The phrase "composition changes" means that something is being altered or modified by adding, removing, or rearranging parts or elements. Full definition
This also means that the weight scale is a poor indicator of fat loss; fat calipers or even just seeing body composition changes in the mirror are much better indicators of fat loss. (fitbodyhq.com)
Body composition changes in elite male bodybuilders during preparation for competition. (completehumanperformance.com)
For another, breast milk composition changes over the course of the day, and even over the course of a feeding. (strocel.com)