Greetings, OmaLou:
This has been studied pretty extensively, by myself, others, and especially researchers in France, who have been using edible clay regularly for centuries.
I recommend reading the guide I've produced called "A Comprehensive Beginner's Guide to Edible Clay", which covers the available scientific research in a bit more detail. While the actual science is not conclusive, it is pretty definitive:
http://www.eytonsearth.org/earthcures/a-comprehensive-beginners-guide-to-using-edible-clay/I, and others, have done proper heavy metal testing regularly by well qualified medical doctors SPECIALIZING in environment medicine. These guys, as well as functional doctors, are really the only ones actually qualified to do this kind of specialized work.
My heavy metal loads are always well below average, and I figure I've probably been exposed to clay more than almost anyone, since I work with it daily, bath in it and use it as needed as a nutrient.
To answer your question, yes we batch test all of our calcium bentonite for heavy metals, trace minerals, and do a microbial study.
Our clay has far less lead than certified food grade, USP bentonite. That said, that all soils have some lead and trace amounts of heavy metals. I have to find a heavy metal free clay, and I've done studies on most clays commonly used in human and animal health.
All of that said, in today's world, I don't recommend that clay be used as a stand alone detox product. I have a daily detox routine that I use to help keep my body's detox channels open, my detox genes up-regulated, and my organs in great shape to help prevent retention toxicity.
If a person has any sort of retention toxicity issue, it won't matter if they use digestive detox aids. Most people that I correspond with that have this type of issue either have liver or kidney retention toxicity. When this happens, the body down-regulates the genes responsible for Phase 1-2 detoxification.
Dr. Christopher Shade is probably the expert in the biochemistry here (and yes, he and his trained MDs use clay in their mercury detox protocols).
Furthermore, most "bentonite" contains particles that are close to 40 microns in diameter. In the nano world, these would be considered huge boulders. Most of these clay particles sorpt anything with a positive charge, and then carry it out of the digestive tract safely. Even if they were loaded with lead, the lead would never reach the blood stream, and even if it did, it would not be bioactive because it is rendered inert.
Even all of this considered, it is true that stomach acid reduces clay's sorption power (which is one reason why I try to teach people to DRINK MASSIVE AMOUNTS OF WATER WITH CLAY USE), and can even cause clay to dump some of its material-- which won't happen if you drink massive amounts of water with clay. (this has been studied by Argiletz in France, and the amounts released by 20 micron illite particles was pretty insignificant). But, where the body runs into most of its heavy metal retention issues is actual via the BILE, which is alkaline.
Clay is an amazingly effective, health-promoting nutrient... acting not only as a direct detox agent, but as a systemic catalyst for detoxification... it's an amazing digestive system healing substance... even MORE amazing at transforming the bio-terrain of the colon... but even so, there ARE people who can run into problems with it. They are rare, but it does happen.
For example, clay is loaded with iron, so you have these over-educated "armchair" doctors who try to sound the alarm about iron as well. But the truth is that laboratory testing has shown that people with a "regular" ability to utilize and eliminate iron do NOT retain iron from clay ingestion.
BUT, those people who have chronic medical conditions with iron absorption and/or retention CAN run into issues with clay. Why this occurs with them, I have not been able to figure out.
It is possible that a person might have a similar response to other materials in clay due to some metabolic or other abnormality. I am extremely pragmatic when it comes to health.
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The first place I would look when a person has a healthy lifestyle and still shows abnormal heavy metal levels is kidney retention toxicity. If the kidneys are not filtering any specific toxin properly (and this can actually and quite literally be toxin-specific, like only mercury).
I would see if there is a strong detox reaction preferably to 1 tsp. of micronized zeolite, taken in 8 ounces of water, which tends reacts very particularly to kidney retention toxicity (yes, zeolite is even more loaded with lead which is fused to the crystal).
I would do the same also (but at a different day/time) with lemon water first, then lemon water with cayenne.
Kidney retention toxicity can happen DECADES after an insult you might not even know about. I've seen kidney retention toxicity happen out of the blue thirty years after chemotherapy has been completed (non-targeted chemo).
If a person does have retention toxicity, again, it really doesn't matter how low the exposure is, it will build up in the body.