Ketone bodies: beyond their role as a potential energy substrate in exercise

In the mid-19th century, the ketone bodies (KBs) acetoacetate (AcAc) and β-hydroxybutyrate (βHB) first came to prominence due to their high abundance in the urine of patients afflicted by diabetic coma. This increase in KBs resulted from enhanced hepatic conversion of free fatty acids to KBs. This mechanism is also activated during starvation, when KBs replace glucose as an energy substrate for the brain, thereby preserving precious gluconeogenic resources and eventually increasing survival. However, experiments in perfused rat muscles and human forearm muscles also nicely demonstrated that elevating blood KBs proportionally increases muscle KB uptake. But under normal physiological conditions uptake rates are very low due to low blood concentrations, and are even lower during prolonged exercise due to elevated free fatty acid (FFA) oxidation. Hence, over recent decades AcAc and βHB have been largely disregarded as energy substrates during exercise under normal physiological conditions. However, the availability of oral ketone supplements, allowing to readily establish exogenous ketosis at millimolar levels, has provided an elegant approach to re-evaluate the role of KBs in energy metabolism while avoiding the metabolic perturbations caused by a ketogenic diet or prolonged fasting. Accordingly, in a seminal publication Cox and his co-workers in 2016 demonstrated that exercise markedly enhanced blood βHB clearance. This was associated with muscle glycogen sparing and improved endurance exercise performance, reportedly by yielding a thermodynamic advantage over muscle carbohydrate oxidation. The idea of KBs as a 'superfuel' was born. However, despite compelling evidence that elevation of blood KBs increases muscle KB uptake, the extent to which KB disappearance from the blood reflects KB-supported mitochondrial respiration remains unknown. In this issue of The Journal of Physiology, Petrick and colleagues (2020) report the findings from a series of elegant in vitro experiments wherein the significance of KBs as an energy substrate for muscle tissue was investigated. They evaluated whether adding βHB or AcAc to mitochondria and permeabilized muscle fibres isolated from animal and human muscle tissue could directly drive mitochondrial respiration. Both βHB and AcAc independently stimulated mitochondrial respiration. However, the thermodynamic efficiency, defined as the P/O ratio, was identical to carbohydrate-derived pyruvate oxidation. Furthermore, the capacity of KBs to generate ATP was up to threefold lower than for pyruvate and was unaffected by prior exercise. As metabolic pathways never operate in isolation the authors also evaluated the metabolic interplay between KBs and pyruvate. Increasing the availability of pyruvate gradually suppressed the potential of KBs to drive mitochondrial respiration, which reached zero at saturating pyruvate concentrations. These results clearly indicate that KBs are submissive to pyruvate. As such, acutely raising KBs with the intention of facilitating ATP production in muscle cells is conceivably irrelevant whenever carbohydrate availability is ample. However, the current experiments demonstrate that AcAc supports mitochondrial respiration when other substrates are deficient. This indicates that KBs might be of some significance as a 'back-up fuel' whenever carbohydrates are insufficiently available, such as during prolonged exercise in the fasted state resulting in hypoglycaemia. Interestingly, another recent study showed that mitochondrial respiration increased by more than 30% following 24 h incubation of human myotubes with 0.5 mm βHB, but not with 1.5 or 5.0 mm, in the presence of sufficient amounts of pyruvate (Mey et al. 2020). This suggests that prolonged exposure to low concentrations of KBs overrides the inhibitory effect of pyruvate on KB-supported mitochondrial respiration. It is therefore tempting to speculate that KBs might become a relevant energy substrate when consistently increased by a long-term ketogenic diet, a nutritional strategy often implemented by ultra-endurance athletes. Research on exogenous ketosis in exercise and training is still in its infancy. However, available findings clearly indicate that under normal physiological conditions KBs provide no thermodynamic advantage in mitochondrial ATP production. This conclusion is supported by studies from different laboratories showing that exogenous ketosis does not have an impact on performance whenever evidence-based recommendations for carbohydrate intake are in place. Moreover, we recently demonstrated that ketone ester intake improved high-intensity exercise performance at the end of a 3 h cycling race when blood KB concentrations had returned to baseline following a transient peak earlier in the event. In addition, the ergogenic effect only emerged when the accompanying ketoacidosis was negated by oral bicarbonate supplementation (Poffé et al. 2020). The physiological mechanisms underlying this 'delayed' effect remain to be elucidated, but alterations in strong ion fluxes are possibly implicated. Recent studies, including the study by Petrick and co-workers (2020), have primarily addressed the acute effects of ketone bodies. Nevertheless, the potential of intermittent exogenous ketosis to modulate long-term training adaptation probably yields a much more attractive perspective. An earlier publication by our group in this journal showed that consistent post-exercise ketone ingestion during a 3-week period of endurance training overload markedly blunted the development of overreaching symptoms (Poffé et al. 2019). Before and after training a wide spectrum of physiological parameters was measured, hours after blood KB levels had returned to normal to exclude potential acute regulation by KBs. Nonetheless, ketone intake during recovery, among other effects, suppressed overnight sympathetic tone, alleviated bradycardia, reduced the rise of the stress hormone GDF-15, as well as markedly stimulated muscular angiogenesis (authors' unpublished observations). The report by Petrick and colleagues now justifies a shift in focus away from the potential input of KBs in energy production to their role as potent signalling molecules that may affect neural, hormonal and muscular adaptations to exercise and training. The potential of KBs to modulate training adaptation probably goes far beyond their marginal role as a 'shadow' energy substrate in active muscles during exercise. The authors declare that they have no competing interests. Both authors have approved the final version of the manuscript and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed. This work was funded by Research Fund Flanders (Fonds voor Wetenschappelijk Onderzoek – Vlaanderen; research grant no. G080117N).