Gluten Free Watchdog comment on the recent gluten cross contact study “Preparation of Gluten-Free Foods Alongside Gluten-Containing Food May Not Always Be as Risky for Celiac Patients as Diet Guides Suggest”

Gluten Free Watchdog comment on the recent gluten cross contact study “Preparation of Gluten-Free Foods Alongside Gluten-Containing Food May Not Always Be as Risky for Celiac Patients as Diet Guides Suggest”

The study, “Preparation of Gluten-Free Foods Alongside Gluten-Containing Food May Not Always Be as Risky for Celiac Patients as Diet Guides Suggest” was recently published online by the journal Gastroenterology.  It is available in full at

Personal note: I have been out of the country and a little late to the party in terms of commenting on this study. Thank you to everyone who reached out over the past week for your patience. A comment from Adrian Rogers, Senior Research Scientist at Romer Labs is also included in this post.

Study Aim:

According to the authors, the primary aim of the study was “to quantify gluten transfer when GF foods are prepared alongside gluten-containing foods.” To this end, the gluten content of gluten-free foods prepared under three scenarios was assessed, namely:

  • Gluten-free pasta cooked in shared water previously used to cook wheat pasta
  • Gluten-free bread toasted in the same rolling toaster or pop-up toaster used for wheat bread with visible wheat crumbs present
  • The same unwashed knife used to first cut a frosted wheat-based cupcake and then a gluten-free cupcake

Study Findings:

  • When gluten-free pasta was cooked in water previously used to cook wheat pasta, all 12 samples of pasta contained greater than 20 parts per million of gluten.
  • When gluten-free bread was toasted in a shared rolling toaster (which may be used commercially in restaurants, hotels, hospitals, etc.), 16/20 slices tested contained a level of gluten below the limit of quantification of 5 parts per million of gluten and 4 contained between 5 and 10 ppm of gluten.
  • When gluten-free bread was toasted in a shared pop-up toaster, all 20 slices tested contained a level of gluten below 5 ppm.
  • When gluten-free cupcakes were cut with an unwashed knife previously used to cut wheat-based cupcakes, only 2/30 cupcakes tested above 20 ppm of gluten. 

Praise for this study:

There is a tremendous need to increase our understanding of gluten cross contact, including when and how it occurs in both home and commercial kitchens to help inform evidence-based recommendations.  The authors should be universally applauded for the study conception and the development of the various scenarios for testing. An accredited lab—Bia Diagnostics—conducted the testing using the fully scientifically validated R5 ELISA Mendez Method. There is no reason to doubt the results of the extractions tested.  

Concerning aspects of this study:

  • A single extraction was tested from each homogenized sample.
    • The kit instructions from the assay manufacturer R-Biopharm explicitly state, “In order to ensure a high analytical performance: Each sample material should be analyzed in duplicates.” When a sample is homogenized the goal is to evenly distribute any gluten present. However, this does not always happen and in fact may not always be possible (try putting three chocolate chips into a batch of sugar cookie dough and evenly distributing them using a blender). Cross contact by nature can be very spotty, including when it comes to crumbs. The standard extraction amount is 0.25 grams.  A slice of bread may weigh at least 28 grams. Even if the bread is homogenized in a blender there may be 0.25-gram portions without any wheat crumbs present. Testing at least two extractions provides some insight into whether the sample is homogeneous in terms of gluten.
  • Some of the results should give everyone pause and should not necessarily be taken at face value.
    • For example, does it make intuitive sense that 16 of 20 slices of gluten-free bread toasted in a rolling toaster (where the bread slice actually lands surface down on wheat crumbs) contain a level of gluten below 5 ppm? Might it be the case that what would be eaten after using such a toaster might differ from what was tested at the lab? Could the wheat crumbs have been dislodged from the surface of the gluten-free bread during collection and placement into plastic bags prior to sending to the lab? Was the lab able to remove all crumbs from the bag? Was gluten distributed evenly throughout the sample? Initial test results should have prompted additional testing of extractions from each bread slice.
  • Should certain study conclusions have been made?
    • For example, the study states, “Toasting in a shared toaster was not associated with gluten transfer above 20ppm.” Is this really the case? Does testing a single extraction from a slice of bread provide enough data to make this statement? If additional extractions had been tested, would the findings and conclusions have been the same?

Final Thoughts

  • Cross contact must continue to be addressed and assessed.
  • One study should not change recommendations.
    • Please don’t use a shared toaster oven or pop up toaster without cleaning the grill or using toaster bags. And please do not use a shared rolling toaster when eating away from home.
    • Please don’t use shared utensils that haven’t been washed between uses.
  • More studies are needed.
    • If you conduct or participate in such a study, please make sure that more than one extraction is tested per homogenized sample.
      • If the results don’t make sense, additional testing may be warranted; Alternative reasons for your results should be considered.

Comment from Adrian Rogers, Senior Research Scientist, Romer Labs: More testing is needed to get a picture. This could be a collaborative project with different researchers repeating the same set of experiments in duplicate or triplicate (Tricia’s note—this means testing 2 or 3 extractions from each homogenized sample). It is all about numbers–a statistician should be consulted to make sure the results are statistically relevant given the nature of the type of contamination.

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Comments (8)

  • Tom Baker MD Reply

    This careless disregard of gluten cross contamination in restaurants is nonsense.
    I personally was sick for five days from traces of gluten at a restaurant salad bar.
    The employee was reaching across several bowls filling them and crumbs fell into several bowls nearby.

    October 8, 2019 at 11:31 am
  • Helen Weems Reply

    Thank you for your clear-eyes and scientific assessments of the GF literature, as always, Tricia.

    October 8, 2019 at 1:23 pm
  • Maya Reply

    I would be curious to know an approximate range of average of how much gluten is in a single crumb of wheat bread. Information like this helps in terms of understanding what the riskiest types of food are when traveling or eating out and specifically what to request in terms of preparation.

    Also, it would be interesting to know how much is really in Tupperware, cutting boards, etc? I suspect if you can’t remove or dislodge the gluten by scrubbing vigorously, it stands to reason that it’s not going to get dislodged and end up in food. I changed all my kitchen equipment eventually, but it was aggravating and expensive and I question if it was truly necessary, but there doesn’t seem to be an actual study on this, as opposed to just assumptions and anecdotes. I personally didn’t feel better after having changed my cutting boards, toaster oven rack, etc. but I am symptomless so it’s not entirely surprising.

    October 8, 2019 at 2:16 pm
  • Kathy Reply

    I was upset by this study, because it shares confusing points of view and makes us with celiac look like were are being over cautious, it’s very much a good news bad news article but over all in my opinion white washes the real danger of cross contamination issues at home and in public.

    October 8, 2019 at 8:33 pm

    Thank you, Tricia–as usual your final thoughts are “spot on”—and for what it is worth I agree whole heartedly with them all. Perhaps the results of this study are valid but to me they just do not make sense—gluten is water insoluble yet ALL of the gluten free pasta samples cooked in the same water contained >20ppm gluten –even after 12 uses: decidedly different from the toaster results where obviously visible crumbs are present. With the rolling toaster 4 of 20 samples tested between 5-10ppm—perhaps these were the first 4 tested and “used up” most of the crumbs—leaving much less for the remaining 16—does not sound likely but cannot explain these results otherwise, except for yours, that it is a homogenization/distribution factor; also at least at home it is very unlikely someone is toasting 16 pieces of bread consecutively, but only one or two and randomly one may get more crumbs than another– likewise the other results with the pop-up toaster and the knife just do not make intuitive sense–obviously, the usual caveat—await further studies—with yours and Adrian Rogers recommendations

    October 11, 2019 at 7:45 pm
  • Joan Williams Reply

    I was diagnosed with Celiac 40 yr ago. I’m now 80 & in very good health but think about what will happen to my health if I go to a nursing home. I think will be sick all the time as cooks won’t be trained to not use the same spoon to stir GF & non GF & that is the least of the worries.
    I had spinal surgery 3 years ago & was in swing bed for recovery & PT in our local Hedrick Medical center owned by St. Lukes. Gluten Free food was non existent-was told that when they stocked GF it got out dated & threw away. I was very vigilant about what I ate there but not much to pick from & I let all employess that came in my room know my thoughts on their uncaring way of gluten free patients & assured them I would steer my GF friends away from this hospital.
    I don’t care if this gets posted as long as you read it & realize the situation in hospitals.

    October 29, 2019 at 2:35 pm

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