Fat Cat vs. Skinny Cat

By Karen Werner-Petak, DVM

“Hey Doc, my cat is really getting fat! She only eats a little bit of dry food every day. Do you think she could be hypothyroid?” This is a common question that veterinarians are asked regularly, mainly because there is an epidemic of overweight indoor cats these days.

It is rare to ever see a cat that is truly hypothyroid. Most cats are overweight, or even obese, because of their sedentary lifestyles and because of the cat food that they eat. For years, many of us have believed that dry food was better for our cats. Good marketing, convenience, and affordability have helped to make dry foods the main diet of American cats. But only in the past several years has better information on ideal diets for cats become the focus of more discussion.

Cats are carnivores (that is, they eat only meat). Unlike dogs and humans, they cannot digest plant-based proteins. In the wild, they eat animals that they kill, and their nutrients come from the prey’s muscle and intestinal contents. Almost all dry foods are too high in plant-based proteins. Dry foods are made mostly from corn, wheat, soy, and rice because of the low cost. So, even though the packages say that they meet the required protein levels for cats, the inability to utilize these proteins means that cats don’t receive the protein levels that they should. Dry foods are also cooked at high temperatures to kill off germs, which lessens the biological value of the proteins. The only reason cats even eat these foods is because they are tricked into liking them—the food is coated with animal fats and is crunchy.

The bottom line is that most of these ingredients are just carbohydrates (“carbs”) to a cat. Since cats can’t use these carbs for energy, the carbs are stored as fats. These carbs affect a cat’s blood glucose (“sugar”) and insulin. Plus, some foods contain soy, which affects a cat’s thyroid gland.

We now know that a better diet for cats is a meat-based, high-protein, low-carb canned food. Certain home-cooked or raw foods are also healthy for your cat. Starting your kitten on these types of diets at a young age will help prevent the typical weight gain seen in so many cats. Kittens must be given a wide variety of foods before they reach six to eight weeks of age; because, at this point they have chosen the kind of food that they will always crave. If they only receive dry food up to this time, it can be very hard to change their food to wet food later in life, but it is possible.

Also, weight can be managed by getting your cat to exercise more by promoting outdoor activities, using a laser light to help them play and run, and putting their food higher up so they have to jump for it. All of this makes them burn more calories and increase muscle tone.

But what happens when the opposite occurs? If you notice your cat losing weight, even though he or she seems to be eating well, but is maybe drinking a little more water than before, what could be going on? At this point, most people would take their cat to the veterinarian. Several diseases can cause this extreme weight loss; the most common are diabetes mellitus [dye-a-BEE-tus MELL-ih-tus] and hyperthyroidism [hy-per-THY-roid-ism].

Obesity is one of the more likely reasons for diabetes to occur. The high amount of carbs consumed with a regular dry diet causes too much insulin to be produced over too long a period. This leads to the body not being able to make enough insulin to keep blood sugars at a normal level. The body’s cells can’t receive sugar from the blood because there is no insulin to transfer it to those cells. The body can’t detect that there is sugar in the blood, so it still thinks it is hungry. Excess sugar is lost in the urine, drawing extra water with it, increasing the urine that is produced, increasing the cat’s ability to keep up with the loss of fluids.

If sugar is found in the urine, and your cat has elevated blood sugars, then diabetes is diagnosed. Most cats have non-insulin dependent diabetes, similar to type 2 diabetes in humans. The cat’s diet should be changed right away to a canned, high-protein, low-carbohydrate meal. Insulin is also started. Often, once the diet is changed, many cats will temporarily go into remission and stay there for some period, no longer needing their insulin. Constant monitoring of symptoms, along with regular visits to the veterinarian, is essential for healthy management of this disease.

Another leading reason for extreme weight loss is hyperthyroidism [hy-per-THY-roid-ism]. This is the most common endocrine disease of cats. About 98% to 99% of these cases are caused by a benign [bee-nine] adenoma [ah-de-NO-ma], which is a nodule (lump) of the thyroid gland that is working overtime. Less than 1% to 2% of these nodules are caused by cancer. No one yet is sure why we are seeing more of this disease, but, once diagnosed, it must be treated promptly before more serious side effects occur, including loss of muscle tone, fast heart rate, irregular heartbeat, vomiting, diarrhea, and behavioral changes. The best therapy, radioactive iodine (radioiodine [RAY-dee-oh-EYE-oh-dine], can actually cure the disease. The injected radioiodine destroys all abnormal thyroid cells, whether in the neck, chest, or other areas, and leaves behind inactive normal cells. A board-certified specialist in radiology, who has the state-approved facilities to handle the radioiodine, must perform it at his or her specialty clinic. Once injected into the cat’s body, your cat will be radioactive for several days and will be able to go home with you once radioactivity levels are safe. This treatment is very easy for the cats to handle, and they almost always begin to gain weight after their treatment.

Other methods of treating this disease include surgical removal of the thyroid gland, or daily dosing of methimazole [meh-THY-mah-zole]. Methimazole will reduce the active thyroid levels in the body, but it must be given by mouth or as a gel rubbed on the inside of your cat’s ear, once or twice daily, for life. This drug only manages hyperthyroidism, which means that your cat will need regular blood work for life. There are several side effects with this drug, since it was made for humans, not cats.

Unlike humans, cats do not typically have to go on oral thyroid hormone replacement after therapy. We are not sure why cats don’t seem to need this replacement.

As to the original question posed at the beginning, cats are rarely found to be hypothyroid. That is almost never the reason why they become overweight, as you can now understand. Please ask your veterinarian about changing your cat’s diet if you think he or she is overweight. Though it was once thought to be cute when your cat was pudgy, we now know that it can lead to many other serious problems.

Dr. Karen Werner-Petak has been a practicing veterinarian in Houston for over thirty years. She currently serves on the Texas Veterinary Medical Association’s Ethics and Grievance Committee. She is particularly interested in dermatology, endocrinology, gastrointestinal and cardiac cases in the dogs and cats that she sees. She has been married to Dr. Steven Petak for 32 years and has two children, Kate and Alex.