It has already been well established that people who are obese are more likely to develop cancer than people of healthy weight. “There are a lot of different theories on why,” says Tufts veterinary nutritionist Cailin Heinze, VMD, DACVM. “One of the main ones,” she comments, “is that obesity is a chronic inflammatory condition. It’s thought that some inflammatory chemical triggers that shouldn’t normally be floating around in your bloodstream in relatively high amounts could set things in motion by causing changes in cells that lead to the growth of malignant tumors.”
In fact, Dr. Heinze reports, “chronic inflammation is known to actually become cancer. For example, in cats we think inflammatory bowel disease can progress into lymphoma.”
Dr. Heinze also explains that there are “a lot of biochemical signaling molecules in the body, like leptin, that tend to be higher when people are overweight, and those higher levels have been associated with the genesis of tumors.”
But what about dogs? Dogs who are obese appear more prone to developing mast cell cancers and mammary (breast) tumors — both very common malignancies among our canine friends — and also bladder cancer. Admittedly, however, “there hasn’t been a ton of research on this,” Dr. Heinze says.
Nor has there been research on whether obese dogs who get cancer are less likely to survive than healthy weight dogs who develop cancerous tumors. In people, the prognosis with some cancers is most definitely worse if you’re overweight.
“This has been seen for breast cancer in particular,” says the doctor.
“Fat tissue produces more hormones, and most breast cancers are very sensitive to hormones.” That is, certain hormones nourish various types of breast cancer. There’s also the question of how much chemotherapy you give an overweight person or animal with cancer. “How do you dose the chemo in someone who’s obese?” Dr. Heinze asks. Do they need more chemotherapy, perhaps to fight a cancer that might grow more aggressively because it is awash in hormones that encourage its growth? Or should you dose based on lean body mass rather than actual weight/body size?
Putting the hypothesis to the test
To learn whether dogs, too, are more likely to die sooner from cancer if they are overweight instead of healthy weight, Dr. Heinze reviewed data on more than 300 dogs of different breeds who were treated at Tufts either for lymphoma or osteosarcoma (bone cancer) — two of the more common cancers that befall the canine species. It’s an important issue to investigate, she says, because it goes right to the heart of how to treat your dog if she develops a cancerous tumor. “Should you try weight loss?” she asks. “Just let them be? What’s the best-case scenario for treatment? There’s always that question, and we don’t really know the answer. In both people and cats, there are a lot of data that show if you’re underweight, your survival is cut short, but that’s a little more intuitive. If you’re underweight, you’re going to waste away more quickly. But we wanted to look at it from the other end.”
It was also important to investigate, Dr. Heinze says, because in previous studies, 30 to 50 percent of dogs with cancer were found to be overweight. The issue affects a lot of dogs.
The results, published in the Journal of Veterinary Internal Medicine, were not as dramatic as the hypothesis about fat’s involvement in cancer might suggest. When it came to lymphoma, as expected, those were who were underweight died more quickly. They lived, on average, just under six months as opposed to healthy weight dogs who got the disease; they lived an average of more than seven months.
But there was no statistical difference in survival times between healthy weight dogs and overweight dogs. They each lived an average of seven to seven and a half months.
It was the same story for osteosarcoma. The overweight dogs with the disease did not die any sooner than the healthy weight dogs.
Why the results do not reflect what was anticipated
“Our caseload wasn’t as fat as we expected,” Dr. Heinze says. “We had a lot of overweight dogs, but not a lot of obese ones. Ideal body condition is a 4 to 5, and they tended to be a 6 or 7 rather than an 8 or 9. So what we learned was that being overweight did not appear to have an impact on survival time with cancer; it didn’t seem to be a disadvantage. What we didn’t learn was whether obesity would influence survival time. We couldn’t tell from this study. It shows us there’s more we need to do, studies that specifically compare healthy weight dogs to obese dogs.”
Along with more studies comparing dogs of different weights who already have cancer, Dr. Heinze says we need more studies to study the issue of whether a dog is more likely to get cancer in the first place if she is overweight—bigger studies than ones that have already been conducted.
To that end, the Morris Animal Foundation is spearheading what is known as the Golden Retriever Lifetime Study. It will look at some 3,000 golden retrievers for about a decade with cancer as the endpoint and will hopefully be able to assess not just the weight of the dogs who get the disease earlier but also their eating patterns and other aspects of their lifestyles.
It is what epidemiologic researchers call a prospective cohort study, which looks at dogs (or people) before they ever get sick and determine from all the data what it was in their diets or daily activities that may have contributed to the development of — or protection from — disease. Think: Harvard’s Nurses’ Health Study or the Framingham Heart Study, which studies people, and even their descendants, over long periods of time.
Prospective studies provide much stronger clues to health and disease than retrospective studies, which go through the records of dogs (or people) and try to piece together after the fact what it was about their lifestyles that predisposed them to various ailments. That’s what Dr. Heinze’s study did.
She is very excited about the Golden Retriever Lifetime Study. “We had a nice little piece of research,” she says, “but this huge study has enrolled dogs before they are one year of age and will follow them throughout their lifespans.” It will cost millions of dollars and may yield very telling information. “It’ll probably be 10 years before any of the findings are published,” she says. “But it has a chance to answer some really important questions,” including the effect of a dog’s weight on cancer risk and cancer survival.