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PURPOSE: Myokine irisin may increase energy expenditure and affect metabolic disorders. We aimed to evaluate the association between irisin and gender, exercise, diet and body composition, and its day-night rhythm. MATERIALS AND METHODS: Observational, cross-sectional, cohort study with an additional 24h prospective observational arm (Day-night rhythm sub-study) and two prospective interventional arms (Mixed meal sub-study and Exercise sub-study) executed in the Military School of Combat Offi cers, Thessaloniki, Greece. Healthy, young students of the Military School of Combat Offi cers, Thessaloniki, Greece were subjected to anthropometric and body composition measurements (lean body mass and fat mass) and completed questionnaires regarding their eating and physical activity behavior. Sub-groups were subjected to 24h monitoring, standardized meal ingestion and 30 min aerobic exercise. Main outcome measures were circulating levels of irisin, leptin, adiponectin, insulin and related biochemical parameters RESULTS: Ιrisin levels were signifi cantly lower in males than females (72.5 ± 6.4 vs. 100.8 ± 6.3 ng/ mL, p=0.02) after adjustment for lean body mass, which was its major determinant. Irisin secretion followed a day-night rhythm (p<0.001) with peak at 9 pm. Irisin levels increased at the end of exercise (105.8 ± 14.3 vs. 84.1 ± 10.0; p<0.001), but signifi cance was lost after adjustment for changes in creatinine and CPK. Irisin levels were not affected by food intake or consumption patterns nor after a standardized meal. CONCLUSIONS: In healthy, young individuals circulating irisin is correlated with lean body mass but not with adipokines or insulin resistance. There is a day-night rhythm of irisin secretion. Exercise increases acutely circulating irisin levels, but food intake has no effect on them. [ABSTRACT FROM AUTHOR]

Metabolomics can be of great value in the study of exercise metabolism. However, because of the high intraindividual and interindividual biological variability of the human metabolome, special considerations should be taken into account when designing an acute-exercise metabolomic study. To study different exercise parameters, e.g., different exercise modes, intensities, etc., a crossover study design, where each participant acts as their own control, is preferable to a parallel design, one involving different groups of participants. Moreover, the study should include a no exercise, control trial. Before each trial, participants should follow carefully designed preparatory steps to control for possible confounding factors, i.e., maintain repeatable and constant conditions for all individual trials of the study to minimize variation due to factors other than the one(s) being studied. This chapter focuses on the design of human metabolomic studies, where the intervention is an acute metabolic challenge, such as an exercise bout or a test meal, and presents some basic steps for screening potential participants, performing preliminary tests, preparing for the trial day, and performing the trial.