My cat is slowly but consistently gaining weight (now 11.5lb). I've been measuring calories and spreadsheeting her meals to slowly lower her calorie intake, but in a recent Vet visit I was introduced to the Resting Energy Requirement for cats, and shown that I've been feeding her significantly less than the amount specified here. I don't understand how its possible that I'm feeding her below what's required for daily bodily functions, yet still gain weight


  • 9yo kitty
  • much less active in the past 2 years (we moved from a place with a large back yard to an apartment)
  • I measure out 158kcal of food (13kcal of which are treats)
  • roughly half her food is wet
  • I spread her food out into 3 meals over the day
  • weight: 11.5lb (2 years ago she was about 10lb, which was also overweight)
  • RER = 70 * BW_kg ^ 0.75 == 250kcal/day
  • We get about 20mins a day of play time
  • I also hide some of her dry food around the place, so that she walks around more
  • Does kitty go outside at all? Maybe she's finding alternative nutrition.
    – Boba Fit
    Commented May 8, 2023 at 16:31
  • Only in the hallway of the apartment, under my guidance. I don't believe she's getting any extra food elsewhere
    – glitchyme
    Commented May 8, 2023 at 16:33
  • The RER does not include any information about the activity of the cat. So it can be only some approximation. If the vet excluded any health issue (i.e. growing tissue/cancer) and because a 9yo should be out-grown by now, I would assume your cat move less than the RER "calculates". It is the same as humans: more movement, same food = loose weight, or same movement, less food = loose weight but also muscles, which could be followed by a lesser need of food for basic body functions Commented May 8, 2023 at 17:55
  • Isn't MER (Maintenance Energy Requirement) the modified function that takes activity level into account? I thought RER is energy requirement for basic function (digestion, respiration, brain function, etc.)
    – glitchyme
    Commented May 8, 2023 at 18:25
  • What were you feeding her previously (calorie-count)? It's generally advised to start with that as the baseline and decrease slowly to avoid hepatic lipidosis (fatty liver disease)
    – Allison C
    Commented May 9, 2023 at 14:11

2 Answers 2


Metabolism is a weird thing that is unpredictable in many cases. Reference points like RER can be an approximation at best, but the actual energy requirement of your individual cat can be influenced by a great many things:


Some cats - just like some people - are genetically predisposed to accumulate more body fat than others when eating the same amount of calories. It doesn't even have to be any genetic mutation. It's scientifically proven that a parents' experience of starvation or scarcity activates certain genes (that have always been there, but inactive) that increase body weight. And this gene activation can be inherited by children of these parents, even if the children never experienced starvation or were born long after the parents suffered scarcity.

Gut biome and "wrong" nutrients

The gut biome also plays a vital role in digestion. It's a chaotic mix of many different species of bacteria, many of which can influence how well your cat can extract nutrients (= energy) from the food it eats. That means if you feed the same 158kcal of food to different cats, they may absorb a different amount of energy from the same food.

Pet foods use a wide variety of ingredients like grain, potatoes, animal fat and plant oil. Depending on your cats gut biome she might digest some of these ingredients better. Trying different foods with different ingredients could have an impact on your cats body weight. Especially for overweight cats you should look for grain free or low-grain foods.

Medical condition

There are some medical conditions that can influence body weight much more than food intake alone. One example is a thyroid dysfunction, but you should ask your vet to test for well-known conditions if you haven't done so already. Some conditions can also lead to an accumulation of water in the soft tissue, which of course increases body weight, but is very distinct from body fat.

Wrong information on the packaging

Science is not infallible and only represents our current understanding of how things work. Several years ago scientists thought that cats cannot absorb the calories contained in starches (like grain or potatoes) or sugar very well. Current studies found the opposite: that cats absorb the calories from starches extremely well, possibly even better than humans.

Depending on which studies were taken into account, either the caloric value of the RER or the cat food or both may be wrong.

So what should you do?

My first advice is to switch to a grain free or low grain cat food. Starches and sugar in cat food seems to have a much bigger impact on body fat and the risk for diabetes than other factors like lack of physical activity.

Since the weight gain was very gradual, I don't expect any serious medical condition, but it doesn't hurt to consult your vet again and maybe schedule some tests.


If it was my cat, I would not worry too much about it. This is a middle aged cat. If she's a bit chubby, well... OK, it might shorten her life a little. But she's probably a lot happier rather than being hungry all the time.

Maybe you can encourage her to be more active. Figure out what activities she will welcome. If active toys grab her attention, great! Or if you can find active games that the two of you can play together, also great! An occasional (once or twice a week) bit of catnip might make a difference.

Some cats become very inactive as they age. Especially when they are indoor cats. Especially when there is nothing to draw there attention, no other pets and no children in the house. My brother has two cats. One of them goes outside and does cat things. He insists on it, hanging at the door until somebody lets him out.

The other cat lives in the triangle between the food bowl, the litter tray, and her bed. She does not welcome activity. Even a game of "bottom of the stairs" is clearly resented. But then, she's something like 20 years old. So people tend not to push her very hard.

  • For what it's worth, this was probably my favorite thing I read all day: "If she's a bit chubby, well... OK" :)
    – Vladimir
    Commented May 29, 2023 at 5:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.