The simple answer is, "That depends." People's levels of sensitivity to gluten cross-contamination vary tremendously, and it also matters whether you have a one-time exposure or whether you're consuming gluten every day over a period of weeks or months.
But it may take less gluten than you think ... possibly a lot less. Bear with me there's lots of number-crunching involved in this.
Major Glutenings Can Be DamagingYou certainly can get very sick and also damage your intestines by eating even a tiny amount of conventional bread or other gluten-containing substance.
For example, one study showed that consuming just 1/5th of a slice of regular bread (about 625 milligrams of gluten) one time is enough to cause nasty symptoms, including severe diarrhea and vomiting, and increase villous atrophy in your small intestine.
That makes a delicious-looking gluten-filled cookie a bit less appealing.
Of course, many of us have experienced symptoms from way less gluten than that. Two older studies found symptoms coupled with increasing intestinal inflammation (but not necessarily villous atrophy) in people who consumed just 24 to 30 milligrams of gluten about 1/145th of a slice of conventional bread (otherwise known as a crumb).
Small Amounts of Gluten Daily Add UpConsuming small amounts of gluten more than 50 milligrams, or about 1/70th of that slice of bread on a daily basis also can add up to increased symptoms and intestinal damage.
A 2007 study led by Dr. Alessio Fasano, who heads the University of Maryland's Center for Celiac Research, found that people who consumed 50 milligrams of gluten each day had renewed villous atrophy after 90 days, while those consuming zero gluten or 10 milligrams of gluten each day did not.
Dr. Fasano and his colleagues say that many or most people with celiac disease can handle up to 10 milligrams of gluten the equivalent of 1/8th of a teaspoon of flour, or 1/350th of that slice of bread in their diets each day without experiencing adverse effects. The study frequently is cited as evidence that celiacs can handle "gluten-free"-labeled foods with up to 20 parts per million of gluten in them.
(For even more math that explains the concept of "parts per million" and how it translates into how much gluten you're eating, have a look at this article: What Does It Mean for Products to Have Less than 20 Parts Per Million of Gluten?.)
Still, an analysis by the U.S. Food and Drug Administration (FDA) alluded to a real drawback of that University of Maryland study: It chose people to participate whose intestines were well-healed, meaning those people likely were less-sensitive to gluten cross-contamination than people who hadn't healed on the gluten-free diet (many people's intestines fail to heal completely, even after years on the diet). Even so, one person in that study who was consuming 10 milligrams of gluten per day not 50 milligrams, the highest level developed a full "clinical relapse" and dropped out of the study due to intolerable symptoms.
So, What Does This Mean To Me?You're probably wondering by now what all of this means to you after all, if you're careful, you're probably following a strict gluten-free diet with no cheating, which means you're 100% gluten-free ... right?
Well, no. It's just about impossible to be 100% gluten-free because "gluten-free" foods actually contain gluten. Grain products those gluten-free breads, cereals, waffles and crackers are the worst offenders.
Therefore, someone eating a "typical" gluten-free diet that contains normal amounts of grain products is consuming about 6 to 10 milligrams of gluten each day (again, up to 1/8 of a teaspoon of flour, or 1/350th of a slice of gluten bread) as part of those "gluten-free"-labeled grain products. That's according to the University of Maryland, which considers this amount safe.
The FDA came to a different conclusion in its analysis: It found that for the most sensitive people, intestinal damage begins at 0.4 milligrams of gluten per day (1/200th of a teaspoon of flour or 1/8,750th of that slice of bread), while symptoms begin at 0.015 milligrams of gluten per day (less than 1/500th of a teaspoon of flour or 1/233,333th of that slice of bread). The agency based those conclusions on various studies, including two case studies involving recurrent symptoms in people who consumed communion wafers once each week.
Bottom Line: Your Mileage May VaryThere's plenty more research to be done on this. Still, there's little doubt that sensitivity to trace amounts of gluten represents a spectrum.
At one end of that spectrum, you've got people with silent celiac disease, who don't get symptoms even if they eat huge amounts of gluten. At the other end, you've got people who are extremely sensitive to gluten cross-contamination and who cannot achieve good health unless they take steps to eliminate all sources of gluten in their diets, including many or all "gluten-free"-labeled foods.
Determining where you fall on that spectrum is a matter of trial and error. But it's clear that some people react to foods that others can eat with no problem.
To Sum It All Up Although a few won't react at all, most people will get really sick from the equivalent of 1/5th of a slice of gluten-containing bread (or more).
People who are doing well on the gluten-free diet (i.e., whose intestines have healed) may be able to handle eating normal amounts of gluten-free-labeled foods (which contain a little bit of gluten) without symptoms or intestinal damage.
People who continue to have symptoms, and whose intestinal damage hasn't healed completely despite eating a careful and strict gluten-free diet, may need to drop potential sources of trace gluten from their diets, including gluten-free-labeled products, in order to heal their intestines and eliminate their symptoms.
Catassi C. et al. A prospective, double-blind, placebo-controlled trial to establish a safe gluten threshold for patients with celiac disease. American Journal of Clinical Nutrition. January 2007; 85(1):160-166.
Fasano A. "In Defense of 20 Parts Per Million": A Letter from Alessio Fasano, M.D., and The University of Maryland Center for Celiac Research. Aug. 22, 2011.
Food and Drug Administration. Health Hazard Assessment for Gluten Exposure in Individuals with Celiac Disease: Determination of Tolerable Daily Intake Levels and Levels of Concern for Gluten. Issued May 2011.
How Much Gluten Is Safe For Me? University of Maryland press release, Aug. 4, 2011.