The objective of this review paper is to evaluate the impact of undertaking aerobic exercise in the overnight-fasted v. fed-state, in the context of optimising the health benefits of regular physical activity. Conducting a single bout of aerobic exercise in the overnight-fasted v. fed-state can differentially modulate the aspects of metabolism and energy balance behaviours. This includes, but is not limited to, increased utilisation of fat as a fuel source, improved plasma lipid profiles, enhanced activation of molecular signalling pathways related to fuel metabolism in skeletal muscle and adipose tissue, and reductions in energy intake over the course of a day. The impact of a single bout of overnight-fasted v. fed-state exercise on short-term glycaemic control is variable, being affected by the experimental conditions, the time frame of measurement and possibly the subject population studied. The health response to undertaking overnight-fasted v. fed-state exercise for a sustained period of time in the form of exercise training is less clear, due to a limited number of studies. From the extant literature, there is evidence that overnight-fasted exercise in young, healthy men can enhance training-induced adaptations in skeletal muscle metabolic profile, and mitigate against the negative consequences of short-term excess energy intake on glucose tolerance compared with exercising in the fed-state. Nonetheless, further long-term studies are required, particularly in populations at-risk or living with cardio-metabolic disease to elucidate if feeding status prior to exercise modulates metabolism or energy balance behaviours to an extent that could impact upon the health or therapeutic benefits of exercise.

Substantial research has been completed examining the impact of carbohydrate (CHO) intake on endurance exercise, whereas its role in resistance-based exercise performance, adaptation and cell signalling has yet to be fully characterised. This empirical shortcoming has precluded the ability to establish specific CHO recommendations for resistance exercise. This results in recommendations largely stemming from findings based on endurance exercise and/or anecdotal evidence despite the distinct energetic demands and molecular responses mediating adaptation from endurance- and resistance-based exercise. Moreover, the topic of CHO and exercise has become one of polarising nature with divergent views – some substantiated, others lacking evidence. Current literature suggests a moderately high daily CHO intake (3–7 g/kg per d) for resistance training, which is thought to prevent glycogen depletion and facilitate performance and adaptation. However, contemporary investigation, along with an emerging understanding of the molecular underpinnings of resistance exercise adaptation, may suggest that such an intake may not be necessary. In addition to the low likelihood of true glycogen depletion occurring in response to resistance exercise, a diet restrictive in CHO may not be detrimental to acute resistance exercise performance or the cellular signalling activity responsible for adaptation, even when muscle glycogen stores are reduced. Current evidence suggests that signalling of the mammalian target of rapamycin complex 1, the key regulatory kinase for gene translation (protein synthesis), is unaffected by CHO restriction or low muscular glycogen concentrations. Such findings may call into question the current view and subsequent recommendations of CHO intake with regard to resistance-based exercise.