Do Dynamic Fat and Fat-Free Mass Changes follow Theoretical Driven Rules in Athletes?

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Maximizing fat mass (FM) loss while preserving or increasing fat-free mass (FFM) is a central goal for athletic performance but the composition of body weight (BW) changes over time with training are largely unknown.


We aimed to analyze FM and FFM contributions to BW changes and to test if these contributions follow established rules and predictions over one athletic season.


Seventy athletes (42 men; handball, volleyball, basketball, triathlon, and swimming) were evaluated from the beginning to the competitive stage of the season and were empirically divided into those who lost (n = 20) or gained >1.5% BW (n = 50). FM and FFM were evaluated with a four-compartment model. Energy densities (ED) of 1.0 kcal·g−1 for FFM and 9.5 kcal·g−1 for FM were used to calculate ED/per kilogram BW change.


Athletes that lost >1.5% BW decreased FM by 1.7 ± 1.6 kg (P < 0.05), whereas FFM loss was nonsignificant (−0.7 ± 2.1 kg). Those who gained >1.5% BW increased FFM by 2.3 ± 2.1 kg (P < 0.05) with nonsignificant FM gains (0.4 ± 2.2 kg). The proportion of BW change as FM for those who lost or gained BW was 90% (ED: 8678 ± 2147 kcal·kg−1) and 5% (ED: 1449 ± 1525 kcal·kg−1), respectively (P < 0.001). FFM changes from Forbes Curve were inversely related to observed changes (r = −0.64; r = −0.81, respectively for those who lost or gained BW).


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.

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