This paper reviewed 11 studies of hydrolysate-based diets used to help diagnose or treat dogs with cutaneous adverse food reactions. Three of the studies involved experimental sensitizations, 7 involved spontaneous sensitizations, and 1 involved both experimental and spontaneously occurring sensitizations. Diets were from Hill’s, Royal Canin, Nestlé Purina, and Iams. Four studies showed some evidence of immunologic changes as a result of feeding the diet (reduced serum IgE binding to a soy hydrolysate) and decreased intradermal test reactivity to hydrolyzed proteins (3 studies). Four studies reported that feeding of the diets reduced or eliminated the dog’s pruritus. However, in 3 controlled studies, up to 50% of dogs showed increased pruritus after eating a hydrolyzed diet that contained foods to which they were hypersensitive. Issues related to the diets included palatability, gastrointestinal signs (constipation, soft stools, and flatulence), and cost. In the studies, a major reason clients dropped out was the pet’s refusal to eat the diet. The authors concluded that there was evidence that these diets reduced but did not eliminate the allergenicity of the hydrolyzed diets.

Commentary: There are 3 types of food trials for dogs with cutaneous adverse food reactions: home-cooked diet protocols, novel protein diets, and hydrolyzed diets. These are diets in which the dietary protein has been synthetically reduced to small fragments that are theoretically below the molecular weight believed to trigger an immune response. This study is important because it confirms what has been clinically observed by many practitioners: Hydrolyzed diets work for some dogs but not all. Food trials are important in the evaluation of allergic dogs. There is no one perfect diet, and the most important factors to consider are cost, palatability, and, most important, owner compliance. To conduct a valid food trial, parasitic and microbial causes of pruritus must be ruled out; food trials are not first-line diagnostic tests. These trials are also a diagnostic test for dogs with year-round allergies, since food allergies are not seasonal. Next, the client must be motivated enough to commit to a strict 8- to 12-week diet trial. Strict implies no table food, no treats, no flavored tablets (vitamins or heartworm preventive agents), and no flavored chew toys. The dog must eat the diet exclusively; this often is not possible in households with children or visitors. In my experience, these clients need to schedule progress check visits every 2 weeks because they need a lot of moral support and monitoring. At the end of the trial, the extent of the dog’s overall improvement is noted, but the diet is not complete until a provocation test with the previous diet is performed. To confirm a food allergy or food-related increase in pruritus, the dog must relapse when fed its former diet. This is an important and overlooked step—because diet trials are conducted over long periods of time, there can be coincidental improvements that are due to changes in season.

A systematic review of the evidence of reduced allergenicity and clinical benefit of food hydrolysates in dogs with cutaneous adverse food reactions. Olivry T, Bizikova P. VET DERMATOL 21:32-41, 2010.