Yeah, xanthan gum....the only darned thing I could find related to food. Obviously, it's not something I keep in the pantry. But, it is an additive used for thickening different food products rather than what we might use at home, such as flour or cornstarch and water. And, get this - it is a bacteria that we've all seen at some point. It's causes "black rot" and it can be found on broccoli, cauliflower and leafy vegetables. It forms a slimy substance that science has made use of as a thickening agent in a variety of foods such as salad dressing and as a stabilizer in cosmetics. (Now, if that doesn't get you salivating, I don't know what will.)
Most xanthan gum is grown on glucose or, more recently, whey-based lactose. The lactose-produced xanthan gums are used primarily in shampoos and salad dressings. (Makes you just want to jump into the shower and hit the salad bar, doesn't it?)
For those who must have gluten-free diets, xanthan gum is often substituted for the gluten they cannot have in breads and batters. Believe it or not, though, xanthan gum is often derived from glutenous products, so celiacs must be aware of which products are made with gluten-free xanthan.
People with corn, soy or wheat allergies also need to be aware of what kind of xanthan gum is being used because they could react to even the minute amounts present in whatever product contains this agent, including toothpastes. Ingredient lists don't generally list the source of the xanthan gums, so it is best to be well researched before trying any product containing them or avoid products containing xanthan gums altogether.
My source of information was Wikipedia.