Hydroxypropyl cellulose is a modified form of cellulose, which is a natural substance that is found in plants. It is often used as a thickening agent, stabilizer, and emulsifier in a variety of food and cosmetic products, including some protein powders.
In protein powders, hydroxypropyl cellulose may be added as a stabilizer to help prevent clumping and improve the texture and consistency of the powder. It may also be used as a thickening agent to help improve the mouthfeel of the powder when mixed with liquids.
Hydroxypropyl cellulose is generally considered safe when consumed in recommended amounts.