Magnesium stearate is a salt, which means the magnesium cation is attached to stearic acid to form a salt known as magnesium stearate. This is similar to magnesium citrate, which is a salt comprised of magnesium and citric acid. Magnesium is never found by itself and is always attached to something, forming a salt.
Stearic acid is the most common saturated fatty acid consumed in normal diets. It is one of the most common fatty acids found in nature after palmitic acid. In 2009, Hunter et al. found that a high stearic acid vegetable oil may have a beneficial effect on LDL cholesterol. The levels needed to see those effects are not currently in any of our products.
Some misinformation on the internet claims magnesium stearate may disrupt the integrity of cell walls and have a suppressive effect on immunity. A study was done in test tubes with cells from mice, and they bathed the cells in stearic acid to see what would happen. Human cells cannot be "bathed" in stearic acid, and mouse cells can't de-saturate fatty acids like human cells can. That's why the mouse cells showed a disruption, but this study is completely non-applicable in humans.
Magnesium stearate is often included when manufacturing supplements because it serves as a filler, diluent and flow agent. It helps keep ingredients in proper form and prevent powders from sticking and clumping to surfaces during production.
It's generally recognized as safe (abbreviated GRAS) by the FDA and approved for use in supplements. The GRAS statement says: "There is no evidence in the available information on magnesium carbonate, magnesium chloride, magnesium sulfate, magnesium hydroxide, magnesium oxide, magnesium stearate, dibasic magnesium phosphate and tribasic magnesium phosphate that demonstrates, or suggests reasonable grounds to suspect, a hazard to the public when they are used at levels that are now current and in the manner now practiced, or which might reasonably be expected in the future."
Shane Durkee, Chief Innovation Officer at Swanson Health Products, said the following about using magnesium stearate when manufacturing supplements: "If we can produce a quality product without it, we'll leave it out. If it increases the product quality to put it in, we'll use it because it is not only safe, but sometimes beneficial."