Magnesium stearate is just the magnesium salt of stearic acid. They didn't use to control the source of the stearic acid back in the day and it was bought and sold as a commodity whether it came from beef tallow or if it came from vegetable oil. Then, after mad cows happened, they started controlling the sourcing and kept vegetable oil sourced stearic acid separate from beef tallow sourced stearic acid and it became popular to say Magnesium Stearate (vegetable sourced) on product labels for a while. Now some say it and some don't, but basically it might still be vegetable sourced even if it doesn't say so. If they use beef tallow sourced stearic acid they have to keep records of the sources and have BSE certificates to guarantee it is free from mad cows disease and even that it is from countries that allow the import of cow derived ingredients and it gets hard because one country allows imports only from certain other countries and it varies from country to country. If you get my point, it becomes too hard to control a commodity like stearic acid if you use beef tallow derived stearic acid so a lot of it is veggie sourced even if it doesn't say so on the label.
As far as soy allergy, anything derived from refined vegetable oil isn't considered to have allergic potential because it doesn't contain any protein and therefore doesn't have to be listed as an allergen or that it contains soy because the allergins are in the protein, not the oil.
By the way, Magnesium stearate is considered to be a safe lubricant for powders to make tablets and capsules with and, unless too much is used, shouldn't really cause any problems with the finished products. If too much is used then it can waterproof the powder and make disintegration or dissolution slow and block absorption of the actives. However, most products are formulated with just a small amount (like 1 to 3 percent or so) and the products are tested for disintegration to make sure there are no problems.
Edited by MrSpud, 19 March 2010 - 04:41 AM.