The short answer is a qualified "yes."
Natural bristles are hair, and hair is attacked by chemicals that dissolve fats and fat-like substances (the technical term is lipids). You feel the same "defatting" effect on your skin when you get a solvent like lacquer thinner on it. Some organic solvents are more aggessive than others: the common alcohols like isopropyl and ethyl (especially when concentrated), acetone, and some of the components in "lacquer thinner" which is usually a blend of different solvents.
Soap and water, or detergent and water, will do the same, but they are less aggressive than the organic solvents, but you can't use them to clean enamels or oils.
Of the organic solvents, mineral spirits is probably the least aggressive. It is all I ever use on the brushes I use for enamels or oils. After the bristles are completely dry, I then treat them with ordinary hair conditioner—the kind you use on your own hair after shampoo.
Never use the same brush for oils or enamels that you use for acrylics. That's the fastest way I know to kill a brush.