Hi to you all,
As a newbie, I’m hoping wiser heads than I will be able to help:
We’ve had our female Maine coon (2 y.o.) for 24 days. She’s boney. Can’t see her ribs, but you can sure feel her bones.
Is there any wet cat food (without corn), but contains taurine in the UK?
In the UK if you’re thin (a human) you can buy a pack of Complan, with it’s long list of vitamnins etc.
I read on the web some cat owners buy baby food, but I’m concerned about onion powder.
Fingers crossed:)
Hi White Fang and welcome to the Straight Dope!
I reported your post for a forum change. Sorry, but I know nothing about cats. Hope you kitty feels better. 
Is your cat a lolcat? Otherwise this thread might best work in MPSIMS instead of ATMB. Hope your kitty gets trim soon.
He could use the fat pig that passes for a cat in this household as a role model.
Sorry, can’t help with UK cat food.
get fat and lazy mice…
BTW White Fang, have you wormed the cat?
If you’ve only had her for 24 days, perhaps she just wasn’t looked after prior to you getting her.
Moved to IMHO from ATMB.
White Fang, you must learn one of the most important rules of this message board. It is forbidden to start a thread about a kitty without posting a picture of said kitty! (You can’t attach a photo. You’ll need to give us a link to a photo on an external site.) We do love those kitty pics! (Well, some of us do, anyway.)
I’m no help on the cat food issue, but a visit to the vet might be in order. Where did you get your kitty? Is she a rescue?
Are you positively sure that it is thin and not just normal (ie, not overweight)? Here is one guide to assess body condition in cats. And here is another chart, again with pictures and explanations.
Second, check the cat with a vet. Like they said above, the cat may need to be dewormed, and this may help her gain some weight. Also, the vet may better assess if the cat really needs to gain weight or is OK with the current weight. She/he may also do a physical exam and find something else that may be causing the animal to perhaps not eat or not gain weight.
Third, IIRC, all major cat brands, wet and dry, now come with taurine. If you want to spoil the cat a bit, try feeding some food labeled for pregnancy/nursing and/or kittens. Both of those are more calorie-dense than regular adult cat food.
See a vet for recommendations for a “recovery” cat food, if it’s called for. It’s very high in protein and fat.
Hello White Fang! Hope your kitty settles in well with you!
Seconding the votes for:
Talk to the vet re: appropriate size of cat (she could just be naturally thin) and about worms and/or other disorders. Even something that doesn’t directly influence weight can matter - My friends have a cat who had a stressful month, then got outside and was lost for a week, and they’re pretty sure he didn’t eat much during that time. He finally found his way back home, and they fed him constantly, but he never gained back any muscle mass, and got all limpy. When they got him vetted out, they discovered he had severe arthritis, and was in pain, therefore not moving enough to gain back any muscle, and not active enough to be hungry. Now that he’s on pain meds and getting the arthritis cleared up, he’s moving around, hungry, and slowly gaining weight again.
Consider making your own protein/calorie dense cat food from people foods - get advice from the vet on what foods (usually straight meats and other “pure” not processed foods) are safe for cats, and put them in a blender with tuna oil or canned cat food. Presto - super kitty weight-gaining food with no weird unknown ingredients!
Use “pregnancy” food rather than kitten food. She’s a girl cat, and the pregnancy food contains stuff intended to keep girls’ bones solid, and keeping them in good condition and with enough nutrients for the stresses of nursing. That’s a much better choice than kitten food, which often is much higher in supplements and stuff intended for growth/development - which you aren’t looking for.
Also, pictures!