Food allergy & intolerance
Adverse Food Reactions
Food Intolerance / Insensitivity
Feeding the best food and ingredients is great unless your dog or cat has an adverse reaction to the food. An Adverse Food Reaction (AFR) “is an abnormal response to an ingrested food or food additive.”  Adverse food reactions can be caused by a food allergy or food intolerance/sensitivity. Food intolerance or sensitivity is more common than a food allergy.
The processing of food or the lack thereof can have an affect on foods since the effect of heat may disrupt some antigens, uncover others, or create new ones.  Signs of Adverse Food Reactions (AFR) can include vomiting, diarrhea, increase in the number of bowel movements, “intermittent addominal pain, intermittent diarrhea, weight loss, flatulence, irritable demianor, soft feces and increased frequency of defecation”.  Itching and skin conditions can be caused by many things: environmental allergies, food intolerances/sensitivities, fleas, and food allergies. Itchy dogs with more than three bowel movements per day are more likely to have an adverse reaction to food. 
Rotating foods every two to three months can do more then add to our dog's enjoyment of his food, it may help to avoid the development of particular food sensitivity and food allergies. The greater the length of time and frequency a particular food is consumed the greater likelihood an intolerance/sensitivity can develop over time. 
Food allergies reflect a more immediate immunological response whereas food intolerances/sensitivities build up over time with exposure to offending ingredient(s). “the body produces the antibody IgE to fight off a food allergy and reacts immediately and violently (anaphylaxis). However, these types of true food allergies are rare.
Food Intolerance / Sensitivity
“Food intolerance mimics food allergy except that it can occur on the first exposure to a food or food additive, because nonimmunologic mechanisms are involved.” 
Food intolerances/sensitivities can manifest in (GI) gastrointestinal issues, chronic itching, gas rumblings, chronic gas, chronic burping, chronic skin, ear and foot infections, especially with the presence of yeast.  Excessive licking of surfaces can also be a sign of digestive disorders.
I have found that a small amount of an offending ingredient causes issues. Reactions can be caused by the differing proteins, oats, millet, quinoa, lentils, barley, salmon oil, herring oil, etc.
“Irritable bowel syndrome is a disease of dogs characterized by chronic recurrent abdominal pain and large bowel diarrhea (Guilford, 1996a). Feeding changes will often alleviate the signs of irritable bowel disease, implying that food sensitivity plays a role in this syndrome. … In affected dogs, the adverse reactions to these nutrients are most likely due to food intolerance rather than a food allergy.” 
Food Allergy Tests
Typically, testing for food allergies involves either a skin prick test or a blood test. (Antibodies IgE and IgG) “Throughout the veterinary profession, these tests for food allergies are considered unreliable and inaccurate.”
“Some tests (e.g., measurement of food-specific serum IgE) sugest that an adverse reaction to a particular food (identified in an elimination-challenge trial) may be due to a type-1 hypersensitivity response rather than another type of allergic reaction or a food intolerence. However, at the present time, intradermal testing, radioallergosorbent assays (ELISAs) for food hypersensitivity are considered unreliable in patients with dermatologic (Jerrers et al, 1991; Kunkle and HOrner, 1992) and GI disease (Foster, 2003).” 
Food Elimination-Challenge Trials
Since a skin prick or blood tests are not considered accurate for determining food allergies, food trials are recommended. A food elimination-challenge trial requires feeding a limited diet to see if the elimination of certain foods alleviate clinical signs of an adverse reaction. If clinical signs are eliminated the patient is feed the original food to see if clinical signs of an adverse reaction return. If clinical signs return the elimination of the food once again should alleviate the clinical signs and confirm the food as the source of a adverse food reaction (AFR). “The elimination diet trial should be performed for a minimum of 6 to 8 weeks and ideally 10 to 12 weeks… Although maximum improvement may take up to 10 to 13 weeks,” 
A limited ingredient food (Limited Ingredient Diet - LID) does not have a standard definition. It generally means the food is limited to one animal protein. Some manufactures state that their food is limited to one animal protein and one carbohydrate. And still others define it further as having an overall lower number of ingredients. Many pet owners select a commercial limited ingredient food in an attempt to avoid foods that may be the cause of food allergies or food intolerance/sensitivities. A problem found with several limited ingredient diets is that they often contain additional food ingredients not listed on the label.
” ten of twelve pet foods tested herein as limited antigen diets may not reliably rule out a diagnosis of AFR, and the use of homecooked diets should be considered whenever the dog fails to respond to dietary restriction” https://onlinelibrary.wiley.com/doi/epdf/10.1111/jpn.12045
“ten of the twelve selected commercial dry limited antigen diets, all novel protein diets, were unsuitable for use in diagnostic food elimination trials because they contained ingredients belonging to one or two zoological classes (mammalian, avian or fish) not listed on the label.” https://onlinelibrary.wiley.com/doi/epdf/10.1111/jpn.12045
“It is noteworthy that dietary trials confirm or rule out adverse reactions to food but do not indicate the underlying mechanism (allergy or intolerance).” 
Food Intolerance Test
Since food intolerance or sensitivity is more common than a food allergy I start with a NutriScan Food Intolerance/Sensitivity test to eliminate the guesswork and putting dogs through weeks/months of food trails and continually going to the vet to manage symptoms. NutriScan tests a dog’s saliva for antibodies IgA and IgM. With the NutriScan kit you collect saliva with a small cotton dental rope. You can do this at home or at the vet’s office. Then ship the kit back to Hemopet for testing. “Nutriscan is the only clinically predictable diagnostic test for dogs, cats and horses to identify the commonly seen food intolerances and sensitivities in saliva.”
I have used and recommend food sensitivity & intolerance test from NutriScan. I ran two NutriScan tests on my Dalmatian and one test on my shepherd mix. My Dalmatian had reactions to almost everything which required me to change and limit her diet. I ran the second test on my Dalmatian about 10-months after the first test and the numbers were dropping (although far from good) on the foods I had eliminated from her diet. It was very helpful after getting results and eliminating foods from both of my dogs diet. Less gas, less bouts of diarrhea, and overall saw an improvement in well-being. My two dogs always loved to eat grass and we would say that they just liked their salad and that it was normal. But after running a NutriScan food intolerance test on each of them and eliminating several foods from their diet their grass eating reduced about 95%. (after eating grass they were not throwing up). For me it is essential to identify and eliminate any food from the diet that can create intolerance. And NutriScan helped eliminate the guesswork with weeks of food trails.
I have seen two other food intolerance “tests” that are marketed. One of the “tests” reports to use “bio resonance technology” and the other reports to use a “biofeedback technician” in their biofeedback “test”, “which has the ability to read the energetic resonance that emanates from the hair and saliva samples”. Neither of these “tests” reports any actual science to back up their claims from what I can determine. I personally do not consider either of these “tests” accurate or reliable for determining true food intolerance. I have a strong bias in support of the NutriScan test created by Dr. W. Jean Dodds. Dr. Dodds is a veterinarian who has spent more than five decades as a clinical research veterinarian.