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_2max, and work efficiency by treadmill spiroergometry. Plasma levels of thyroid hormones, testosterone, and cortisol were analysed by standard laboratory methods.

Results: CST increased VO_2max (+ 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_adjFFM. RMR_adjFFM 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_2max 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_2max 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.