Magnesium Stearate or Stearic Acid.
Every few years the topic of magnesium stearate or stearic acid and its putative toxicity arises. This topic is typically brought up by companies trying to support their own particular philosophy of product formulation, which does not include stearic acid. Over the years they have promulgated a misleading story about the dangers of stearic acid and how bad it is for you and for a product�s formulation. However, when one looks closely at the data and studies that are often cited by those making these arguments, it is clearly inaccurate to characterize stearic acid used in dietary supplements as "dangerous". These "anti-stearic acid" arguments constitute a gross mischaracterization of these studies and take much of the data completely out of context. The argument is typically couched with sensational headlines, (such as "poisonous flow agents") designed to get your attention and divert one from the actual science with respect to stearic acid.
Stearic acid is an 18 carbon saturated fatty acid naturally found in many foods. The magnesium salt of stearic acid (magnesium stearate, also called vegetable stearate) is typically used in the formulation of dietary supplements. The material has a slippery consistency and is used as a flow agent to help raw material flow evenly and more easily into capsules. Stearic acid is allowed for use in dietary supplements and there is nothing inherently toxic or dangerous about stearic acid. However, when you read the one or two sentences that are often cited from the paper by Tebbey et al. (Immunology 1990), such as "Stearic acid inhibits T-cell dependent immune responses", it can sound scary. Nonetheless, when one reads the complete study it is clear that the authors were not studying dangers of stearic acid, or even making any connection between the consumption of stearic acid and any possible ill health effects. The study was actually looking in vitro (not in vivo) to see how B cells and T cells may metabolize compounds differently. The authors believed that these two cell types handled stearic acid differently and were performing experiments in an attempt to further understand the metabolism of these cell types. As it turns out, B cells had the ability to desaturate stearic acid, while T cells did not have that ability. The study was not performed to test the dangers of stearic acid and is not relevant to the quantities of stearic acid consumed from supplements (which is small) and was never tested in an in vivo system. These cells were directly exposed to stearic acid in a model system that does not reflect the in vivo situation.
In fact the authors were interested in the potential usefulness of stearic acid as it pertained to allograft rejection and autoimmune problems. To draw a connection between the results reported in the Tebbey et al. paper and a "danger" of stearic acid in supplements is simply bad science. Generalizing data from an unrelated in vitro study, which was never designed or intended to test the argument they are making, may make a nice marketing story; but that does not make it right.
A deeper look at stearic acid helps to further highlight the silliness of this argument and demonstrates the safety of the compound. Stearic acid is commonly found in many foods such as beef, cheese, milk, and coconut oil as well as foods that are considered to have interesting health benefits such as dark chocolate. The amount of stearic acid present in the foods we routinely consume is vastly greater than the few milligrams of stearic acid that are present in dietary supplement formulations. Additionally, once consumed, stearic acid is converted in the body to oleic acid, a monounsaturated fatty acid. This metabolic conversion may in fact be one of the reasons that stearic acid typically does not raise LDL cholesterol levels. This conversion of stearic acid to oleic acid in the body also makes it even harder to relate in vitro studies, that apply stearic acid onto cells, to any kind of in vivo situation.
Another example of taking results wildly out of context is an often cited reference to a study by Ulloth, et. al.. In fact the full citation is typically not given, but it appears to relate to an article that appeared in the Journal of Neurochemistry in 2003. The authors were looking in vitro to see if they could induce cell death with palmitic and stearic acid when given to certain cells at concentrations similar to what is seen after traumatic brain injury. Of course the "traumatic brain injury" aspect of the study is not mentioned in the highlighted quote. How stearic acid, used in supplements, is at all related to neuronal cellular metabolism following traumatic brain injury remains a mystery. Nonetheless, taken out of context, it certainly sounds like stearic acid may be bad for you. What it comes down to is this: stearic acid used in supplements is safe and there have been no published data to demonstrate otherwise. The often quoted references used to discredit steric acid sound important and dangerous, but they are taken out of context. To use them to support the argument that stearic acid should not be used in dietary supplements misrepresents the data in the studies from which they were taken.
References
Tebbey PW, Buttke TM. Molecular Basis For The Immunosuppressive Action of Stearic Acid on T cells. Immunology, 1990 70; 379-386
Ulloth JE, et al. Palmitic and stearic fatty acids induce caspase-dependent and -independent cell death in nerve growth factor differentiated PC12 cells. Journal of Neurochemistry 2003 84; 655-668

So...it's not so bad?

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"We can judge the heart of a man by his treatment of animals." ~Immanual Kant~

Lets look at this from a logical point of view.

The very purpose of using magnesium stearate in supplements is to lubricate the equipment so that there are less breakdowns/more profit for the manufacturer.

I don't have a problem with that, however, it is logical to think that this "lubricant" would also coat the intestinal villi, thus preventing nutrients from being absorbed in the gut, and that the more pills you take, the greater the effect of this greasy coating..

Fact: Magnesium stearate is an unnecessary ingredient.

Fact: Magnesium stearate contains no magnesium.

When I buy supplements I want what I pay for and no extra ingredients such as flow agents, artificial colors, artificial sweeteners, titanium dioxide, or fillers of any kind.

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Let Food Be Your Medicine And Medicine Be Your Food.(Hippocrates)

Agreed.

On the capsule or tablet level, it is minuscule. However if you take as many as I do on a daily basis, it adds up. I agree that it is not toxic. I also agree that it can build up on the intestinal walls and cause all sorts of problems. I'd like to know if our friendly bacteria eat it or enzymes dissolve it. Does it compete with bacteria for space on the intestinal wall?

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- Jim

"No one can make you feel inferior without your consent." �Eleanor Roosevelt

As far as I understand, magnesium stearate does not interact with friendly flora, and the only way to remove it from the villi is to eat plenty of fibre. As this roughage travels through the intestines, it takes this coating with it. As I understand, it is a white coating.

Nobody knows if it is toxic or not as there hasn't been any longterm studies done.