One study that many people have used as evidence against magnesium stearate is a 1990 experiment entitled “Molecular basis for the immunosuppressive action of stearic acid on T cells.” This baffles me, and I suspect that anyone using this study to indict magnesium stearate hasn’t actually read it.
In the experiment, scientists isolated T-cells and B-cells from mice, put them in a Petri dish, and bathed them in a solution containing stearic acid (along with some other components). They observed that the T-cells incorporated the stearic acid into their cell membrane, eventually de-stabilizing the membrane enough that the cell died.
First of all, this study has nothing to do with magnesium stearate. They just used the plain old stearic acid that you’d find in your beef, chocolate, or coconut oil, so this study could just as easily be used against those foods. If you’re going to be concerned about this study (which you shouldn’t be), you’d have much bigger sources of stearic acid to worry about than the magnesium stearate in your supplements.
Second, the study has nothing to do with stearic acid consumed in the diet. Under normal conditions, your T-cells are not bathed in stearic acid, even if you consume superhuman amounts of coconut oil, tallow, and cocoa butter.
Finally, the researchers used T-cells from mice, and in this case, the results cannot be applied to humans. The mouse cells incorporated stearic acid into their membranes because they lacked the ability to de-saturate fatty acids. However, human T-cells do have the ability to de-saturate fatty acids, so even if you did bathe your T-cells in stearic acid, they would be able to maintain their membrane function. (1)
In case you got lost, here’s a summary: this study has no relevance whatsoever to human consumption of magnesium stearate, I have no idea why the study is being referenced in this manner, and you shouldn’t be concerned about it.
Another criticism is that because stearate is often derived from cottonseed oil, it can be contaminated with pesticides. Keep in mind that magnesium stearate is a highly purified substance, and goes through an intensive refining process before appearing in your supplements. So far, I haven’t come across any reports indicating that magnesium stearate retains substantial amounts of pesticide residue.
As for the concern that cottonseed oil is often genetically modified, the source of crude fat shouldn’t make a difference in the final form of the stearate. Stearic acid is an 18-carbon molecule with a specific chemical structure that will be the same whether the stearic acid is from a genetically modified cotton plant, a bar of Hershey’s chocolate, or a grass-fed ribeye steak.
Absorption of Nutrients
Another criticism is that magnesium stearate might inhibit nutrient absorption. One in vitro study conducted in 2007 found that tablets containing magnesium stearate dissolved more slowly than tablets without magnesium stearate when placed in artificial gastric juice. (2) The study authors concluded that in vivo studies are needed to determine whether this finding has any practical significance. However, an earlier study found that although magnesium stearate increased the time it took for a drug to dissolve, it had no effect on overall bioavailability, as evidenced by blood levels of the drug in test subjects. (3) Further, another study found that levels of magnesium stearate didn’t affect tablet dissolution at all. (4)
All of this indicates that although magnesium stearate might affect the rate of tablet dissolution in some circumstances, it doesn’t affect the overall bioavailability of the drug or supplement.
I’ve seen this claim pop up in a few places around the internet, so I’ll address it briefly. Some critics of magnesium stearate claim that it can induce formation of harmful biofilms in the intestine. This assertion appears to be based on the fact that soap scum contains magnesium and calcium stearate, so they insist that just as soap scum creates films on your sink or shower, magnesium stearate creates films on your intestines.
It should be pretty obvious that the intestinal lumen is a vastly different environment from a shower door, but some people still seem to be concerned. Rest assured, there is no conceivable reason why this would take place, and I haven’t seen a single scientific article that even hints at this possibility.
A 2012 study entitled “Magnesium stearate: an underestimated allergen” reported on a 28-year-old woman who had an allergic reaction to magnesium stearate, resulting in hives. I’m very curious about this, because an allergy to either magnesium or stearate seems highly unlikely, but unfortunately I don’t have full-text access to that study. But needless to say, if you develop hives (or another allergic response) after consuming magnesium stearate, you should probably avoid it in the future.
As a final note, a rat study determined that you’d have to take 2500mg of magnesium stearate per kilogram of bodyweight per day to start seeing toxic effects. (5) That means a 150lb person would have to consume 170,000 mg per day, which is so far beyond any amount you would encounter in supplements that it’s a non-issue.
Overall, I haven’t found scientific evidence to substantiate the claims against magnesium stearate, and the small amounts found in supplements shouldn’t be a problem for the majority of the population.
Data accumulated by Chris Kresser http://chriskresser.com/harmful-or-harmless-magnesium-stearate/