Resting metabolic rate and energy efficiency in response to an intensive 84-day combat-swimmer training in the German Armed Forces.

Purpose: According to the 'constrained model', there are compensations in resting metabolic rate (RMR) at high levels of physical activity (PA). Here, we have used a standardized combat-swimmer training protocol (CST) to investigate whether changes in RMR (i) confirm the 'constraint model', and (ii) differ between successful participants and dropouts.

Methods: Controlled 84d CST in 44 male soldiers with 13 finally successful. Fat mass (FM) and fat-free mass (FFM) were measured using Quantitative Magnetic Resonance. RMR was assessed by indirect calorimetry, VO, and work efficiency by treadmill spiroergometry. Plasma levels of thyroid hormones, testosterone, and cortisol were analysed by standard laboratory methods.

Results: CST increased VO (+ 6.9%) and exercise efficiency at low workloads of 10 and 12 km/h (+ 8.7 and + 6.5%; both p < 0.05). As energy balance was moderately negative (-356 ± 383 kcal/d), FFM and FM decreased (-2 and -16%; both p < 0.05). There was a considerable inter-individual variance but no change in in the mean values of RMR and RMR. RMR before CST had a negative association with its decrease with CST (p < 0.005). Concomitantly, plasma hormone levels were unchanged. When compared with dropouts, successful participants had a higher VO at baseline (5.2 ± 0.6 vs. 4.9 ± 04 l/min; p < 0.05) that increased with CST (+ 4.4 vs. -0.4%; p < 0.05) at similar changes in body composition and energy balance.

Conclusion: While CST increased VO and exercise efficiency as a compensation, there was an inter-individual variance in exercise-related compensation of RMR with no differences between 'completers' and 'non-completers'. Trial registration DRKS00018850, November 27, 2019.