feline nutritional allergies

Feline nutritional allergies aren't that uncommon, that's the nice way of saying they're out there, but very hard to diagnose and treat.  Basically, I find that any overgrooming or odd skin changes cat can be allergies, but can also include kitties that vomit, wheeze, or have soft stools.  Obviously, other disease process cause these exact same symptoms, so working up labwork including blood, urine, and stool will be necessary to rule out other sources of the symptoms.   The treatment for food allergies is to "food trial" on a novel protein food (read none of the proteins kitty has eaten previously may be in this food) or a hydrolyzed diet (read the proteins have been cut down to small pieces the cats immune system isn't supposed to read.  There are very few truly novel protein foods out there, most having in common at least soy(see below).  To this end some try niche diets (a diet made by a company for your cat), or cook for them ( a hard to balance proposition).  I recommend the hydrolyzed diet as the easiest of these approaches.
A niche diet is for your pet, it is literally the first one to eat it. It balances on paper, but most niche companies don’t do animal feeding trials to measure actual nutrition performance. A few organic brands have gone through animal feeding trials for some of their formulas, but most have been presumed balanced based solely on book values. When choosing any food, look at the food’s AAFCO statement for these words: animal feeding tests substantiate this food is complete and balanced. If the words animal feeding tests are missing, the tests haven’t been done.

When feeding your kitty who may have a food allergy, be patient. If you are able to find a true novel protein diet, it may take up to eight weeks to see a convincing response. One study in Europe showed that it may take as many as three consecutive diet trials to catch 95% of food allergic patients!

It is difficult to find a truly novel protein for cats because ordinary cat foods contain so many similar flavors and ingredients. Duck, for instance, often cross reacts with turkey and chicken. Beef often cross reacts with venison, bison, etc. Add to that the difficulty that many OTC formulas that say "soy free" or "grain free" have been shown to have significant enough levels of these ingredients to aggravate a truly allergic cat. It’s not so much they’re “sneaking it in” as there is inadvertent contamination. Feed mills are dusty places and it’s easy to have cross contamination of production lots.   The dog food market has a few that use kangaroo or rabbit and pass the" no soy… really, for real… no soy" tests. However, those diets just aren’t there for cats, and dog food doesn't have the essential amino acids required to keep kitties healthy.  There are a very few over the counter diets that have proteins limited to salmon, chicken, duck, and venison and also live up to their “no soy” claims. Again, realize that none of those proteins are completely novel for all cats.  
For pet owners who really want to be sure they are using a novel protein that has no contaminants and they want to avoid commercial manufacturers, could try the web site balanceit.com. They sell supplements that make a homemade diet complete and balanced (at least on paper). It was designed by board-certified veterinary nutritionists. You basically can make your own diet out of your own protein source and their supplement will make it balanced. They also have some free recipes using human supplements, but I think the custom formulas are better for cats (this is a niche site).
Before commiting to any of the "food trials" understand that NOTHING else can sneak in, even a tiny morsel of chick in a poultry-allergic feline will result in a failed trial.  The symptoms are fast to appear, and slow to disappear.
For dogs the trials are slightly easier as they do have some good options, including the hydrolyzed formulas that cats have.
If you have questions, please give me a call.