Problems with my 4 month old kitten

Question
I have a kitten that has had all of his shots including the new Calivax shot,
tested for FIV, and up til now has been extremely healthy. When I got him I
fed them NutroMax dry food with Fancy Feast, then on to Fancy Feast dry w/
Fancy Feast wet then on to Friskies dry and Friskies wet all of this because I
have an older cat that is blowing up with weight because of the kitten food so
I stopped it. Now the kitten started throwing up a couple of days ago I didn't
think much about it because it has always been common among the cats I
have had. Yesterday I noticed he seemed skinny (hard to tell sometimes
because he is long hair and my daughter never lets me near him) I checked
him and his stomach was not full like normal so I got worried. I pulled his
skin and noticed he was dehydrated. I will be taking him to the vet tomorrow
but could all of this be because I keep changing his food? If so what do I do
in a mixed household to keep one's weight down while gorging the other?

Answer
Congratulations of having a new kitty and on getting him all of his vaccinations. That is the mark of a good pet owner.

Now, the second mark of a good pet owner is one that is informed about their pet's nutrition and health.
Way too many veterinarians do not discuss diet with their clients and when they do they give a cursory answer to "What should I feed my pet?"
The reason for this most of the time is because not many vets have had any training in nutrition in school and sometimes don't take it upon themselves to learn more.
Having said that, many technicians, like myself have.
So let me educate you about diet and nutrition from a Vet tech and pet owner to a pet owner.
The reason I am saying it this way is this: You,like millions of pet owners, are pulled and pushed by advertisers.
How would you know what is the best food out there when they ALL claim to be the best?

Let me tell you these facts first of all about our pet food industry and an animals energy needs.
1. Animals eat to satisfy their energy needs, what ever those might be-from playing with a cat toy to a dog that hunts.
2. Pet food manufacturers make their foods affordable by using poor quality ingredients and over selling and over adding too many nutrients to make up for the low quality of the food.
3. When your pet eats this food in the prescribed amounts given on the bag or can, they are being way over fed to make up for the lack of nutrition from the poor quality ingredients.
I liken this to my clients as you trying to get a pound of protein from a box of saltine crackers.
4. Obesity in animals is a very serious problem and is brought on by the above statement. IF an animal is to meet their energy needs with poor quality food, then they are going to have to eat a LOT of it to get that protein out of the crackers.
Are you following me here?

Okay the next problem we have here is you are feeding one food after another to a young kitten whose digestive system isn't even quite honed yet. Kittens grow for up to one year old-even if they look 'grown up' by 7 months of age. You are introducing way too many potential allergens, too much fat and salt and way too many new ingredients to this poor kitties diet way too fast.
My question to you is why do you keep switching foods?
The cat probably liked the Nutro just fine.
It is always a problem to feed too different ages with two different foods.
Adult cats need adult food, not kitten food. Kitten food is way too high in proteins and fats for an adult. The result from an adult eating kitten food can be weight gain, fatty liver and in the long term, kidney failure. Too much protein in adult cats has been implicated for years of causing kidney failure in adult cats and even kittens.
Kittens need more protein, but not so much that they grow too fast. Cats are obligate carnivores, which means they HAVE to have protein to survive. They cannot live on an omnivorous diet like we can (and dogs can too).

The best thing to do is feed the kitten twice a day-morning and night- and leave the adult food down. Pick up the kitten food when he is done.You may have to stand over the bowl for a few minutes but when he is full he will walk away.
He is probably dehydrated from vomiting. His gut is most likely inflamed from all the food changes so fast (in four short months) and that is why he is vomiting. Hopefully he won't progress into an inflammatory bowel problem.

Now here is my recommendation for you:

Throw out all the food you are feeding and get two bags of food at the pet store. One 4 lb bag of Science Diet Kitten Growth and one bag of Science Diet Adult light.
You can measure out a 1/3 cup a day of the light for the older cats and feed the little one a 1/4 twice a day of the dry while you are with him. Once he leaves the bowl put it up for the night. If there is some left over, put the bowl in the microwave so the other cats don't get to it.

Feeding Science Diet will do many things- but you have to wait to see what is wrong with your kitty at the vets tomorrow first.
He may want him on a bland rice and chicken diet for a few days to settle his poor tummy and digestive tract down.

Science Diet has over 50 yrs of veterinary research behind it and is what most vets feed their own pets.
*It is made of the highest quality food available and is made with a fixed formula, so it is consistent batch after batch. No other food from a pet store does that anymore.
*It is nutrient dense which means your cats are getting their energy needs met with way less food than before. Some owners think their cats don't like the food because they leave more in the bowl but are maintaining their weight and look better- but the reason for that is that they are satisfying those energy needs with less food.
*So in that respect it saves you money. If you feed it according to the package directions then your bags of food will last quite a while. These results are seen usually after the first month of feeding since it takes that long for the new food to kick into the whole body.
*The stool is smaller due to the digestibility of the food too. Less mess in the cat box.
*Feeding Science Diet seems to promote a long life in animals also.
My cat lived to be 19 and my dog to almost 16 and she was a large breed.

If you follow these recommendations you will have a healthy, happy kitten and a healthier happy adult cat or cats. I know, I have been feeding it for over 30 yrs.
Another thought too is that he could have a case of roundworms going that just hatched. He should be wormed every two weeks as the cycle is 10 days or so for a new hatch.

Let me know what the vet says and I hope he does well.