An astounding 61% of cats in the U.S. are overweight or obese. And that number continues to rise. We’ve known for a long time that obesity increases an animal’s risk of disease and reduces their life expectancy, so it’s important to understand how excess weight can affect your cat’s health. Many factors are involved in how your cat’s body uses energy and stores fat. Diet and activity are important, but they don’t tell the whole story. The gut microbiome is also a big factor in your cat’s ability to achieve and maintain a healthy weight.
What Is Obesity?
Obesity is defined as a condition in which the accumulation of excess body fat has reached a level that negatively affects the body’s health.
Because more body fat leads to a higher body weight—and weight is easier to measure than body fat—the typical guidelines refer to weight: A cat who is 10%–20% above their ideal body weight is considered overweight. A cat who is more than 20% above their ideal weight is usually considered obese.
A 2022 survey by the Association for Pet Obesity Prevention (APOP) found that in the U.S., 61% of cats are overweight or obese (up from 60% in 2018).
Obesity and Inflammation
We know obesity reduces life expectancy. In fact, the mortality rate for obese cats is 2.8 times that of lean cats. That’s mainly because excess body fat puts a cat at higher risk for a variety of diseases.
Here are some of the health conditions for which overweight and obese cats are at greater risk than healthy-weight cats:
- diabetes mellitus
- heart disease
- respiratory disease
- feline lower urinary tract disease (FLUTD)
- liver disease
- skin disorders
- cancer
- osteoarthritis
- lameness
One reason excess body fat contributes to so many health problems is that fat tissue secretes hormones that create inflammation and oxidative stress (an imbalance between free radicals and antioxidants). Both low-level inflammation and oxidative stress are known to contribute to many diseases.
So while obesity is often considered a metabolic disorder (having to do with using and storing energy from food), it is also a chronic, low-level inflammatory condition. Since chronic inflammation contributes to insulin resistance, it’s no surprise that obesity is the most significant risk factor for diabetes.
What Causes Obesity?
A cat’s ability to maintain a healthy body weight is determined by multiple interconnected factors. Diet and activity level are important, but the numbers of calories consumed and calories burned don’t tell the whole story.
A study based on medical records for nearly 3,000 cats identified several factors that increase a cat’s risk of being overweight, including a dry food diet, “being a greedy eater,” and inactivity. In addition, spaying/neutering typically results in weight gain.
Genetics definitely influence an individual animal’s body weight. For example, some cat breeds, such as Siamese and Abyssinians, tend to be leaner than mixed-breed cats. But we still have a lot to learn about the involvement of particular genes in cat obesity.
Certain diseases can lead to obesity. Hypothyroidism (underactive thyroid gland) and insulinoma (insulin-secreting tumor) can both cause weight gain, for example. (Weight gain can also result from hyperadrenocorticism, or Cushing syndrome, but this disease is relatively rare in cats.)
Another important factor for body weight is the gut microbiome, the complex community of bacteria and other microorganisms that live in the lower gastrointestinal tract (GI tract). The gut microbiome is influenced by both diet and exercise, and in turn it influences a wide variety of bodily systems, including the immune system.
What your cat eats also feeds the bacteria in their gut. A healthy diet promotes beneficial bacteria, which protect against weight gain. But a diet that’s too high in carbohydrates, for example, promotes the kinds of bacteria that contribute to inflammation, which can lead to obesity.
Your cat’s gut microbiome plays an especially important role in the health of their immune system, because 70%–80% of the body’s immune cells live in the gut. A healthy gut microbiome means a strong immune system that can help protect your cat from disease.
How the Gut Microbiome Is Involved in Obesity
Multiple mechanisms are involved in the relationship between the gut microbiome and body weight.
Digestion and Metabolism
The gut microbiome has a lot of control over your cat’s digestion. The bacteria in the gut are responsible for breaking down food and converting it into glucose, the molecule that cells use for energy. So those gut bacteria influence the calories your cat takes in and the calories they burn. In addition, the gut microbiome can actually signal the body to speed up or slow down the metabolism.
Differences in the Gut Microbiomes of Overweight Cats
Studies of humans and other animals have consistently found that the gut microbiomes of overweight individuals look different from those of lean individuals. Specifically, the gut microbiomes of overweight animals have certain characteristics that seem to promote weight gain.
In fact, when the gut microorganisms from obese mice are transplanted into germ-free mice (mice without any gut microorganisms), the recipient mice are more likely to become obese, even without any change to their diet.
Researchers continue to investigate multiple theories about exactly how these differences may promote obesity. One theory is that the gut bacteria of obese animals may be more efficient at extracting energy from food. Another possibility is that certain kinds of obesity-associated gut bacteria may increase low-grade inflammation.
Different Bacteria in Different Proportions
Compared to normal-weight cats, obese cats have significantly altered populations of several important types of gut bacteria. One study identified several “hallmark” features of the obese cat gut microbiome, including dramatic increases in certain Bifidobacterium, Olsenella, Dialister, and Campylobacter species, and significant decreases in Firmicutes and certain Phascolarctobacterium and Erysipelotrichaceae bacteria.
This study also found that the gut microbiomes of obese cats have lower Firmicutes/Bacteroidetes ratios and “dramatically decreased microbial diversity.” Such findings add to the extensive evidence (from both human and animal studies) that gut microbiome imbalance (dysbiosis) is a significant contributor to obesity.
If your cat is overweight, how do you know whether an imbalance in their gut microbiome might be involved? Our Gut Health Test is an easy, noninvasive way to find out. You send us a tiny sample of your cat’s poop, and we analyze all the different kinds of bacteria present as well as their proportions. The personalized test report explains whether your cat’s gut microbiome is missing any important bacteria, contains any harmful bacteria that might be causing trouble, or suffers from an imbalance among the different bacterial populations.
Gut Microbiome Imbalance
When compared to healthy individuals, animals with obesity, diabetes, and other metabolic disorders tend to have intestinal dysbiosis (imbalance of the gut microbiome). In general, they have smaller populations of beneficial bacteria and larger populations of harmful bacteria.
The gut microbiomes of overweight and obese cats also tend to have low bacterial diversity (a measure of how many different kinds of bacteria are present). Reduced diversity is one reason the use of antibiotics—which kill off a lot of beneficial bacteria along with the harmful ones—has been identified as a risk factor for obesity. (Rebalancing the gut microbiome after antibiotic use can counteract the increased risk of obesity associated with these medications. We’ll discuss rebalancing a little later in this article.)
Without all the right bacterial populations in the right proportions, the gut microbiome can’t perform many of its important functions. For instance, we know that dysbiosis leads to issues with metabolism regulation, including increased insulin resistance.
Leaky Gut
Dysbiosis also leads to thinning of the mucous lining of the intestines, contributing to a condition called “leaky gut,” in which tiny holes develop in the barrier that normally encases the GI tract. Leaky gut has been identified as a factor in a number of symptoms and conditions, including weight gain.
Those tiny holes in the gut lining allow food molecules to pass through and enter the bloodstream, where they may be misidentified and attacked by the immune system as foreign invaders. That misdirected immune response generates inflammation and can lead to a range of unhealthy outcomes.
How to Help Your Cat Maintain a Healthy Weight
Achieving and maintaining an appropriate weight can help your cat live a longer, healthier life. Here are some important ways you can influence your cat’s weight.
Diet
Diet is the best way to manipulate the gut microbiome to promote a healthy weight, specifically by promoting beneficial bacteria over the kinds of bacteria that are associated with obesity.
Reduce Calories Very Gradually
If your cat is overweight, the logical first step toward weight loss might seem to be cutting back on their calories. But it’s very important to consult your veterinarian before reducing the amount of food you’re giving your cat. Cats can develop a potentially fatal condition called hepatic lipidosis (or fatty liver syndrome) if they don’t eat enough or if the amount they eat is reduced too quickly. Overweight cats are especially vulnerable to this dangerous condition.
Your veterinarian can evaluate your cat’s body condition, calculate an ideal weight and an appropriate daily calorie count for your cat, and help you make a plan for gradually reducing their food intake, if necessary.
Cats Need a High-Protein, Low-Carbohydrate Diet
Cats are obligate carnivores, which means they have to consume animal meat in order to get all the nutrients they need. Cats (and their gut microorganisms) do best on a diet that’s high in protein (containing more than 40% protein on a “dry matter” basis) and very low in carbohydrates.
Some kibble diets are too high in carbohydrates, so they don’t promote the growth of all the beneficial bacteria your cat’s gut needs. To support your tiny carnivore’s microbiome, be sure that the first two ingredients on the back panel are meat-based ingredients. With too many carbs on board, the kinds of bacteria that like to eat them are also the kinds that promote unhealthy levels of inflammation in the body. These bacteria multiply when fed a diet that’s high in carbs.
A high-carbohydrate diet also leads to high concentrations of insulin in the blood, a symptom characteristic of type 2 diabetes. The good news is that weight loss on a lower-carbohydrate (usually canned food) diet can allow overweight cats to regain normal insulin sensitivity.
Prebiotics Feed Beneficial Bacteria
Prebiotics are special soluble dietary fibers—like inulin, fructo-oligosaccharides (FOS), and mannan-oligosaccharides (MOS)—that provide the ideal food for beneficial gut bacteria. By promoting the growth of “good” bacteria in the gut, prebiotics can actually help counter the effects of a high-fat diet.
Bacteria “eat” prebiotic fibers by fermenting them, and the fermentation process generates end products called postbiotics—substances that also support the body’s health in numerous ways.
Fermented Foods Are Rich in Postbiotics
The various molecules produced when bacteria consume prebiotics are called postbiotics. Some especially important postbiotics are short-chain fatty acids (SCFAs), including acetate, propionate, and butyrate.
SCFAs are important for multiple aspects of intestinal health. They also directly influence the body’s metabolism, including the conversion of food into energy that cells can use. SCFAs influence body weight, appetite control, cholesterol levels, and insulin sensitivity. In human studies, higher levels of SCFAs have been found to improve type 2 diabetes. There’s evidence that SCFAs can help prevent and counteract obesity.
That’s why fermented foods are so good for us (and our pets): they’re a great source of beneficial bacteria, but they also contain significant amounts of the health-promoting postbiotics those bacteria produce. Consuming fermented foods is associated with reduced levels of inflammation.
You can help your cat get the benefits of postbiotics by adding a little fermented goat’s milk or unflavored yogurt to their diet. (Start with a very tiny amount and work up gradually to about a teaspoon a day.)
Exercise
Excess weight may make a cat less inclined to move around, but encouraging more play will benefit your cat in multiple ways. In addition to helping your cat burn calories, exercise is good for other aspects of their metabolic health. For instance, regular exercise prevents sudden spikes or dips in your cat’s blood glucose levels, helping to maintain proper insulin regulation.
Play is a big factor in the quality of life of indoor cats, providing mental stimulation as well as exercise. Physical activity also triggers the release of hormones that make your cat feel good. And we know that mood and the gut microbiome affect each other via the gut–brain axis. Exercise has also been found to influence the gut microbiome directly. For example, exercise causes significant shifts in Firmicutes and Bacteroidetes populations.
Fecal Microbiota Transplant (FMT)
An especially effective way to restore balance to an imbalanced gut microbiome is to introduce a complete set of healthy gut microorganisms via fecal microbiota transplant (FMT).
FMT is the transfer of stool from a healthy donor to the GI tract of a sick recipient. That transfer can be done via colonoscopy, enema, or endoscopy (under anesthesia) or via oral capsules. The stool from the healthy donor contains a diverse, well-functioning community of bacteria that take up residence in the recipient’s gut.
FMT even has the potential to correct obesity. Several studies in mice have found that transplanting fecal material from lean mice into the intestines of obese mice transfers metabolic benefits, allowing the recipient mice to achieve a healthier weight.
In addition, research has shown that fecal transplants can reverse insulin resistance in diabetic animals. That’s great news for patients with type 2 (insulin-resistant) diabetes, which is the type that’s more common in cats.
Our KittyBiome Gut Restore Supplement is a safe, noninvasive way to give your cat the benefits of FMT—including better weight control—in an oral capsule. If your cat’s Gut Health Test report indicates that missing bacteria, harmful bacteria, or an imbalance could be contributing to your cat’s weight gain or other symptoms, FMT capsules can help reestablish a healthy balance.
Questions for Your Veterinarian
How much should my cat weigh?
How can I tell if my cat is obese?
What kind of cat food is best for weight loss?
Does my senior cat need exercise?
What types of low-calorie treats can I give my cat?