Magnesium stearate is the magnesium salt of the fatty acid stearic acid. It is widely used as a diluent or filler in the manufacturing of drug or dietary supplement capsules, tablets and powders. In this regard, its lubricating properties allow powders to flow freely through encapsulation machines without sticking. Magnesium stearate, and the related filler stearic acid, have been widely maligned on several internet health sites as "toxic" or "unhealthy," particularly those selling 'stearic acid-free' or 'magnesium stearate-free" alternatives. However, several of the claims against magnesium stearate and stearic acid are inconsistent with the available scientific evidence, or are simply incorrect:
Magnesium stearate and stearic acid are not trans-fats. Stearic acid is a naturally-occurring saturated fat. By definition, a saturated fat is fully "saturated" with hydrogen atoms, and contains no double bonds between carbons of the fatty acid backbone. Trans-fats contain at least one carbon-carbon double bond in a trans- configuration. Trans-fats are a synthetic byproduct of the hydrogenation of unsaturated fats.
Accumulation In The Body
While the amounts of stearic acid and magnesium stearate used in dietary supplements are generally low, critics point out that these compounds may potentially "accumulate" to dangerous levels after prolonged usage. Stearic acid is the most common fatty acid in the human body; when people store excess carbohydrates as fats, the fatty acid synthase enzyme first synthesizes stearic acid from glucose metabolites. Humans synthesize other fatty acids from enzymatic modification of stearic acid. Due to the human body's metabolic familiarity with stearic acid, and its relative abundance in the body, it is unlikely that small amounts of dietary stearic acid or its derivatives will "accumulate" or escape normal metabolism.
There have been studies that have drawn an association between stearic acid and the function of immune cells, however, it is important to present these findings in the correct context. An oft-cited study of depressed natural-killer cell function in rats fed a high-stearic acid diet is published in "Biochimica et Biophysica Acta"; however, it is important to note that the rats ate 20 percent of their body weight in fat each day, and the effect of stearic acid was comparable to that MCT, the fatty acids from coconut oil. Similar studies of human cells in vitro from the journal "Immunology" show an immunosuppressive effect of free stearic acid, but on isolated T-cells. Therefore, most of the evidence of the immunosuppressive activity of stearic acid is only relevant in certain specific contexts, and not to its general metabolism. As one of the most abundant fatty acids in the human body, it would be unexpected if the immune system was dysfunctional in ts presence.