Purpose: We investigated the effect of a 31-d ketogenic diet (KD) on submaximal exercise capacity and efficiency. Methods: A randomized, repeated-measures, crossover study was conducted in eight trained male endurance athletes (V˙O_2max, 59.4 ± 5.2 mL⋅kg⁻¹⋅min⁻¹). Participants ingested their habitual diet (HD) (13.1 MJ, 43% [4.6 g⋅kg⁻¹⋅d⁻¹] carbohydrate and 38% [1.8 g⋅kg⁻¹⋅d⁻¹] fat) or an isoenergetic KD (13.7 MJ, 4% [0.5 g·kg⁻¹⋅d⁻¹] carbohydrate and 78% [4 g⋅kg⁻¹⋅d⁻¹] fat) from days 0 to 31 (P < 0.001). Participants performed a fasted metabolic test on days −2 and 29 (~25 min) and a run-to-exhaustion trial at 70% V˙O_2max on days 0 and 31 following the ingestion of a high-carbohydrate meal (2 g⋅kg⁻¹) or an isoenergetic low-carbohydrate, high-fat meal ( 0.05). The KD impaired exercise efficiency, particularly at >70% V˙O_2max, as evidenced by increased energy expenditure and oxygen uptake that could not be explained by shifts in respiratory exchange ratio (RER) (all, P < 0.05). However, exercise efficiency was maintained on a KD when exercising at 0.05). Time-to-exhaustion (TTE) was similar for each dietary adaptation (pre-HD, 237 ± 44 vs post-HD, 231 ± 35 min; P = 0.44 and pre-KD, 239 ± 27 vs post-KD, 219 ± 53 min; P = 0.36). Following keto-adaptation, RER >1.0 vs <1.0 at V˙O_2max coincided with the preservation and reduction in TTE, respectively. Conclusion: A 31-d KD preserved mean submaximal exercise capacity in trained endurance athletes without necessitating acute carbohydrate fuelling strategies. However, there was a greater risk of an endurance decrement at an individual level.

This study investigated the effect of the racemic β-hydroxybutyrate (βHB) precursor, R,S-1,3-butanediol (BD), on time-trial (TT) performance and tolerability. A repeated-measures, randomized, crossover study was conducted in nine trained male cyclists (age, 26.7 ± 5.2 years; body mass, 69.6 ± 8.4 kg; height, 1.82 ± 0.09 m; body mass index, 21.2 ± 1.5 kg/m²; VO₂peak,63.9 ± 2.5 ml·kg⁻¹·min⁻¹; W_max, 389.3 ± 50.4 W). Participants ingested 0.35 g/kg of BD or placebo 30 min before and 60 min during 85 min of steady-state exercise, which preceded a ∼25- to 35-min TT (i.e., 7 kJ/kg). The ingestion of BD increased blood D-βHB concentration throughout exercise (0.44–0.79 mmol/L) compared with placebo (0.11–0.16 mmol/L; all p < .001), which peaked 1 hr following the TT (1.38 ± 0.35 vs. 0.34 ± 0.24 mmol/L; p < .001). Serum glucose and blood lactate concentrations were not different between trials (all p > .05). BD ingestion increased oxygen consumption and carbon dioxide production after 20 min of steady-state exercise (p = .002 and p = .032, respectively); however, no further effects on cardiorespiratory parameters were observed. Within the BD trial, moderate to severe gastrointestinal symptoms were reported in five participants, and low levels of dizziness, nausea, and euphoria were reported in two participants. However, this had no effect on TT duration (placebo, 28.5 ± 3.6 min; BD, 28.7 ± 3.2 min; p = .62) and average power output (placebo, 290.1 ± 53.7 W; BD, 286.4 ± 45.9 W; p = .50). These results suggest that BD has no benefit for endurance performance.

Military pilots risk their lives during training and operations. Advancements in aerospace engineering, flight profiles and mission demands may require the pilot to test the safe limits of their physiology. Monitoring pilot physiology (e.g. heart rate, oximetry and respiration) inflight is in consideration by several nations to inform pilots of reduced performance capacity and guide future developments in aircraft and life support system design. Numerous challenges, however, prevent the immediate operationalisation of physiological monitoring sensors, particularly their unreliability in the aerospace environment and incompatibility with pilot clothing and protective equipment. Human performance and behaviour are also highly variable and measuring these in controlled laboratory settings does not mirror the real-world conditions pilots must endure. Misleading or erroneous predictive models are unacceptable as these could compromise mission success and lose operator trust. This narrative review provides an overview of considerations for integrating physiological monitoring systems within the military aviation environment. Advancements in military technology can conflictingly enhance and compromise pilot safety and performance. We summarise some of the opportunities, limitations and risks for integrating physiological monitoring systems within military aviation. Our intent is to catalyse further research and technological development.