Helping an overweight cat lose weight safely

Why a cat can’t just diet fast

The Merck Veterinary Manual calls feline hepatic lipidosis, or fatty liver, the most common acquired and potentially lethal liver disease of cats. It describes how it starts: an illness or a lack of food that stops an overweight cat eating sets it off, and fat floods the liver faster than the liver can use or clear it.

Dieting is one of those triggers. Merck says obese cats undergoing weight loss are at increased risk of hepatic lipidosis and must keep eating enough calories and nutrients. If a cat won’t eat the new diet, Merck’s advice is to find an alternative: “Starvation is never a safe or humane way to cause weight loss.” The 2014 American Animal Hospital Association (AAHA) guidelines flag the same risk for cats on very low calorie plans.

In practice, that means:

• never withhold food to speed things up;
• make sure your cat actually eats any new food;
• call your vet if your cat eats much less than usual or stops eating.

Is your cat overweight, and by how much?

Vets use a 9-point body condition score (BCS). The World Small Animal Veterinary Association (WSAVA) puts the goal for most pets at 4 to 5 of 9. Higher scores mean you can’t easily feel the ribs, the waist fades, and a fat pad grows under the belly. Merck calls a cat 10–20% above its ideal weight overweight and more than 20% above it obese.

AAHA’s 2021 guidelines equate each point above 5 with being about 10% overweight. WSAVA’s cat chart adds a useful footnote: a score of 6 of 9 may be acceptable in some cats, especially older cats. Your vet’s hands-on score is the one to use; WSAVA notes that feeling the body is needed for an accurate score, especially in medium- and long-haired animals.

Target weight and daily calories

Our calculator estimates the ideal weight by dividing today’s weight by 1 + 10% for each point above 5. The band around it uses 5–15% per point, the spread seen in a study of 28 dieting dogs; no comparable cat figure was found, so treat the band as rough. Calories for loss come from that target weight, not today’s weight: AAHA 2021 uses 0.8 × the resting energy requirement (RER, 70 × kg^0.75), and AAHA 2014 calls 80% of ideal-weight RER effective and well tolerated. The floor is 60% of RER; AAHA 2014 suggests bringing in a specialist veterinary nutritionist for any pet that needs less to lose weight.

A score of 9 is the top of the validated scale, so the second cat’s target is the least certain. AAHA 2014 notes that no proven method exists for severely obese pets and that this approach may overestimate the ideal weight. Your vet will refine the target as the score improves.

Some calculators set a diet from today’s weight. For the house cat, a maintenance factor on 6.5 kg gives about 370 kcal, roughly twice the 187 kcal plan.

How fast: 0.5–2% a week

AAHA 2014 sets the desired rate for cats at 0.5–2% of body weight a week (1–2% for dogs). Merck calls about 1% a week a reasonable goal. For the house cat, measured against today’s weight:

So the realistic span is about 13 weeks to a year. The second cat, starting at BCS 9, takes about 16.7 to 67.1 weeks. Weekly changes this small need a scale that reads in grams or ounces; a bathroom scale that reads in whole pounds will miss them.

If your cat loses faster than 2% a week, AAHA 2014 says to add 10% more calories and keep watching. If loss stalls, AAHA 2014 suggests first checking that the plan is being followed — every meal and treat counted — and only then cutting calories by 10–20%. AAHA 2014 also says the rate should be modified for growing pets under a year old, so our calculator doesn’t plan weight loss for kittens.

A plan to make with your vet

1. Book a check-up first. Your vet scores body condition by hand, sets the target and looks at your cat’s overall health.
2. Measure every meal, ideally by weight, and count treats inside the day’s calories.
3. If the plan calls for a new food, make sure your cat actually eats it; if not, ask your vet for another option.
4. Weigh every 2 weeks, as Merck suggests, on the same scale.
5. Adjust with your vet: 10% more food if the loss is too fast, a review if it stalls, a new target as the body condition score improves.
6. Call your vet if your cat stops eating or eats much less than usual.

This is general information, not veterinary advice — talk to your vet before starting any weight-loss plan for a cat.

Next step

Enter your cat’s weight and body condition score in the pet weight-loss planner to see the estimated target, the daily calories and the weeks at each rate. Then turn those calories into cans and cups with the cat calorie and food calculator, and see how much to feed a cat for splitting wet and dry food.