Hilary A. Jackson,     Glasgow, Scotland

Indications for and Use of Elimination Diets

Elimination diets are designed primarily for the diagnosis and management of adverse reactions to food ingredients in dogs and cats. The following definitions are used in this discussion:

The ideal elimination diet used in the diagnosis and management of adverse food reactions should contain an ingredient that is either novel to the animal or in a form that does not incite an adverse response. Commercially available diets generally are designed for the management of true food allergy and thus often are referred to as hypoallergenic diets, although the incidence of immunologically mediated disease in the dog and cat currently is unknown. In humans most food allergies are mediated by immunoglobulin E (IgE) antibodies (Sampson, 2004), and many food allergens have been characterized at the molecular level; however, those inciting specific reactions in dogs and cats are largely unknown. Food allergens in humans generally are glycoproteins with a molecular weight of more than 10,000 D because smaller molecules are less likely to bridge IgE molecules on the mast cell surface and incite a hypersensitivity reaction. The currently available commercial elimination diets contain either novel or hydrolyzed proteins with added carbohydrate and nutrients to balance the diet for long-term feeding. It is assumed that the dietary protein is the allergenic substance, although there is a report of dogs with spontaneous food allergy reacting to cornstarch (Jackson et al, 2003). This chapter considers a number of key issues related to elimination diets.

Novel Protein Diets

Most novel protein diets have one carbohydrate and one protein source. After a detailed history of the patient’s diet is reviewed, a trial diet with a novel protein is selected on the assumption that the protein has not been fed before. This approach does assume complete recall by the owner of all diets, foods, and treats to which the patient has been exposed. It also assumes that commercial diets are not contaminated with foreign proteins, which may not necessarily be the case (Raditic et al, 2011). A recent study found that three out of three over-the-counter limited-antigen diets tested positive for soy protein and one quarter tested positive for beef protein; neither soy nor beef was on any of the diet ingredient lists (Raditic et al, 2011). The presence of cross-reacting protein epitopes in different foods may also be a factor. For example, if the author is treating a patient with chicken intolerance, then intolerance to other poultry meats is considered possible, and turkey- or duck-based diets also are avoided.

Hydrolyzed Protein Diets

Hydrolyzed protein diets are available for humans, specifically for the management of infants with milk allergy. Hydrolyzed protein is available as either a partial or a complete hydrolysate. Before a claim of hypoallergenicity can be made, the complete hydrolysate formula should be demonstrated to be tolerated by 90% or more of infants with milk allergy in double-blind, placebo-controlled conditions (Baker et al, 2000). Complete milk protein hydrolysates have free amino acids and peptides of less than 1500 D, whereas partially hydrolyzed formulations are not hydrolyzed as extensively. The hydrolyzed veterinary diets available at the time of this writing would be classified as partial hydrolysates. Typically these include common sources of protein such as chicken or soy. A newer product on the market based on feather protein represents both a novel and a highly hydrolyzed protein source.

Only a small number of studies have been performed to address the issue of whether these diets are clinically effective and have reduced immunogenicity in the food-allergic animal. These studies have been reviewed by Olivry and Bizikova (2009), who concluded that there was limited evidence of reduced immunologic and clinical allergenicity in dogs with suspected adverse food reactions; furthermore, between 20% and 50% of dogs actually experienced a worsening of clinical signs when fed these diets.

Additionally, these diets are recommended for the treatment of canine and feline adverse food reactions based on the assumption that most adverse food reactions in these species are mediated by IgE, as they are in humans. Since this is not necessarily the case, the size of the protein hydrolysate may not be critical or, paradoxically, hydrolysis could enhance uptake and presentation to the immune system.