Zeitgebers of skeletal muscle and implications for metabolic health.

Metabolic health is a crucial area of current research, and is an outcome of innate physiology, and interactions with the environment. Environmental cues, such as the Earth's day-night rhythm, partly regulate diurnal hormones and metabolites. Circadian physiology consists of highly conserved biological processes over ∼24-h cycles, which are influenced by external cues (Zeitgebers - 'time-keepers'). Skeletal muscle has diurnal variations of a large magnitude, owing in part to the strong nature of physical activity throughout the day and other external Zeitgebers. The orchestration of whole-body and skeletal muscle metabolism is a complex, finely tuned process, and molecular diurnal variations are regulated by a transcription-translation feedback loop controlled by the molecular clock, as well as non-transcriptional metabolic processes. The mitochondrion may play an important role in regulating diurnal metabolites within skeletal muscle, given its central role in the regulation of NAD+ /NADH, O2 , reactive oxygen species and redox metabolism. These molecular pathways display diurnal variation and illustrate the complex orchestration of circadian metabolism in skeletal muscle. Probably the most robust Zeitgeber of skeletal muscle is exercise, which alters glucose metabolism and flux, in addition to a range of other diurnal metabolic pathways. Indeed, performing exercise at different times of the day may alter metabolism and health outcomes in some cohorts. The objective of this Symposium Review is to briefly cover the current literature, and to speculate regarding future areas of research. Thus, we postulate that metabolic health may be optimized by altering the timing of external cues such as diet and exercise.