Association between Pet Ownership and Obesity: A Systematic Review and Meta-Analysis

Obesity is a major risk factor for lifestyle-related diseases, including cardiovascular disease, type 2 diabetes, and hypertension. Several studies have investigated the association between pet ownership and obesity, but the findings have been inconsistent. This systematic literature review and meta-analysis assessed the association between pet ownership and obesity. Using PubMed and Scopus, we overviewed the literature published until December 2019 and selected pertinent data for meta-analysis. Two independent reviewers extracted the data. Pooled relative risks (RRs) with 95% confidence intervals (CIs) for obesity were calculated using the random-effects model with inverse-variance weighting. The 21 included articles were cross-sectional studies. Five publications (nine analyses) that reported adjusted RRs for BMI ≥ 25 were included in the meta-analysis. No significant association existed between pet ownership and obesity (pooled RR = 1.038; 95% CI, 0.922–1.167; I2 = 51.8%). After stratification by age group (children vs. adults), no significant association was detected (pooled RR = 0.844; 95% CI, 0.604–1.179; I2 = 64.1% vs. pooled RR = 1.099; 95% CI, 0.997–1.212; I2 = 25.2%). Similarly, no significant association was observed between dog ownership and obesity, indicating no association between pet ownership and obesity. However, no infer causation can be reported because all studies included in this meta-analysis were cross-sectional. Therefore, further prospective studies are needed.


Introduction
In recent years, the association between pet ownership and human health outcomes has been increasingly investigated. A survey of 22 countries, including Europe, the Americas, and Asia Pacific countries, reported that nearly half of the participants in the study were pet owners [1]. Keeping a pet has been associated with health-promoting effects, such as reduced risk of asthma [2] and allergies [3] among children. The frequency of annual doctor visits and medication administration is also reduced among pet owners, resulting in lower medical expenses [4]. Owning a dog is also associated with a 24% reduction in all-cause mortality [5], and reduced risk of cardiovascular disease [6]. However, there is some skepticism regarding the health benefits of pets; for example, a meta-analysis showing reduction of all-cause mortality included studies with insufficient confounding controls [5], and another meta-analysis reported inconsistent results. [7] The prevalence of obesity, one of the risk factors of cardiovascular disease, is globally increasing. Approximately 39% of the global population aged 18 years and older have a body mass index (BMI) ≥25, and 13% of whom have a BMI ≥ 30 [8]. Obesity is also a risk factor for other lifestyle related diseases, such as hypertensions and type 2 diabetes [9].
The number of studies investigating the association between pet ownership and obesity has grown since 2000. However, the research findings have been inconsistent. Regarding the association between pet ownership and obesity in children, Timperio et al. reported that pet ownership meant a lower risk of obesity (OR = 0.5; 95% CI, 0.3-0.8) in a cross-sectional study in 2008 of 281 children aged 5-6 years [10], whereas Westgarth et al. reported that pet ownership was not associated with obesity (OR = 1.07; 95% CI, 0.86-1.34) in a cross-sectional study in 2012 of 6634 children aged 7 years [11]. Regarding adults, Timperio et al. reported that pet ownership was not associated with obesity (OR = 1.1; 95% CI, 0.9-1.5) in a cross-sectional study examination of 2000 adults in 2008. [10] In contrast, Parslow et al. mentioned that pet ownership was associated with adult obesity (OR = 1.16; 95% CI, 1.00-1.34) in a cross-sectional study of 5079 adults [12]. Therefore, we conducted a systematic review and meta-analysis, to incorporate all available previous studies, and to evaluate the association between pet ownership, including adults and/or children and obesity.

Search Strategy
In this study, we performed a search in PubMed and Scopus from their inception until December 18, 2019. Studies evaluating the association between pet ownership and obesity were identified using a combination of the following keywords: "body mass index," "obesity," "body size," "waist circumference," "overweight," "metabolic syndrome," or "adipose tissue," and "dog ownership," "cat ownership," "pet ownership," "cat owner," or "dog owner," and "not obesity" or "not veterinary." The specific search strings for the databases are shown in Table S1. Two researchers independently searched for published studies through these databases. This study was conducted in accordance with the Preferred Reporting Studies for Systematic Reviews and Meta-Analyses (PRISMA) Statement checklist (Table S2).

Inclusion Criteria
For the systematic review, the inclusion criteria were set as follows: (1) exposure was having a pet (any kind of pet); (2) outcome indices were related with obesity (obesity, BMI, body weight, body shape, waist circumference, overweight, body fat, and metabolic syndrome); (3) participation of healthy individuals; (4) epidemiological studies; (5) language was restricted to English. Articles were excluded from the review when: (1) exposure was not pet ownership; (2) outcomes were unrelated to obesity; (3) participants were not community-dwelling people; (4) studies were involved in animal or cell studies, systematic reviews, meta-analysis, or conference reports.

Characteristics of the Selected Articles (Systematic Review)
The characteristics of the selected articles are listed in Table 1. These studies were published from 1992 to 2019, mostly in Western countries; eight of them were conducted in the United States, six in Australia, three in the United Kingdom, one in Norway, one in Finland, one in Canada, and one in the Czech Republic. All of them were cross-sectional studies. When age-stratified analyses were included, there were 17 and eight analyses involving adults and children as participants, respectively. BMI was the most common evaluation method for the outcome. For analysis of the association between pet ownership and obesity, most of the studies compared the proportion of overweight (BMI ≥ 25) or obesity (BMI ≥ 30) (16 analyses). After considering confounding factors, most of the studies revealed the association using logistic regression analysis (nine analyses). Regarding the result of the association between pet ownership and overweight/obesity, three analyses reported that pet owners were more obese, 17 analyses found no association, and five analyses reported that pet owners were less obese. Eighteen of the 25 analyses reported results of the association between dog ownership and being overweight/obesity, where two, 13, and three analyses reported that the dog owners were more obese, had no association, and were less obese, respectively. The mean quality assessment score by the Newcastle-Ottawa scale was 6.3 for 21 articles.

Characteristics of the Selected Articles (Systematic Review)
The characteristics of the selected articles are listed in Table 1. These studies were published from 1992 to 2019, mostly in Western countries; eight of them were conducted in the United States, six in Australia, three in the United Kingdom, one in Norway, one in Finland, one in Canada, and one in the Czech Republic. All of them were cross-sectional studies. When age-stratified analyses were included, there were 17 and eight analyses involving adults and children as participants, respectively. BMI was the most common evaluation method for the outcome. For analysis of the association between pet ownership and obesity, most of the studies compared the proportion of overweight (BMI ≥ 25) or obesity (BMI ≥ 30) (16 analyses). After considering confounding factors, most of the studies revealed the association using logistic regression analysis (nine analyses). Regarding the result of the association between pet ownership and overweight/obesity, three analyses reported that pet owners were more obese, 17 analyses found no association, and five analyses reported that pet owners were less obese. Eighteen of the 25 analyses reported results of the association between dog ownership and being overweight/obesity, where two, 13, and three analyses reported that the dog owners were more obese, had no association, and were less obese, respectively. The mean quality assessment score by the Newcastle-Ottawa scale was 6.3 for 21 articles.

Figure 2.
Overall meta-analysis of the association between pet ownership and obesity risk (body mass index ≥ 25). CI, Confidence Interval; OR, Odds Ratio.
Assuming that the participants' age affected heterogeneity, stratified analyses were also conducted for adults (in five analysis) or children (in four analysis). For adults (Figure 3), the pooled OR of the five analyses was 1.099 (95% CI, 0.997-1.212) [10,12,25], indicating that there was no significant association between pet ownership and obesity. Insignificant heterogeneity was observed (I 2 = 25.2%, p = 0.254). For children (Figure 4), the pooled OR of the four analyses was 0.844 (95% CI, 0.604-1.179) [10,11,17], indicating no significant association. Significant moderate heterogeneity was indicated (I 2 = 64.1%; p = 0.039).   Furthermore, we conducted a similar analysis focusing on dog owners because we suspected that dog-walking had a preventive effect on obesity. For all ( Figure 5), there was no significant association between dog ownership and obesity (for seven analyses, the pooled OR = 1.001; 95% CI, 0.869-1.152; I 2 = 42.9%; p-value for I 2 = 0.105) [10,11,17,25]. Stratified by age group, no significant association was also observed between dog ownership and obesity. For adults, pooled analysis of three results showed the pooled OR = 1.082 (95% CI, 0.966-1.210; I 2 = 0%; p-value for I 2 = 0.973) ( Figure  6) [10,25]. For children, the result is presented in Figure 4. Furthermore, we conducted a similar analysis focusing on dog owners because we suspected that dog-walking had a preventive effect on obesity. For all ( Figure 5), there was no significant association between dog ownership and obesity (for seven analyses, the pooled OR = 1.001; 95% CI, 0.869-1.152; I 2 = 42.9%; p-value for I 2 = 0.105) [10,11,17,25]. Stratified by age group, no significant association was also observed between dog ownership and obesity. For adults, pooled analysis of three results showed the pooled OR = 1.082 (95% CI, 0.966-1.210; I 2 = 0%; p-value for I 2 = 0.973) ( Figure 6) [10,25]. For children, the result is presented in Figure 4. Furthermore, we conducted a similar analysis focusing on dog owners because we suspected that dog-walking had a preventive effect on obesity. For all ( Figure 5), there was no significant association between dog ownership and obesity (for seven analyses, the pooled OR = 1.001; 95% CI, 0.869-1.152; I 2 = 42.9%; p-value for I 2 = 0.105) [10,11,17,25]. Stratified by age group, no significant association was also observed between dog ownership and obesity. For adults, pooled analysis of three results showed the pooled OR = 1.082 (95% CI, 0.966-1.210; I 2 = 0%; p-value for I 2 = 0.973) ( Figure  6) [10,25]. For children, the result is presented in Figure 4.   Figure 6. Meta-analysis of the association between dog ownership and obesity risk (body mass index ≥ 25) in adults. CI, Confidence Interval; OR, Odds Ratio.

Discussion
We conducted a systematic review and meta-analysis to summarize published articles (until December 2019) that investigated the association between pet ownership and obesity. A total of 21 articles were included in the systematic review, five of which were included in the meta-analysis. There was no significant association between pet ownership and obesity regardless of age group. Furthermore, no significant association was observed between dog ownership and obesity.
The number of studies that directly investigated the association between pet ownership and obesity is limited, and only a few have reported RRs adjusted for confounding factors. Regarding physical activity, only one of the five articles was adjusted for this. Among the elderly, 17% of dog owners who walked at least three times or more per week were obese compared to 29% among dog owners who did not walk three times per week [31]. Additionally, 19% of non-dog owners who walked at least three times or more per week were obese compared to 25% of non-dog owners who did not walk at least three times per week. These results suggested an association between walking and obesity, regardless of dog ownership. However, many dog owners maintained their walking habits beyond 3 years [31]. The importance of distinguishing the daily amount of physical activity from an increase in the amount of physical activity associated with dog ownership is important in our analysis. The fact that the physical activity levels were not sufficiently evaluated in almost all of the studies included in the meta-analysis may have affected the findings.
Obesity is caused by an imbalance between energy intake and expenditure. According to the World Health Organization, one of the causes of obesity is the global increase in the consumption of energy-dense foods that are high in fat [8]. A study of 3185 dog owners in 11 European countries reported that increasing positive attitudes towards a healthy diet decreased the likelihood of being overweight/obese (β = −1.662, Standard error = 0.441, p < 0.001) [33]. Only one of the studies included in this meta-analysis considered the influence of diet [17]. However, the questionnaire used in this study only assessed the eating habits of participants, based on the recall of healthier/less healthy foods that were consumed on the previous day with yes/no options [17,34]. The questionnaire did not assess the nutritional intake. Therefore, further studies are needed to assess the quantitative dietary intake using tools, such as dietary records.
In our study, pet ownership was treated as an exposure. However, the information on the type of pets could not be obtained in some studies. A study by Garcia et al. reported that 64%, 68%, 63%, and 68% of the non-pet, dog, cat, and bird owners, respectively, had a BMI ≥ 25, indicating that the type of pet may be related to pet owners' obesity [20]. In addition, although most of the studies included in the meta-analyses treated dog ownership as exposure, the dog breed (i.e., giant, large, medium, small, toy) was not considered. We conducted a meta-analysis only in dog owners but did not find any association between pet ownership and obesity. Therefore, pet type or pet breed may mask the association between pet ownership and obesity.
This study had several limitations. First, a limited number of studies were included in the meta-analysis. In the future, as the number of studies involving different populations increase, the null results reported in the present study may change. Second, as all the studies included in the meta-analyses were cross-sectional, the study design may have led to the null results in the present study. Therefore, prospective studies are needed to elucidate the association. Finally, as most of the included studies were conducted in Western countries, the results cannot be generalized to other populations, such as Asians and Africans.

Conclusions
Systematic review and meta-analysis did not suggest the preventive effect of pet ownership on obesity. However, as all studies extracted by the systematic review process were cross-sectional, a causal relationship cannot be inferred. Therefore, further prospective studies are needed.