I like Dr. Mercola. He has a lot of credibility, and has shone light upon many extremely important health issues. Overall, his site offers a great service. But there are times when he takes things out of context and makes them sound scary, when they’re simply not. The most recent example is regarding claims that magnesium stearate in supplements has detrimental effects.
What is Magnesium Stearate?
Magnesium stearate is a lubricant and flow agent commonly used in supplements. When you are encapsulating natural ingredients that are sticky, to get them out of the machine and into the capsule in consistent amounts, (or at all), you need to lubricate the particles so they flow more easily. Otherwise they’re simply not going in, and/or you get different amounts in each capsule or tablet.
Magnesium stearate is composed of two molecules of stearic acid…a long-chain saturated fat commonly found in butter… attached as a salt (ionically bound) to one atom of magnesium. And this is what you get when they break up in your gut after being swallowed...nothing more…nothing less. Magnesium is arguably the most versatile mineral that we need. Stearic acid has been shown to be helpful in reducing cholesterol, (see http://www.ajcn.org/content/60/6/986S including the links at the bottom).
How much is too much?
The most important consideration for magnesium stearate is that “the dose makes the poison”. Just like we can’t live without the mineral iron, but it’s poisonous if you get too much. Or that Chromium Picolinate caused genetic damage in hamsters…when they were subjected to a human-equivalent dose of 6000 capsules in a day . Likewise, there is a huge difference between ingesting thousandths of a gram, i.e. milligrams (mg.) of magnesium stearate in a supplement, (usually less than 10 mg), versus large gram amounts that would never be realistically ingested. And yes, magnesium stearate is regarded in transport to be a hazardous substance, because if you spill a 25 kilo pail and inhale it, that may be hazardous. But, magnesium stearate is GRAS (Generally Recognized As Safe) for ingestion up to 2500mg per Kg of body weight per day, i.e. this means a 175 lb. person theoretically could safely consume approx. 200,000mg in a day…(even though they may not feel great after). 
Mercola mentions the immuno-suppressive effect of magnesium stearate on T-cells, referring to a 1990 test-tube study which used albumin- bound fatty acids, and did not specify the realistic human-equivalent dosage.  Are we talking mg’s, or 25 grams…which you would never ingest?
Biofilms and absorption…
The article also offers nothing to back-up its claim that magnesium stearate creates biofilms in your gut and blocks absorption of nutrients. Firstly, biofilms are created bymicrobes like bacteria adhering to each other, and/or a surface…not fats and a mineral. Now, if you go to You Tube, you can see a demonstration of how absolutely insoluble in water magnesium stearate is, and how mixing it with reactants inhibits normal reactions. Yet this isn’t surprising, and it is dose-dependent. Stearate is a fat, and fats don’t mix with water. If you take a tablespoon of butter, it isn’t scary when you put it in a glass of water and it won’t mix, or if it prevents other ingredients it is mixed with from dissolving. But this may change if you mix 25 mg (a speck) of butter. One way or another, to address this solubility issue our body releases bile into our digestive tracts to break-up and emulsify fats so we can absorb them and the other nutrients they are mixed with. This is actually a good example of why you have to be careful about drawing direct conclusions about living systems from test-tube studies…the living systems are most always way more complex. Our guts are adapted to absorb nutrients from complex mixtures of water, fats, carbs, proteins, vitamins, minerals, etc.
Mercola definitely purports that supplements that don’t contain magnesium stearate are what you want, (i.e. his supplements). But if you look at Mercola’s Joint formula it lists calcium stearate as an excipient.  This is analogous to saying “We make absolutely sure we don’t use sunflower oil…we use safflower oil!” (i.e. Calcium and magnesium stearates have extremely similar properties.)
The post also uses recommendations from the Codex Alimentarius Committee to back up its condemnation of magnesium stearate. Yet there are few groups more vocal than Mercola’s about the purposefully restrictive and destructive nature of this committee for the dietary supplement industry. And, after everything Mercola has rightfully posted about the dangers of sugar, his Kid’s Multi uses sucrose. Is this less than perfect? Yes. But it is a practical consideration…if you are going to get a kid to eat a multiple vitamin, it better taste reasonable, and of course he would say, “there’s so little in it”.
Those who villainize magnesium stearate always say its use is motivated by profit, i.e. to keep costs down. Yet how many of these critics are using it to promote their own supplements? And how many of you find that supplements are expensive and don’t want them to be more expensive?
Speaking realistically, unless you are hyper-sensitive or have major problems digesting fat, consuming mg amounts of magnesium stearate will be inconsequential, i.e. even if you take a handful of supplements one or more times per day. But, as always, go with the result. If you are getting good results from the supplements you are using, and they contain magnesium stearate, don’t worry about it. If you can digest butter, you will likely not notice magnesium stearate that you consume. But if you are getting intestinal upset, or are not happy with the results you are getting, try something else! (At Optimum Health, we do carry a hypoallergenic line that specifically excludes it from their formulations.)
In Optimum Health’s supplements we have always specifically requested that our manufacturers use the absolutely smallest amount of excipients or fillers possible, and will usually simply add more of the ingredients if we can to avoid or reduce them. Yet, because you, our customers, are requesting it, we are currently looking into other excipients. But be perfectly aware that they are all likely to have attributes that can be criticized as well.
-  Townsend Letter for Doctors & Patients; issue 149 December 1995, pp. 72-81.
-  ^ D. Søndergaarda, O. Meyera and G. Würtzena (1980). "Magnesium stearate given peroprally to rats. A short term study". Toxicology 17 (1): 51–55. DOI:10.1016/0300-483X(80)90026-8. PMID 7434368
-  http://forum.lef.org/default.aspx?f=35&m=17242 Molecular basis for the immunosuppressive action of stearic acid on T cells. Immunology 1990 Jul;70(3):379-86Tebbey PW; Buttke TM Department of Microbiology and Immunology, East Carolina University School of Medicine Greenville 27858-4354.
-  http://media.mercola.com/assets/pdf/product-labels/Joint-Formula_Label-30caps-web.pdf