Ghrelin is related to energy balance that regulates food intake. We explored whether Ghrelin would be linked to differences in fitness in adolescent girls, whose menstrual cycles are sensitive to changes in energy balance.

Methods: Seventy-two girls ages 14 and 21 years, including 5 with amenorrhea, were studied in the follicular phase. Luteinizing hormone (LH) was sampled every 10 minutes over 24-hours and ghrelin was measured hourly between 23:00 and 03:00 h. Body composition was measured by dual X-ray absoprtiometry. Fitness was characterized by reported frequency of exercise per week and by maximal oxygen consumption (VO₂max) with "high" versus "low" fitness groups defined from VO₂max norms for this population. Data was analyzed with SAS© software.

Results: Ghrelin was related to percent body fat (p = 0.038, R² 0.07), weekly exercise (p = 0.032, R² 0.07), and 24-hour mean LH (p = 0.002, R² 0.13). The Ghrelin relationship with LH was more pronounced in the low fitness group. In multiple regression models, 24-hour LH was an independent predictor of Ghrelin after adjusting for percent body fat, fitness, exercise or age. Ghrelin was higher in Caucasian girls than in African-American girls after adjusting for covariates at 02:00 h (p = 0.017).

Conclusion: Twenty-four hour LH is an independent predictor of night-time Ghrelin concentrations in post-pubertal adolescent girls. Diverging patterns in Ghrelin may reflect differences in exercise patterns and/or may be influenced by ethnicity. This data introduces Ghrelin as a biomarker of individual differences in energy balance during the menstrual cycle and across ethnicities.