Is magnesium stearate a safe supplement additive?

Magnesium stearate (which contains stearic acid) is a controversial supplement additive. Some regard it as a risky excipient that should be avoided, and others believe it to be completely safe.

Identifying magnesium stearate in supplements

1. Magnesium stearate may also be called vegetable stearate or vegetable lubricant (if not manufactured from animal fat). It may be referred to by the E number E470b.
2. Stearic acid alone is also a common supplement additive, and maybe be referred to by E number E570.

Experts concerned about its risks

1. Dr. Joseph Mercola is an osteopathic physician popular in the alternative online health world who believes magnesium stearate to be dangerous, describing his objections in A Dangerous Ingredient in Your Supplements.
2. Dr. Ron Schmid is a naturopathic doctor who raises similar concerns as Mercola.
3. Suzy Cohen, RPh is a pharmacist whose magnesium stearate criticism has recently become popular, and raises similar concerns as the prior two articles.

All three of these people sell supplements that are free of magnesium stearate. That doesn't make me dismiss their arguments, but does give them a motivation to spend time arguing against its safety.

Experts who argue for its safety

1. The FDA considers magnesium stearate to be Generally Recognized as Safe.
2. In 2015 the Joint FAO/WHO Expert Committee on Food Additives left the "acceptable daily intake" of magnesium stearate unspecified, with the only concern being laxative effects at high doses.
3. Chris Kresser (a licensed acupuncturist) argues against the several claims about the dangers of magnesium stearate.
4. Dr. Joseph Dever (a PhD in molecular and environmental toxicology) and Dr. Michael Kemp (a PhD in nutritional science and a registered dietitian) argue that there's no evidence for harm or significant impact on bioavailability.
5. Byron J. Richards is a nutritionist that argues that Mercola and other critics are mistaken about magnesium stearate's risks, and that it has, at worst, neutral health effects.
6. Dr. Ray Sahelian also argues that it is a safe supplement additive.

The latter four people are all involved commercially with supplements, and Chris Kresser sells third-party supplements on his store. As with the critics, this doesn't make me dismiss their arguments, but gives them some motivation to make them.

Stearate in food compared to the magnesium stearate in supplements

Both Richards and Sahelian point out that chocolate and other common foods contain far more stearic acid than the amounts used in supplements. However, food forms of stearic acid (where it is usually combined with other fatty acids) could have different health effects than the isolated magnesium stearate used in supplements.

Critics raise concerns about the sources of stearate production, which can include pesticide-treated genetically-modified cottonseed oil, and claim that metal catalysts used during the manufacturing might survive in the end product. The possibility of impurities is mentioned in the Food and Agriculture Organization of the United Nations' chemical and technical assessment. It lists the possibility of chlorides, sulphates, cadmium, lead, and nickel remaining as contaminants. Manufacturers should be testing for these, but it's hard to know whether they actually are.

The FAO assessment also mentions dioxin contamination, with the reasonable attitude that it's probably not a concern because of the regulation of animal fats and vegetable oils used as sources.

Magnesium stearate and supplement absorption

Critics frequently cite risks of magnesium stearate decreasing the absorption of supplements, as in Suzy Cohen's YouTube demonstration.

The most frequently cited research is a study in Pharmaceutical Technology where capsules containing stearates dissolved more slowly in water. This is used to suggest that stearates will prevent supplements from dissolving properly. I haven't been able to find the text of the study, but if the results are similar to the study that followed, they do suggest that magnesium stearate can slow the dissolution of supplements into a water/methanol mixture, at least in combination with certain other ingredients.

It's hard to draw any strong conclusions from these for actual human digestion (designed to handle fats and salts). Dever and Kemp point this out with a rabbit study where the in-vitro model (using artificial gastric fluid) produced a lot of variation that didn't occur in real digestion and absorption:

Further complicating the issue are ingredient-specific interactions, such as this study where magnesium stearate levels didn't affect dissolution of propranolol hydrochloride tablets, this study where it did affect dissolution of ranitidine hydrochloride tablets, and this one where it improved actual absorption of frusemide tablets, at least compared to other lubricants.

The variability in the data makes me suspect that risk of "losing 75% of your supplements" (as some critics claim) is exaggerated. I think a more reasonable conclusion would be that magnesium stearate may minorly affect the absorption rate with some supplements and ingredient combinations, primarily because it's not water-soluble.

Magnesium stearate and biofilms

Several critics argue that stearates may promote biofilm formation, inhibiting nutrient absorption. However, I haven't found any data on stearic acid triggering this problem. Richards even points out a study showing biofilms being weakened by stearic acid.

Stearic acid and immune response

Critics of stearic acid claim that it suppresses the immune system's T cells. This claim appears to be based entirely on a study that was only monitoring for a mechanism by which stearic acid in combination with other chemicals interacted with weakened T cells in vitro. This doesn't indicate a connection between dietary stearic acid and T cell problems, especially not in the small amounts contained in supplements.

Magnesium stearate and stearic acid at high doses

As with most substances, magnesium stearate and stearic acid are dangerous when ingested in extremely high amounts. For example, a rat study found health problems with high doses of magnesium stearate. The study concludes that 2500mg per kg of body weight (somewhere around a half pound of magnesium stearate for an adult human) was a low-enough level to avoid the effects.

Some critics of magnesium stearate will point to Material Safety Data Sheets for magnesium stearate which advise medical attention upon ingestion. Referencing those data sheets is spurious because they are designed for occupational safety emergencies. They aren't relevant to normal magnesium stearate consumption.

Stearates and other health indicators

Sahelian points out that stearic acid has shown improvements on thrombosis risk factors in one study and that both thrombogenic and atherogenic risk factors improved in another, suggesting that it can't be entirely toxic.

My conclusion

From what I can tell, most of the criticism of magnesium stearate rests on two studies that don't actually support any real risks. I don't see any reason to conclude it would have any measurable effect on health. It might affect absorption rates in some cases, and theoretically could have small amounts of contaminants. But given the tiny amounts used in supplements, I'm not personally going to worry about those, nor do I think there's any reason to toss existing supplements with magnesium stearate. Of all the excipients I've looked at, it's the one I'd be least concerned with consuming.

However, it's easy enough to avoid if it concerns you. Every type of supplement I've looked at for this site includes at least one option without it.