Cat possibly losing muscle mass after switching to grain free?

My cat was 20.5 lbs when we switched him to young again zero carb food. He was quite overweight and we couldn't feel his ribs or spine at all. Now his spine is easily felt especially when hes laying down. When he's standing you can still feel it but it's not nearly as prominent and feels well muscled. He is about 15 lbs now  and he's been on the diet for 6 months. His energy level seems to be the same and he's very happy, he jumps better than he did before so he doesn't seem weak.

I forgot to add that he's been losing weight within the recommended range of 1% to 2% a week.

It does sound like he's losing weight at a healthy rate with the Young Again cat food. I think it's more of a matter of weight loss rather than muscle mass loss per se. Sometimes losing muscle mass in certain areas of a cat can be symptoms of diseases like hyperthyroidism (losing muscle mass in the paravertebral area is a symptom), but I don't think this is a problem with your cat - he's just losing weight at a healthy rate. 

Just make sure that his spine doesn't begin to jut out or that he doesn't start losing weight too quickly. This can lead to a whole host of other problems.

How old is your cat?

Thanks very much for your post. Everything you said makes a lot of sense, I can be a little paranoid about the health of my cats because I love them so much. My cat is 9 years old.

It's okay - here we all get paranoid about our cats' (hence the many, many threads on the Cat Health/Nutrition forums  ) health, and usually it's a good thing because the issue can get diagnosed early.

Just make sure to watch for signs of aging - especially with weight loss, you have to be careful. Again, make sure to check that his spine isn't protruding out and that his coat is nice and shiny - common diseases in aging cats are hyperthyroidism, diabetes, and IBD, especially in regard to weight loss and muscle mass loss. Glad to hear that he's still active.