October 5, 2023 — Dr. Kelly Diehl sits down with Valerie Benka and Audrey Ruple to discuss their new paper looking at associations between obesity/overweight and gonadectomy status, sex, age at gonadectomy, and breed size. They also discuss how their study is similar, and where their findings differ, from previously published studies.
Age at gonadectomy, sex, and breed size affect risk of canine overweight and obese outcomes: a retrospective cohort study using data from United States primary care veterinary clinics
AAHA Body scoring system charts
0:00:10.6 Dr Kelly Diehl: Welcome to Fresh Scoop, Episode 63. What do we know about the effects of spay/neuter on health and behavior in dogs? I'm your host, Dr Kelly Diehl, Morris Animal Foundation, Senior Director of Science Communication. And today we'll be talking with Valerie Benka and Dr. Audrey Ruple. Ms. Benka is Program Director at the Alliance for Contraception in Cats & Dogs, and Dr. Ruple is the Dorothy A. And Richard G. Metcalf, Professor of Veterinary Medical Informatics at Virginia Tech, and a member of the Golden Retriever Lifetime Study, Scientific Steering Committee. So welcome back, Audrey. Hello to Valerie, because this is Valerie's first time with us, but Audrey is an old hand. Before we get started, I always ask people to tell us a bit about yourself and what led you to your current roles, so I'm going to go ahead and start with you, Valerie. So, tell us a little bit about yourself.
0:01:02.4 Valerie Benka: Sure. So, I am program director for the Alliance for Contraception in Cats & Dogs, or ACC&D for short, and I've been with the organization for about 10 years now. We can talk a little bit more about ACC&D later, but just in brief, we work to advance non-surgical fertility control options for cats and dogs to basically expand the tools in the toolbox for population control. About me personally, I have graduate degrees in animals and public policy and conservation biology, and I did those programs envisioning work that would combine animal welfare and science, and my current job kind of fits that bill really well.
0:01:58.6 DD: And for those... You guys can't... We're on Zoom together, and Valerie has an awesome, handsome red kitty sitting in her lap right now, who's sagely listening to what we're saying. So, Audrey, do you want to tell a little bit about yourself too?
0:02:14.4 Dr. Audrey Ruple: Sure, and thanks for having me back today, Kelly. I'm Audrey Ruple, I have a DVM from Colorado State University and I also have a Master's and a PhD, both of those specializing in epidemiology, but unlike most veterinarians, I really specialized in small animal epidemiology. Most veterinary epidemiologists focus on herd animals, rather than small animals. So, I was able to join the ACC&D team to work on this project in a volunteer position.
0:02:47.0 DD: Well thanks, Audrey. And as Audrey said, she's going to be the cheerleader in the background because Val is going to answer the bulk of my questions today, and to start out with, I think we have a pretty varied audience, and it might be a good idea to define some terms. Can you talk a bit about how dogs are classified when it comes to body weight, like how we measure it in dogs, and then... And the systems that we use and then talk about what gonadectomy means for non-veterinarians. So, Valerie, I know that's a mouthful but see what... I know you can do it.
0:03:27.3 VB: You bet. So gonadectomy is surgical, spay/neuter. The more simple term, and just turning to a body condition score, so veterinarians use body conditions for BCS, speaking of that red kitty, he might be bored right now. They use BCS to evaluate an animal's body condition or body fat. So, it's a more quantitative and universally understood way to identify whether an animal is at a healthy weight. If not, how under or overweight they are, and in small animal medicine, it's used for dogs and cats. So, the standard practice now is to score a dog from one to nine on the BCS score, and for this study, we use data from a period when a one to five score was used. Regardless of the range though, the lowest number is a really seriously emaciated animal, unhealthfully thin, and the highest is an animal who is extremely obese.
0:04:51.1 DD: Okay. And I will just say now for people listening, I'll post in the show notes, there are several visual descriptions, depictions of the body condition score, there are graphs, and there's one from AAHA that's fairly recent that merges like the one to five and the one to nine scale. It's sometimes a little challenging to see, but they try to do... To compare that. So, we'll put that in the show notes for people. But I think Valerie gave us a great run down on how these things because I think just like in people, we know that body weight, like just weighing us, even though we do it every time I go to the vet, if we go to the doctor, right? They make us stand on the scale, is, can be inaccurate in people, right?
0:05:31.7 DD: We have different muscle masses and bone densities and all that good stuff, so thanks for doing that, Valerie. To move on, tell us why ACC&D had a special interest in this question and how they helped facilitate this study.
0:05:51.2 VB: Absolutely. So, as I mentioned before, ACC&D is working to advance non-surgical fertility control and non-surgical sterilants won't really have the same profile as surgical sterilization. There will be some different effects on the body, and along with that, different effects on hormones. So, our goal now, as non-surgical options are being developed, is to really expand the knowledge about spay/neuter and different health considerations related to it, so that we have a stronger baseline against which to evaluate non-surgical options when they come about and along with that support individualized medicine for individual animals and their people.
0:06:50.5 VB: So that's what we were interested in doing, and we worked with a data set from Banfield Pet Hospital, which is a very large nationwide practice with clinics across I think pretty much every state, and which means a vast trove of data and potential for research, so we were really lucky to have Banfield interested in participating in this, and then we were also really lucky to get an expert team, including Dr. Ruple, veterinary epidemiologists and theriogenologists and animal behavior experts, and really the scope of people who are important participants in this discussion.
0:07:50.7 DD: Right. And this is a really... I've seen a lot of people are starting to look at it, but just to wind back the clock a little bit, can you tell us when people first really started to get interested in this question, a link potentially between spay/neuter status and body condition, and I can tell you being, I'll just say it, 61, I remember when I was a kid, right? And we would... It was like, common, I don't know, lore, right? That you're going to get your animals spayed or neutered and they're going to get fat, and however, that was not really critically looked at till recently, I think, but can you maybe walk us through a little bit when people start to really think about this in a scientific way.
0:08:38.1 VB: Sure. Yeah, there are papers, looking, starting to look at the question a couple of decades back. And right now, there's quite a bit of research out there on spay/neuter and health outcomes in dogs broadly speaking. Alongside that we know that spay/neuter these days is very popular in the US with estimates of 70% to 80% of dogs who are sterilized, and we also know that it's clearly beneficial at a population level to prevent unwanted births and litters, but valid questions about the health and behavioral effects, and that's being shown in the research that's coming out. And weight gain is one of the more negative effects, so yeah, there's been a variety of research, and particularly in the past decade or so, I would say there's, there've been quite a number of good comprehensive papers on the topic.
0:09:58.9 DD: And before we get into that, I do have a question, because I think some of what prompted this interest also was a big trend I saw starting in the 90s with early spay/neuter, right? For population control. I think a lot of folks, if you've ever adopted an animal more recently from a humane society, they often come spayed or neutered, right? That was a strategy for population, like we should just spay/neuter them before letting them out, right? And it seems like some of this health outcomes sort of dovetailed with that big trend, right? Toward early... Is that correct, or am I just thinking wrong, incorrectly about that?
0:10:42.9 VB: I think so. That's what it seems like to me. Dr. Ruple, do you have thoughts on that?
0:10:49.9 DR: I agree. I do think that that timing is exactly right on, and not surprisingly, Kelly, I think that you really nailed it, and I think that there is going to be a continued trend in this exploration because we're getting bigger and bigger data sets where we can actually answer some of these questions that we've not been able to answer previously.
0:11:10.1 DD: Right. And Valerie, you mentioned this, but let's go... Let's turn to that. Part of the reason we have Valerie and Audrey on this, they wrote a really great paper, which again, I will put a link to in our show notes in the Journal of the American Veterinary Medical Association, which is really prestigious, so congratulations. And let's take a dive into that paper. So first, Valerie, what question or questions were you trying to answer with your study and the subsequent paper that came out of it?
0:11:40.6 VB: Sure. So, we were trying to answer three specific questions. The first one, and they were all interlinked. The first was, is sterilization associated with development of overweight status or obese status. So BCS of 4 or a BCS of 5 compared to intact dogs, intact, meaning those who have not been sterilized or gonadectomized. And then the second question was, is the age at the time of sterilization associated with development of overweight or obese status compared to those intact dogs? And then the third question looked at the sterilized cohort only and asked, Does the risk of O/O, overweight or obese status vary depending on age at time of surgery? So those were the three questions. And one of the unusual or noteworthy aspects of this paper is that we studied these dogs based on their size groups. A lot of studies have restricted research to pure bred dogs, dogs known to be purebred, others have included mixed breeds, but grouped all dogs who were mixed breeds together, so you would have a 15 pound and a 75-pound dog kind of in the same category, and that leaves out a lot of dogs or kind of glosses over the potentially unique trends and needs of different sizes.
0:13:36.0 VB: So, we used a different approach by doing weight categories and then putting different purebred and mixed breeds in those weight categories to evaluate them.
0:13:46.3 DD: Right. And so, walk us through... We're going to go through the methodology, which can be... Bear with us everyone, will get through it and it'll be really clear. But take a deeper dive, talk to me more about, Valerie, about these groups. because I think your point is really well taken when we talk about even mixed breed dogs and lumping them together, because we all know there that dogs of different sizes mature at different rates. So, take us through. So, start with your groupings.
0:14:19.6 VB: Right. So, we grouped dogs into five weight categories, which are also... We use the same weight categories that the dog aging project uses. So, the toy and small breed was dogs who were 0 to 22 pounds. Then medium was 22 to 44 pounds, standard 44 to 66, large 66 to 88, and then giant was above 88 pounds. So that allowed us to group animals, again, a variety of different breeds of mixed breeds together, but looking at their sizes and the potentially unique trends or needs of the different sizes of animals. And one of the things that I just want to sort of mention about how we approach this study as well was by working with Banfield Pet Hospital. It allowed us to look at dogs who were just being seen at a regular clinic for regular vet checks. These weren't dogs that necessarily had specific health issues or were being seen as a specialty referral veterinary hospital, but they were just sort of average dogs going in for their average check-ups, which made them we hope a more kind of representative example of dogs across the country.
0:16:04.0 DD: Right. I think that that is really important. And remind me, and I know you put this in the paper, but I forget. Did... The purebred dogs were put in with the mixed breed because you divided by weight, right? So just to make sure, and can you remind me like how many purebred dogs did you have versus mixed breed before we move on. I don't remember.
0:16:31.0 VB: That is a really good question. I don't recall. Offhand there were, once we applied exclusion criteria, which was dogs who had been identified as over-weight or obese prior to the start of the study, we had 155, more than 155,000 dogs in the final sample. Off the top of my head. I don't remember.
0:17:00.5 DD: It probably doesn't make that much difference, I just... I couldn't remember. So just say that number again for folks out there, because this is like a huge number of dogs.
0:17:10.9 VB: It is. Yes. Over 155,000. I believe it was 155,199 dogs.
0:17:21.6 DD: Wow. So again, for everyone listening, Valerie, just to reiterate the point to make sure I have them straight as we move on too. I think we have lots and lots of dogs, we have dogs going to their neighborhood veterinarian, and so again, more representative, a lot of previous papers for variety of reasons, have often come out of referral practices or universities because that's where our papers are usually written, and these large practices and large databases that are now available to us are really, really important because I think sometimes we can get a skewed view of things when we have a very narrow population. So now, Valerie, tell us how you compared and analyzed the groups.
0:18:13.8 VB: Sure. So, we started out by, I mentioned exclusion criteria, which were various things that we put in place ahead of starting the analysis that basically said, okay, this is not a good candidate to be included in the sample because for whatever reason, it would skew the outcomes in a way that was not reflective of general population. So, after that we... I want to note that everything was anonymized, there were no identifying features or characteristics in the data set, so we didn't know anything about the dog other than it's breed and certain year when it was sterilized, etcetera, etcetera. Nothing about the owners. So, to be in the sample, a dog had to have at least one visit to a Banfield Pet Hospital in 2013 and one in 2014, and then they were followed through in the analysis through 2019 or whenever they stopped being seen at Banfield either because they passed away or they just moved out of that hospital.
0:19:34.0 VB: So, we set an index date, which was the start date. And index was either a dog sterilization date, date of gonadectomy, or for intact dogs, the first time that they were seen at Banfield in 2014. And then we followed each dog from their index date forward in time to look at if and when they received an obese or overweight diagnosis, and then at that point, they were censored and they were out of the study.
0:20:09.6 DD: Okay, and what are some of the pros and cons of this approach? And what was especially challenging for you guys as you started to analyze this enormous amount of data?
0:20:26.4 VB: So, I'll start by answering this and then I will hand it over to Dr. Ruple because she is the expert, [laughter] on this methodological approach. So, I will say that our methodology, we did a retrospective cohort study in which Cox proportional hazard models evaluated associations. It differs from those used in several prior studies. It's common to use a cross-sectional approach, which basically evaluates factors associated with being overweight or obese, and then looking back. The benefits of our approach is that it's a prospective study in the sense that dogs were followed over time. They were forward, it was a forward-looking study, and it doesn't carry as the same risk of bias. There are challenges that are... Well, actually, let me pass that to Dr. Ruple. If that's okay.
0:21:44.3 DD: Yeah, yeah. Absolutely.
0:21:49.4 VB: Just say really your thoughts about the benefits of that approach.
0:21:50.6 DR: Okay, so just to emphasize one of the pros that Val has already mentioned, but just to really emphasize this in terms of that prospective approach in that we are confident that these dogs were not overweight at the time that they were spayed and neutered. And that is something that is a real challenge with these cross-sectional approaches that are typically taken, which is just... It's not that there's a negative to the approach, is that this is a potential bias, that, that approach of doing cross-sectional work is that we're not as confident about that, what is the overweight or obesity status at the time of spay-neuter versus when we're doing the study. With this approach, we're pretty confident that we had dogs that were of normal weight at the time of spay and neuter that then became overweight, so the overweight and obesity is something that truly developed after the time of spay and neuter. And so that's a real strength to this approach that we've taken. Val, do you want to start with the challenges and then I'll back in, or did you want me to start there?
0:22:51.3 VB: Yeah. You can give it a go. [laughter]
0:22:54.8 DR: Okay. So, some of the things that I think would be challenges to the approach, for one thing is that we don't know the total demographics of the people involved in this either, and we do know that makes it difficult for us to recognize the true representativeness of this work. Our hope is that this is a truly representative sample and that we will have external generalizability. So, in other words, that the study findings that we have are things that can then be applied to other populations. But we don't have a really great way to measure that. Our study population versus all the other dogs and all the other people that own dogs in our country and other places in the world. Even so, we can't say that because we have a really large sample size, which is a unique and another big pro to this one is to have a sample size this large, we have what we believe, in scientific speak, we talk about this regression to mean the larger the sample size, the more representative it's going to be just by nature of the size of the sample. So, we think that we have a pretty representative sample here.
0:24:07.6 DR: And then another one, which is one that Val has talked about, and something she wrote about it in the paper too, is that BCS score, it's a number, it's a numeric value, but there's still a subjectivity to the way that we ascribe those numbers. And not every veterinarian is going to call a dog the same BCS number as every other vet. Even with really good training, which we know that Banfield Hospital does provide to their veterinary staff. They do have a training system involved, and so we do feel like it's as good of a measure as we could have had, and we do feel strongly that using BCS was the more appropriate way to go rather than using weight as a metric, but there is still potential for subjectivity in that. So, there is a potential for a measurement bias in the way that these numbers are ascribed.
0:25:00.4 DD: Okay, that was really helpful because I know that there are always challenges and pros and cons. But to reiterate the prospective, if you've listened to this podcast before, we've talked about why prospective studies are important and unique. And I appreciate you guys talking about that again. So Val, can you tell us... What did you find and what were your conclusions from this study?
0:25:29.8 VB: We have some interesting findings in this study, particularly for a couple of the size groups. And just a quick reminder that we analyze dogs based on their size assignment, as well as their sex, age at gonadectomy, and a couple of other variables. And those size groupings follow those of the Dog Aging Project. We found that gonadectomy increased overweight or obese for most dogs compared to intact dogs, which was not surprising, not new. But unlike most prior findings, the hazard ratios among the gonadectomized versus intact dogs were larger for males than females. And that we found to be an only one other study that was very small scale. So that means that all other factors being equal, males showed a higher risk of becoming overweight or obese than their female counterparts. The other thing that was interesting was that O/O risk among the sterilized, gonadectomized dogs compared to the intact dogs, it varied according to breed size, but it wasn't linear. So, it wasn't like the smallest dogs had one risk and it just went continuously from there.
0:27:08.4 VB: Toy and small dogs did have the highest hazard ratio, and giant dogs had the lowest. But large dogs, which was the second largest size group and includes some of the most popular breeds, like Labrador Retrievers, Golden Retrievers, German Shepherds, it came close to toy and small dogs in terms of the hazard ratio that we found. One other point just worth making is that, and relevant to the high rate of spay/neuter in the US, is that among dogs who are sterilized, we looked at the best time to do it from the standpoint of O/O risk. So, there are many other reasons why one would choose a certain age, but we were really homing in on the risk of weight gain. And we found that sterilizing a one-year-old dog tended to yield a lower risk or a lower hazard ratio compared to doing it later. We then compared on the rest of one year to six months, so younger age, that varied by breed size. And what was interesting was that large dogs who were sterilized at three or six months had a higher hazard ratio than those sterilized at one year old, and it was statistically significant, and that was different from the other size groups.
0:28:43.6 DD: Okay, so size matters. Sorry guys, that's a really crude joke. But it does. And I think what I really liked about your paper is because of using the size categories not the breed, but sort of the size categories we get out... I think what all of us know, which is if you've looked at longevity data, there's differences in size as we talk about when dogs mature at different groups. But I think what was one of the things you mentioned that was really different, and we'll get into this a little more is I was surprised about the male thing. Because I think I had heard, and maybe, again, this may go back to lore, was that it was a female problem, [laughter] especially. That female dogs were more likely to get overweight or obese after gonadectomy, but you found something quite different. Val, you touched on this, but maybe we can dive a little deeper into where do your findings agree with previous studies and where do they differ? And I know that's a big category, but can you maybe hit some high points on that?
0:29:52.8 VB: Sure. Yeah. I'd be happy to. So, our study was consistent with many others in that sterilized dogs do have a higher rate, or did in the study, have a higher rate of O/O than their intact counterparts. But I would say that there isn't a definitive explanation for why that is. As Audrey mentioned, there's sort of the human behavior, owner behavior component that might be considered. With any study and outcome in animals, there are questions of physiology, questions of hormones. The finding that males showed a higher risk of O/O than females differed for most other studies. The increased hazard ratio among large dogs of both sexes sterilized at six months or earlier was unusual, and in our minds, worthy of further research and analysis.
0:31:01.8 DD: And I was going to ask you about that. What would you like to look at next? What questions arose, you just mentioned one, from this study where it would be good to take a deeper dive?
0:31:15.9 VB: Sure. We are actually in the midst of a second analysis looking at overweight and obese outcomes, specifically among popular breeds as well as mixes of those popular breeds. We took two size categories, we took toy, small dog breeds and large dog breeds, and we are interested in the toy and small because they showed the highest hazard ratio in our first analysis. And then the large job breeds because of this sort of unusual finding as well as the incredible popularity of some of the breeds in that size group. So, we are going to be, or we are looking at specific breeds within those size categories and looking at whether they're any breed-specific trends. And one of the... What we're trying to look at is whether there were certain breeds that were driving outcomes within the larger sized group. For example, if there were one or two popular toy or small dog breeds that were particularly prone to becoming overweight and therefore kind of swaying the results, so we're trying to get a little more granular level to see what those breeds can tell us about the noteworthy findings in our size analysis.
0:32:53.7 DD: Okay, and that's a good point because I think for those of us as a person-owned Labradors and they are really food-driven, I just did... Audrey will appreciate that I did. I had to do something for Dog Aging Project with my dog. My dog is in Dog Aging Project, and it was finding treats in a box. And it had to wait for a certain amount of time, and my dog lied down on the ground during the wait period and still could find a treat as a Labrador. And we know that they have some genetic abnormalities that affect satiety and they're very unfortunately gotten in there. So, I think that's a great question to look at Val because I would wonder with Labradors being really popular, whether they were skewing some of that large breed stuff. And I am going to ask you guys a big question and I should know this answer too, and maybe you guys know or maybe you don't, which is in Europe, they don't gonadectomize dogs very much. Do we know anything about obesity and overweight in Europe or not?
0:34:00.4 VB: There have been some studies looking at overweight and obese outcomes. And there are countries in which overweight dogs are quite prevalent, and there are, I think some countries where it's a lower prevalence based on the data. But it's a real... You raise a really, really interesting point and really interesting area for comparison, just looking at countries where it's popular and where it's less so...
0:34:45.8 DD: And that comes to another thing I was thinking about, which is... And again, Val touched on this a little bit, but what are some of the big questions like we just don't know of gonadectomy, overweight and obesity in dogs.
0:35:04.5 VB: I think the human behavior component is a big question, and sort of the environmental aspects of dog overweight and obesity. I think there's also a lot more research to be done regarding the importance of age and sterilization, particularly as it relates to hormones. And from ACC&D's perspective, that is a big question because it's likely that non-surgical options won't have the same effect on hormones that surgery will.
0:35:55.4 DD: And that's a good reminder, Val, because we're kind of looping back to where you work and the questions of when we are looking because Morris has funded oh, for a long time we've tried. With non-surgical methods, we've got two cat projects in the works right now, but I guess that would...If we're going to apply these methods, let's say on feral cats, that's always been community cats, how do we address them? We certainly don't want to do something that could be really harmful with that. And I know in other areas of the world, there's questions about, there were a lot of underserved communities where you're not going to get your dog spayed or neutered. That's just not going to be feasible, but maybe some other type of contraception, and we certainly don't want to have negative outcomes. So, as we wrap up, Val, what's kind of your take home message for our audience because this was really interesting and it's a lot of information.
0:37:02.5 VB: It is, yeah.
0:37:09.8 VB: It was fun to do, but it's a complex analysis and complex process. So, I think that one of the big take-home messages and one of the things that our team who is doing the analysis talked a lot about, is the opportunity to provide more individualized veterinary care for specific animals based on their profiles, based on their needs, as well as those of their guardian. And more data and more precise data can do that, can help support those decisions that people make about their care for their animals. I think that one thing that is really important that we also emphasized in the paper is that the research that we did came out with some pretty compelling findings. But it can't speak to other variables that might be considered when determining the best age to spay and neuter, so we were just looking at one factor, overweight and obesity. And we are currently envisioning doing further studies and further analysis looking at health outcomes.
0:38:30.8 VB: And certainly, lots of other great researchers are doing that as well with other data sets to get more information about overall outcomes of how we care for dogs. And I think the other message specifically for the veterinary community is that veterinarians are really uniquely positioned to help prevent overweight and obesity in dogs through educating and supporting clients regarding all factors related to an appropriate weight for pets, and that's throughout the life span of a dog. So that's what this paper is trying to support as well. And I think that one of the really exciting factors or aspects of the study for ACC&D was both that Banfield was really interested in sharing their data to allow our team to work on this issue, and also we're really grateful for Morris Animal Foundation for supporting the work.
0:39:48.4 DD: Oh, it's a really fun and great study. And I think it speaks to, well, the Foundation, but a lot of folks who are looking at big databases to try to leverage big data to look at these issues, and I really appreciate. Again, that kind of does it for this episode of Fresh Scoop. But I really appreciate Valerie and Audrey for coming on because this has really, really been interesting, and I appreciate you walking us through a complicated but very important paper. Made a lot of sense to me, so thanks.
0:40:27.1 VB: Yeah, thank you for having us. It was...
0:40:29.5 DR: Thanks, Kelly.
0:40:30.6 DD: Yeah, it was really fun. So just to wrap up for everyone listening, we'll be back again, of course, with another episode next month that we hope you'll find just as informative the science of animal health as we know, and we just learned is ever-changing. We need cutting-edge research information, whether we're treating patients as veterinary caregivers or as pet parents or both. And that's why we're here. You can find us on iTunes, Spotify, Google Podcast and Stitcher. And if you like today's episode, please take a moment to rate us because that helps other folks find our podcast. And of course, to learn more about Morris Animal Foundation's work, go to morrisanimalfoundation.org, and there you'll see just how we bridge science and resources to advance the health of animals. And you can also follow us on Facebook and Instagram and TikTok. We are now on TikTok everyone, I think we have six videos up there, and I am in a couple of them. [laughter] So I'm probably like the oldest person on TikTok. But anyway, I'm Dr. Kelly Diehl. We'll talk soon.