This study looked at the body composition during weight increases and decreases in athletes (Silva AM, Matias CN, Santos DA, Rocha PM, Minderico CS, Thomas D, Heymsfield SB, Sardinha LB. Do Dynamic Fat and Fat-Free Mass Changes follow Theoretical Driven Rules in Athletes? .Med Sci Sports Exerc. 2017 Oct;49(10):2086-2092.). The results were startling:
Athletes that lost BW used 90% of the energy from FM while in those gaining BW, 95% was directed to FFM. When BW is lost, dynamic changes in its composition do not follow established rules and predictions used for lean or overweight/obese nonathletic populations.
Credit to Sigma Nutrition for this subject. Check out their podcasts.