced GGT activity, although it still remained within the reference range for the species. Increased GGT activity has been reported in obese humans, which is directly related to metabolic syndrome and its comorbidities (Saely et al. 2008). However, the reasons that led to the reduction in the activity of this enzyme in feline obesity are still unknown, and further studies are needed to clarify this issue.
Few earlier studies have evaluated oxidative stress in feline obesity. All the methods employed in this study indicated that obese cats had higher TAC, although TOC and lipid peroxidation remained unchanged. On the other hand, overweight cats showed an increase in TAC only by the ABTS+HRP and FRAP methods. Thus, oxidative stress seems to be related to animal weight, since TAC was higher in obese and overweight cats, depending on the method of analysis. Other authors have observed protein oxidation and lipid peroxidation in cats that had obesity induced and maintained for an 8-weeks period, as well as an increase in inflammatory cytokines, indicating an inflammatory condition induced by obesity (Tanner et al. 2007). In our study, the inclusion criterion was that the animals had to have been obese for at least one year, which means that they had already experienced obesity for a long period, making the process more chronic. Therefore, it was assumed that the animals were already adapted to obesity and that the TAC increased in order to fight oxidative damage during this condition.
The TAC evaluation methods showed differences, with overweight cats showing an increase in TAC only by the ABTS+HRP and FRAP methods and obese cats an increase in TAC by the four evaluated methods. A comparison of overweight and obese cats showed a difference only by the CUPRAC method, with obese cats showing higher CUPRAC than overweight animals. The differences observed in the TAC can most likely be attributed to the biochemical assays used in each method. The FRAP method primarily assesses uric acid, bilirubin, vitamin C and polyphenols (Benzie and Strain 1996). The CUPRAC method predominantly assesses non-enzymatic antioxidants from the thiol group (Rubio et al. 2016b), while the ABTS method assesses protein-based antioxidants such as glutathione and albumin (Erel 2004). The difference found between the methods may be related to antioxidant compounds not evaluated in the present study, since albumin and uric acid did not differ between groups. It is known that about 60% of TAC in human plasma is composed of uric acid (Benzie and Strain 1996) and the increase in this analyte has already been reported in human obesity (Abdul-Majeed 2009), in rodents (Tsushima et al. 2013) and in obese dogs, although in the latter species this increase was not enough to prevent systemic oxidative stress (Bosco et al. 2018). Thus, chronic feline obesity induces increased TAC by substrates other than uric acid and albumin.
In the obesity levels evaluated in this study, no evidence was found of change in TOC and lipid peroxidation. This may be explained, at least partially, by the increase in TAC, which could contribute to the inactivation of oxidizing compounds in feline obesity, preventing lipid peroxidation. A previous study demonstrated that obese dogs underwent oxidative stress as a result of increased TOC and lipid peroxidation, while TAC remained unchanged (Bosco et al. 2018). In studies on human obesity by Cătoi et al. (2013) and Pirgon et al. (2013), an increase detected in the oxidative stress index was attributed to increased TOC and reduced TAC. In rodents, a decrease in TAC was also observed among obese animals (Bełtowski et al. 2000; Furukawa et al. 2004). In addition to obesity, oxidative stress has been described in pathologies such as chronic kidney disease (CKD) in dogs (Silva et al. 2013), in cats (Keegan and Webb 2010) and in humans (Rysz et al. 2004), and in feline infectious peritonitis (Pedersen 2014). In CKD in felines, an increase was detected in reduced glutathione: oxidized glutathione (GSH:GSSG) ratios. This suggests the activation of antioxidants to fight ROS, despite the significantly lower TAC of sick animals, precisely because they are unable to maintain a balance between oxidizing substances and antioxidants (Keegan and Webb 2010). Increased lipid peroxidation has already been described in human obesity (Ozata et al. 2002; Konukoglu et al. 2006) and in rodents (Bełtowski et al. 2000; Furukawa et al. 2004). In canine obesity, the increase in TBARS was not detected in animals on a short-term fattening diet (van de Velde et al. 2012). Thus, lipid peroxidation and TOC do not seem to be suitable markers for the evaluation of oxidative stress in natural feline obesity.
It is essential for further research to evaluate the role of oxidative stress and inflammation in feline obesity, given the paucity of studies with this species in the obese condition and the fact that most of the data found are extrapolated from other species. However, since felines have nutritional, metabolic, physiological and pathological characteristics that render many of these extrapolations meaningless, further investigations into this subject are needed to understand its clinical implications in feline obesity.