Coeliac disease (CD) is defined as a permanent intolerance to gluten, a protein found in cereals such as wheat, rye and barley, associated with mucosal disease of the proximal small bowel.1 The disorder is characterized by intestinal malabsorption, histological abnormalities of the small bowel mucosa, clinical and histological improvement on a ‘gluten-free’ diet, and relapse on a gluten-containing diet. Lifelong avoidance of gluten ingestion is the cornerstone treatment for CD.2
The true prevalence of CD is difficult to ascertain as many affected people are asymptomatic. The prevalence of the disease is estimated to vary between 1/100 and 1/500 in different continents.3 The seroprevalence of CD in the general population of both English adults4 and children5 has been reported to be about 1%.
Whilst it is generally accepted that avoidance of gluten is necessary for people with CD, the relationship between the quantity of gluten ingested and the development of symptoms and histological abnormalities is not clearly defined6 and the exact amount of gluten that people with CD can tolerate on a daily basis without suffering any deleterious effects has not been established. Total avoidance is also extremely difficult, if not impossible to achieve, as gluten contamination in ‘gluten-free’ products cannot be avoided completely.6, 7 Thus in CD, it is generally accepted that the term ‘gluten-free’ refers to a level of gluten that is supposed to be harmless, when ingested indefinitely, rather than to total absence of gluten.
Considerable controversy exists among authorities as to what constitutes a ‘gluten-free’ diet’. In 1998, the World Health Organisation / Food and Agriculture Organization’s Commission that sets international standards for foods, Codex Alimentarius, proposed a revision of their 1981 standard8 and suggested that naturally ‘gluten-free’ foods (i.e. food consisting of or made only from ingredients which do not contain any prolamins from wheat or all Triticum species such as spelt, kamut or durum wheat, rye, barley, [oats] or their crossbred varieties) should not contain more than 20 parts per million (ppm) of gluten but that foods consisting of ingredients from wheat, rye, barley, oats, spelt or their crossbred varieties, which have been rendered ‘gluten-free’ should not contain more than 200 ppm gluten.9 Although the current Codex recommendation of ≤200 ppm gluten in wheat starch-based ‘gluten-free’ products has been adopted in a number of countries including the UK and some European countries, it is not universally accepted. In some countries such as the USA, food made from wheat starch is not recommended, and a naturally ‘gluten-free’ diet is prescribed.10 These different practices reflect the fact that the exact amount of gluten that can be tolerated without harmful effects by patients with CD remains unclear.
In this study, we aimed to evaluate systematically the current evidence on the potential threshold amount of gluten which people with CD can tolerate, and to explore the evidence base of the Codex threshold level of gluten in ‘gluten-free’ products that can be tolerated by people with CD.