This study was conducted to determine the protein content, label accuracy, and in vitro digestibility of different protein powders commonly used as dietary supplements by athletes using the Dumas method. Eighteen different protein powders were provided to determine protein and digestibility levels: whey powder, bovine milk casein powder, soy powder, egg white powder, beef protein powder, and pea powder. The protein contents reported on the labels of the protein powder samples examined in the study and the protein levels determined within the scope of the study were 76-72.87%, 86-83.83%, 91-76.01%, 88-99.13%, 85-85.93%, and 84-77.32% for whey, casein, soy, meat, egg white, and pea powders, respectively. The protein powder most compatible with the label information was egg white powder, while the least compatible was soy powder. The protein powder with the highest digestibility rate was meat powder protein (66.21%), followed by casein powder (60.89%), standard bovine milk casein (56.75%), whey protein powder (54.50%), and pea protein powder (53.66%), followed by soy protein powder (43.76%). The protein powder with the lowest digestibility rate was egg protein (36.87%). This study emphasized the importance of informing consumers about the methods used to produce protein powders, as these methods affect digestibility and the rate of protein conversion into body protein. It emphasized that protein powders should be marketed with measurable and rational label information, such as digestibility and the rate of protein conversion into body protein.