Gluten in human and canine health

Introduction

Gluten is the term used to describe plant proteins found in the endosperm of cereal grains and which support the nutritional needs of germination and growth. Scientists have long been interested in gluten, with analysis allowing characterization of its composition, and proteins of various cereal grains are now separated into four categories, namely albumin, globulin, prolamin and glutelin 1. Albumin and globulin are water-soluble, whilst prolamin can be obtained with an alcohol process, and glutelin can be extracted with an alkaline solvent. Prolamin is named due to its high proline and glutamine content, and is also known as gliadin, hordein, secalin, avenin, and zein in grains derived from wheat, barley, rye, oats and corn respectively. Glutelin represents proteins incorporating enzymes and cell walls, and is known as glutenin in cereal grains derived from wheat. Some authors advocate that the term “gluten” should be reserved strictly for gliadin and glutenin, the water-insoluble wheat grain proteins.

As well as its role in supporting germ growth, gluten is also important in food engineering. The disulfide bonds between gluten amino acids are responsible for the viscoelastic and adhesive properties of leavened doughs, and contribute to the interior structure of a chewy bread loaf, and adding extra gluten to doughs is an efficient and economic approach to improve the texture and the mouthfeel of baked goods. However, gluten also contributes to the development of various disorders such as celiac disease in humans. It is well known that consuming cereal grains derived from plants within the Triticeae tribe, including wheat, barley and rye, can induce clinical signs in susceptible individuals. Similar signs can also be triggered by certain cultivars of oat and oat grains when processed in milling plants used for wheat, barley and rye 2. While the term “gluten” is commonly used to refer to all cereal proteins, the gluten derived from corn and rice is not associated with gluten-related disorders in humans, and this article will use the term exclusively to describe proteins derived from barley, rye, oats and wheat.

Gluten-related disorders in humans

“Gluten-related disorders” is a collective term used to describe clinical signs associated with gluten exposure. Based on the differing pathogenesis and etiology, these disorders can be categorized into the following three groups 3.

Allergic reactions

Allergic reactions to gluten can be either type I or type IV hypersensitivity, and can be delayed or show immediate onset after exposure in susceptible individuals. Where immunoglobulin E (IgE) is involved, the crosslink between IgE and gluten triggers cellular cascades in mast cells and basophils that results in degranulation and release of cytokines and inflammatory mediators such as histamine. These induce clinical signs which will vary depending on where in the body the reaction occurs, and include vomiting, diarrhea, pruritus, atopy, asthma and rhinitis. Gluten-related disorders with allergic reaction involvement are collectively known as wheat allergy, one of the most common food allergies worldwide 3.

Autoimmune reactions

The classic example in this category is celiac disease. When a susceptible individual consumes products containing wheat, the presence of gliadin in the gastrointestinal (GI) tract undermines the integrity of enterocyte tight junctions and alters intestinal permeability 4. Leaked gliadin stimulates systemic immune responses and results in the production of anti-gliadin (AG) antibodies. Along with increased intestinal permeability, gluten assimilation also contributes to the development of autoimmune reactions to tissue transglutaminase (tTG) 5. This is a multifunctional enzyme found ubiquitously in the human body, and is responsible for the deamidation and transamidation of glutamine in the GI tract. Because of the high glutamine content in gliadin, tTG forms tight crosslinks with the gliadin, which then generates new antigenic epitopes resulting in the development of autoantibodies against tTG. In addition to the direct effect of gluten, genetic predisposition in celiac patients is known to contribute to their vulnerability; most celiac patients carry specific variants of human leukocyte antigen (HLA) genes, which exacerbate the immune reaction via increased lymphocyte activation 3. The multisystem involvement and autoantibodies make the autoimmune reaction to gluten distinct. Overall, celiac patients suffer from malabsorption and other GI signs due to the damaged enterocytes and villous atrophy that develop after gluten exposure (Figure 1).

Anti-tTG antibodies are thought to contribute to the development of dermatitis herpetiformis (Figure 2) and gluten ataxia, the other two forms of gluten-related autoimmune reactions 6. It is noteworthy that although signs of these disease processes may overlap, the predominant clinical sign is usually driven by the organ or tissue primarily involved. For example, due to the involvement of Purkinje cells and the cerebellum, patients with gluten ataxia usually present with movement disorders, including ataxia, tremor, and myoclonus 6. Analysis of serum anti-tTG and AG antibodies are therefore valuable tools to facilitate diagnosis in suspected cases of autoimmune reactions to gluten.

Other gluten-related reactions

Some individuals develop reactions following gluten consumption where neither allergic nor autoimmune mechanisms can be identified. The clinical signs are similar to those reported in individuals with allergic or autoimmune reactions to gluten, but histopathological examination of intestinal biopsies is usually unremarkable, and although elevated serum AG antibodies are sometimes seen, serum anti-tTG antibodies are not evident 3. Given the similarities in clinical signs, the diagnosis of “neither allergic nor autoimmune reaction” can only be achieved by exclusion, hence this category is also known as non-celiac gluten sensitivity 3.

Despite this well-established categorization, the true prevalence of gluten-related disorders in humans remains unclear, because most individuals who experience pertinent clinical signs usually attempt diet modification on their own without formal medical advice. In suspected cases, diagnostic tools such as serum antibody profile, genetic typing, and intestinal biopsies facilitate the investigation. Although a causal relationship between a dietary component and a disease is usually not easy to establish, an elimination diet trial followed by provocation tests with pure gluten or gluten-containing foods can be helpful to connect gluten exposure to clinical signs. It is noteworthy that withdrawal of dietary gluten can result in false negative tests, including unremarkable serum antibody profiles and resolved histopathological changes in intestinal biopsies from celiac patients 6.

Gluten-related disorders in dogs

Clinical signs associated with dietary gluten have been reported in certain breeds of dogs in recent decades. The link between gluten and an enteropathy seen in Irish Setters has been studied since the late 20^th century, with one study attempting to characterize the condition and to assess the difference between the condition and human celiac disease 7. In this study, Irish Setters referred for difficulty in maintaining body weight were assessed (Figure 3). Participating dogs had unremarkable fecal examinations, serum cobalamin concentrations, and exocrine pancreatic function, but histopathology on small intestinal biopsies revealed villus atrophy of varying severity which could result in malabsorption and consequently difficulty in maintaining weight. However, the study did not thoroughly assess the dietary history of these dogs, and a definitive causal relationship between gluten and the enteropathy could not be established.

Another study reported improved to resolved clinical signs (such as diarrhea) and histopathological abnormalities of intestinal mucosa after a cereal-free diet was introduced to susceptible Irish Setters reared on a wheat-containing diet 8. These dogs then were challenged with pure gluten, and experienced relapsed clinical signs along with recurrent villus damage. However, this study did not report pertinent serological profile in these dogs to support the diagnosis of celiac disease.

Despite the possible causal relationship suggested in these reports, a study could not find significant differences in serum AG antibody concentration between susceptible Irish Setters reared on either wheat-containing or cereal-free diets, and the concentrations of serum autoantibody and other immunoglobulins were not significantly different between the two groups 9. Given the lack of systemic and autoimmune involvement, the enteropathy reported in this breed seems to be different from classically defined celiac disease, and although the pathogenesis and genetic predisposition of the Irish Setter enteropathy remain to be determined, these dogs do seem to benefit from gluten-free diets. These findings, along with recent trends in human nutrition, have made dietary gluten a frequent focus of discussion between clients and veterinarians.

Gluten has also recently been reported to be associated with paroxysmal dyskinesia (PD) in Border Terriers (Figure 4). Paroxysmal dyskinesia is the term given to a group of conditions with episodic movement disorders, including dystonia and tremor, which can be triggered by stimulations such as excitement, exercise, or stress 10. These episodes are noted in young and middle-aged dogs of certain breeds, including Border Terriers, Cavalier King Charles Spaniels, Chinooks, Norwich Terriers, and Soft Coated Wheaten Terriers 10. Despite the abnormal involuntary movements, which can last for minutes to hours, patients seem to remain conscious throughout these episodes, and (unlike some neurological deficits often seen after a prolonged epileptic seizure) recovery from these self-limiting episodes is uneventful, with the dogs behaving normally between episodes 10.

Although the pathogenesis is not fully understood, gene mutations as a possible cause of PD in dogs have been discussed. One study has reported a microdeletion of the BCAN gene in Cavalier King Charles Spaniels 11. This gene encodes for brevican, a proteoglycan constituting the extracellular matrix in the brain and responsible for maintaining synaptic stability, and the BCAN mutation is highly associated with episodic falling syndrome, a paroxysmal movement disorder seen in Cavalier King Charles Spaniels 11.

On the other hand, dietary components seem to play an important role in PD in Border Terriers (previously known as canine epileptoid cramping syndrome, or Spike’s disease). One study has attempted to phenotype PD in the breed, and included eligible dogs based on owner observation, video recording, and medical records 12. Interestingly, the study reported that dermatological signs (including frequent pruritus), as well as mild GI signs (e.g., vomiting and diarrhea) were reported in 50% of dogs assessed. In addition, many dogs that had had a dietary change (to either a “hypoallergenic” diet or one that had a single protein and carbohydrate source) experienced a reduction in PD frequency, which was suggestive of an underlying food allergy or intolerance as the cause. The report also noted that breeders and owners of affected Border Terriers have anecdotally reported fewer movement disorders after changing affected dogs onto a gluten-free diet.

A further small study recruited 6 eligible Border Terriers dogs with PD and performed a strict 9-month elimination diet trial using a veterinary therapeutic diet formulated with a limited number of hydrolysates which appeared to be gluten-free. This resulted in a significantly reduced frequency of PD episodes in dogs of compliant owners, and additionally the concentrations of serum AG antibodies and anti-tTG antibodies in these dogs were also reduced 13. Despite the small sample size, the study demonstrated a positive response to a gluten-free diet in the affected dogs.

In a follow-up study, researchers categorized participating Border Terriers into four groups based on the clinical diagnosis in an attempt to characterize the serological profile of dogs with PD 14. Besides groups of healthy dogs, dogs with presumed PD, and dogs with presumed idiopathic epilepsy, other participating dogs were categorized into the fourth group (individuals with ambiguous clinical diagnoses or with non-neurological conditions). None of the included dogs were fed gluten-free diets. The authors found that elevated concentrations of serological markers, specifically anti-tTG and AG antibodies, were not seen in all the Border terriers with presumed PD. In addition, elevated concentrations of serological markers were not unique to dogs with clinical signs consistent with PD. As with humans with gluten-related disorders, Border Terriers with elevated concentrations of serological markers seemed to present with a spectrum of clinical signs. However, these serological tests do appear to have high specificity for diagnosing paroxysmal gluten-sensitive dyskinesia in the breed 14. Although the diagnostic value of serological markers can be diminished once affected dogs are transitioned to a gluten-free diet, the combination of serological marker tests, complete diet history, and diagnostic tools to exclude extra- and intra-cranial causes can be useful in investigating PD in Border Terriers.

Nutritional considerations in dogs with gluten-related disorders

Currently, epidemiology data on gluten-related disorders in the general canine population remain to be determined. Moreover, further investigation is required to better understand the phenotyping and pathogenesis of such disorders. Despite these knowledge gaps, dogs of certain breeds that present with clinical signs and a diet history consistent with gluten-related disorders can still benefit from dietary interventions according to available data.

Where a gluten-related disorder is suspected, the goal is to offer a complete and balanced diet formulated with gluten-free ingredients that is appropriate for the dog’s life stage, alleviates clinical signs, and provides an adequate calorie intake to encourage or maintain an ideal body condition. In the USA the Food & Drug Administration (FDA) dictate the requirements for “gluten-free” labeling of foods, allowing humans with gluten-related disorders to maintain a healthy diet by careful product selection 15. Despite the convenience, it is noteworthy that the intentional exclusion of certain dietary components can sometimes result in nutritional deficiencies, including dietary fiber and zinc, and professional guidance is recommended to ensure a complete and balanced diet 16. In veterinary medicine, nutritional inadequacy is also reported in home-cooked diet plans formulated by non-professionals 17. Although a complete and balanced customized home-cooked diet excluding gluten can be formulated by a board-certified veterinary nutritionist, feeding such a diet is not always feasible, given the work and resources involved in its preparation 18. Fortunately, there are now plenty of commercial dog foods formulated to meet the nutritional needs of different life stages, lifestyles, and disease processes, and owners can explore the most appropriate diet for their dog under veterinary guidance (Figure 5). Any gluten-free diet manufactured by a reputable company can be considered, and a WSAVA checklist also aids evaluation of pet foods and pet food companies 19.

As readily available regulations regarding gluten-free claims have not been established by the relevant body – e.g., the Association of American Feed Control Officials (AAFCO) – determining whether a commercial pet food product is gluten-free requires a thorough assessment of the ingredient list on the package, which is sometimes an educated guess because quality control of pet food companies varies 20. One study reported that some over-the-counter “limited-ingredient” commercial diets contained animal proteins not declared on the packaging label 21. Although limitations in this study exist, including batch variation and false positivity in such sensitive tests, these findings implied possible cross-contamination during the manufacturing process, which can make over-the-counter limited-ingredient commercial diets less than ideal when an elimination diet trial is needed. In the author’s opinion, a veterinary therapeutic diet that is formulated with a limited number of gluten-free ingredients and is produced by an experienced manufacturer with excellent quality control serves as a good choice when performing such a trial. Potential options include many of the diets formulated to treat adverse food reactions, or hypoallergenic diets designed for dermatology-related food allergies.

Note that commercial diets do vary significantly in many ways, including how it is manufactured and the nutritional profile, so the specific name of the manufacturer and product, the type (dry or wet), and the amount to be fed daily must be provided to the client when a diet is recommended to avoid misunderstanding. Depending on clinical signs, the body system involved, and the patient’s response, an elimination diet trial may last for between 1-2 weeks for gastrointestinal signs and up to 12 weeks for dermatological signs. Ideally, the trial should be followed by provocative testing to assess the patient’s response to food items suspected to be associated with clinical signs. Such tests will help confirm the diagnosis of a gluten-related disorder, and indicate the need to start a lifelong monitoring and gluten-free diet plan for affected dogs.

Finally, while clinical signs of gluten-related disorders have only been documented in Border Terriers and Irish Setters to date, a gluten-free diet is sometimes requested by clients due to their own adverse reaction to gluten, which can be life-threatening in susceptible individuals. Under such circumstances the request should be taken seriously, and due consideration given to identifying a suitable dietary plan for the dog.

Conclusion

Gluten-related disorders have been reported in both humans and dogs, but can be managed with careful exclusion of dietary gluten. Although the prevalence and pathogenesis of gluten-related disorders in dogs remain to be elucidated, the positive response seen in affected animals warrant timely dietary intervention when appropriate. As gluten is a major topic for both human and canine nutrition, a sound knowledge base will allow the veterinarian to best advise pet owners if and when they need to alter their dog’s diet.