Cat Getting Thinner?
If you feel your cat is losing weight and you are not trying to make them thinner this is an important problem to take to the veterinarian. Watch them closely and see if you feel they are eating more or less than they used to or if there are changes in water consumption. Has there been a change in the household lately that could account for the weight loss?
Did you switch foods? Observe their behavior and see if there are other changes that are not usual for them. Your vet will likely want to do bloodwork to check their kidneys, liver, thyroid and blood sugar.
If you are not sure if they are really losing weight or maybe you are just imagining it, then you can try weighing them over several weeks to see if they are in fact decreasing.
Scales are not always super accurate and cats are not always cooperative so you may need to hold them and get on the scale then subtract your weight to measure this. If they are in fact decreasing, you should get it checked out.
Cat Getting Fatter?
This is a common problem in house cats and can become quite medically serious if it gets out of hand. Cats can get type 2 diabetes, urinary tract troubles, arthritis, and have some unfortunate grooming issues when they get overweight.
You should never diet your cat without at least a call to your vet. Cats are super sensitive to calorie restriction and can get liver troubles if they lose weight too fast. Try weighing your cat like directed above to see if they are gaining over time. See if you can get a handle on how much they are eating. Any treats? Yup, they count too.
Cats are pretty tough to exercise but look at the page on hygiene and exercise for some ideas. Make sure they are eating the appropriate amount for their weight by looking on the back of the bag/can you use.
Don’t forget that they may be eating another housemate’s food too. Let your vet know if you are struggling with maintaining an appropriate weight in your cat. They may have some dietary ideas that will make this process easier. Don’t get discouraged, it’s worth the work to keep them feeling good long term.