General:
* Body structure
* Body makeup
* Body build
* Somatotype
* Physiological composition
* Physical composition
* Body mass distribution
Focusing on fat and muscle:
* Fat-to-muscle ratio
* Lean body mass
* Muscle mass
* Body fat percentage
* Fat mass
* Muscle-to-fat ratio
Other options:
* Body density (a measure used to calculate body composition)
* Anthropometry (the study of human body measurements)
* Bioimpedance analysis (a technique used to estimate body composition)
The best synonym for "body composition" depends on the specific context and the focus of your writing.