Context: The myokine irisin may increase energy expenditure and affect metabolism.
Objective: The objective of the study was to elucidate predictors of irisin and study whether circulating irisin may have day-night rhythm in humans.
Design: This was an observational, cross-sectional study with an additional 24-hour prospective observational arm (day-night rhythm substudy) and two prospective interventional arms (mixed meal substudy and exercise substudy).
Setting: The study was conducted at the Hellenic Military School of Medicine (Thessaloniki, Greece).
Patients and interventions: One hundred twenty-two healthy, young individuals were subjected to anthropometric and body composition measurements, and their eating and exercise behavior profiles were assessed with validated questionnaires. Subgroups were subjected to day-night rhythm, standardized meal ingestion, and 30-minute aerobic exercise studies.
Main outcome measures: Circulating irisin levels were measured.
Results: Ιrisin levels were lower in males than females (P = .02) after adjustment for lean body mass, which was its major determinant. Irisin levels followed a day-night rhythm (P < .001) with peak at 9:00 pm. Irisin levels were increased at the end of exercise (84.1 ± 10.0 vs 105.8 ± 14.3 ng/mL; P < .001). Irisin levels were not affected by intake of a standardized meal and were not associated with caloric intake or diet quality.
Conclusions: In healthy, young individuals, circulating irisin displays a day-night rhythm, is correlated with lean body mass, and increases acutely after exercise.
Trial registration: ClinicalTrials.gov NCT01986530.