Actually, leather contains some acid in it already. Acid (of some variety) is what chemically changes a raw, green hide (which will rot) into leather.
The leather used in upholstery, jackets and most all suede is tanned with chromic acid and will actually eat pits into tool steel. Therefore, it is a truly horrible holster material.
Holster leather is vegetable tanned. The tannic acid used in this process is made from either oak or hemlock bark, and/or acorns, and/or leaves. A really strong "tea" is brewed from these materials and water into a tannic acid solution. The green hides are submerged for about 90 days in the stuff until they are fully tanned, and are then dried, finished and split to weight.
In short, an acidic body chemestry might well discolor light colored leather holsters (usually turning them black), but in MOST cases will probably not do significant harm to the leather, except POSSIBLY causing it to dry out prematurely. (Although such a body chemistry can easily rot linen thread, and cause the stitching to fall apart.) However, most makers today use either polyester or nylon thread to prevent such a thing.