G. Gluten Challenge Studies There is little information in the literature on minimal disease-eliciting doses of gluten for sensitive individuals. Gluten challenges have generally been performed in individuals where diagnosis is uncertain (e.g., infants, Laurin et al., 2002) or in individuals with unclear intestinal pathology results (Wahab et al., 2001). Challenges have also been performed to determine the time of disease relapse after a prolonged period of gluten avoidance (Mayer et al., 1989). In most cases, gluten challenges have been performed to elicit or confirm disease rather than to measure the level of sensitivity (Farrell and Kelly, 2002).
There is no standard protocol for gluten challenges, and challenge studies have varied greatly in amount and duration of gluten exposure. Although some studies have been designed to determine the acute effects (i.e., after 4 hours) of exposure to gluten (Sturgess et al., 1994; Ciclitira et al., 1984), most challenges consist of an open challenge to a fixed or incremental dose of daily gluten over a minimum period of 4 weeks. Many challenge studies use a high exposure (> 10 g/day) to gluten, because this is believed to shorten time to disease confirmation or relapse and, therefore, to minimize discomfort to subjects (Rolles and McNeish, 1976). However, some studies have shown that low daily exposures to gluten also can elicit a disease response (Catassi et al., 1993; Laurin et al., 2002; Hamilton and McNeill, 1972).
Catassi et al. (1993) reported that children, whose celiac disease had previously been controlled on gluten-free diet, had evidence of intestinal mucosal or immunological changes (changes in intraepithelial lymphocyte counts and the villous height to crypt depth ratio) following 100 mg or 500 mg of daily gliadin over 4 weeks; this corresponds to 200 mg and 1000 mg of daily gluten respectively (Collin et al., 2004). The degree of inflammation was dose dependent. However, this study had several important limitations, which include the short-term follow up (4 weeks), testing in young children, the small number of subjects (n=20), and the lack of control groups. In addition, although gliadin is believed to be the major immunogenic portion of gluten, T cells from the small intestine of celicac disease patients have been shown to be responsive to peptides from the glutenin portion as well (Van de Wal et al., 1999). Thus, the Castissi et al. (1993) study was also limited by the use of gliadin rather than gluten. Estimating potential harm by extrapolating from gliadin levels may not be representative of the harm from total gluten exposure.
A study currently in progress [The Italian Microchallenge Study] has extended the scope of these earlier findings by evaluating the effects of exposure to either 10 or 50 mg of purified gluten per day for 3 months with a population of 36 celiac disease individuals in a double-blind, placebo-controlled study (Catassi et al., 2005b). Preliminary unpublished results suggest that minimal mucosal abnormalities occur with a strict gluten-free diet, that both 10 mg and 50 mg daily gluten are well-tolerated, but that there is a trend for mucosal changes to occur at the 50 mg dose. These results can be compared to estimated gluten exposures from gluten-free diets containing various levels of gluten contamination (Table III-1, from Collin et al., 2004, reproduced below). Fasano (2005 personal communication) used these values to suggest that a
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