Food Allergy and Hydrolysed Diets

The response to dietary manipulation remains the cornerstone of the diagnosis of food allergy: clinical signs, whether dermatological or gastrointestinal, should resolve on exclusion of the offending dietary component and return with rechallenge. the prime requisite of an exclusion (elimination) diet is to contain only dietary components to which the animal has not previously (or recently) been exposed. options for exclusion diets include home-prepared diets, single-source protein diets, and hydrolysed protein diets. the exclusion diet should be the sole source of nutrition for the duration of the trial. Complete owner compliance is essential, and a role for hydrolysed diets is poten- tially to overcome some of the practical difficulties of diet trials.

Traditional exclusion diets are composed of single protein and carbohydrate sources. if a home-cooked diet is used, it may not be nutritionally balanced, but this is not usually important for the short duration of the trial. However, given the increased diversity of commercial pet foods more esoteric food sources may be necessary. a simple alterna- tive to home-cooked diets is the use of a commercial exclusion diet, and a wide range is now available, containing different sources of protein (e.g., chicken, soy, fish, catfish, venison, or duck) and carbohydrate (e.g., rice, maize [corn], tapioca, or potato). it should therefore be possible to find an appropriate diet in most cases, although manufacturers cannot always guarantee a constant supply of the diets with more exotic components. Hydrolysed diets offer a potential alternative to trying to find a suitable whole, single- protein exclusion diet but are significantly more expensive to produce than standard exclusion diets, and both palatability and owner compliance may be problematic. nev- ertheless, such diets are currently the easiest way to guarantee the feeding of novel antigens, and this approach has become the preferred method for many clinicians. the first veterinary hydrolysed protein diets contained peptides with an average molecular size in the range of 5 to 15 kilodaltons (Hill’s z/d uLtra allergen-free® and z/d Low aller- gen®, purina Ha®, royal Canin Hypoallergenic, affinity advance® Hypoallergenic). origi- nally marketed for the diagnosis and management of dietary sensitivities, hydrolysed diets may also play a role in the managment of acute enteritis, exocrine pancreatic insufficiency and inflammatory bowel disease: but most is known about their use in die- tary sensitivities
Hydrolysed diets are usually based on either chicken or soy protein and are now readily available commercially. in principle, the enzymatic hydrolysis process splits proteins into components of a molecular weight below that which would be expected to elicit an immune response. yet to completely abolish all antigenicity, peptides need to be less than 1 kD, but such oligopeptides often have a bitter taste. thus at the size achieved in the most palatable hydrolysed diets (5 to 15 kD), proteins are still potentially antigenic, but they are too small to be able to cross-link igE molecules on mast cells, and type i reactions are consequently abolished. However, the hydrolysis process itself might expose hidden antigenic epitopes, and type iV hypersensitivity reactions are still possi- ble, especially if the hydrolysed product is made from the native protein that the patient is sensitive to. Very recently, a diet containing free amino acids and much smaller (< 1 kD) oligopeptides (royal Canin anallergenic®) has been marketed; it is too soon to know whether this offers any advantage.