The Effect of Exercise Intensity on Ghrelin Release

Background: Ghrelin is a gut hormone and exists in a des-acyl form (DAG ~78% of total ghrelin(TG)), and in an acylated form (AG ~22% of total ghrelin)7 : Ghrelin has numerous physiological effects, including the regulation of energy balance, insulin sensitivity, vascular health, and body composition. Exercise is a therapeutic option that can potentially optimize ghrelin levels, and sex and obesity may affect the ghrelin response to exercise. Exercise intensity may have an important role on altering not only TG, but may also have a differential effect on AG and DAG. Purpose: To identify the optimal dose of exercise to modulate ghrelin levels, and also explore the role of sex and obesity on the response to exercise. Methods: Peak oxygen consumption (VO2peak) and lactate threshold (LT) were determined via an incremental test on a cycle ergometer. Subjects had their body composition assessed via dual energy x-ray absorptiometry (DEXA) to measure total body fat percentage (BF%) and an abdominal CT scan to measure abdominal visceral fat (AVF). The testing period consisted of three randomized visits: Control (CON, no exercise), Moderate intensity exercise (MOD, power output at LT), and high intensity exercise (HIGH, power output associated with 75% of the difference between LT and peak). The caloric expenditure was kept consistent within each subject for the exercise conditions. AG, TG, DAG, and lactate were measured at the following timepoints: 0, 10, 20, 30, 40, 50, 60, 90, 120, 150, and 180 minutes. Appetite ratings (hunger, satisfaction, fullness and desire to eat) were measured via 100 mm visual analogue scale (VAS) at timepoints 0, 60, 90, 120, 150, and 180 minutes. Brachial flow mediated dilation (FMD) was measured at baseline, 30, 60, 90, and 120 minutes post-exercise. Subjects were split using previously determined cutoffs where females with a BF% >37.1% and males with a BF% > 25.8% were placed in the obese group (OG). We examined area under the curve and data at each individual timepoint for both TG, AG, DAG, FMD lactate, hunger, and appetite. Results: Eight males (age: 42.25 ± 11.0y; BMI: 26.6 ± 5.7; VO2peak: 30.65 ± 8.7 mL/kg/min) and eight females (age: 35.25 ± 11.1 y; BMI: 24.25 ± 4.7 kg/m2; VO2peak: 27.65 ± 5.7 mL/kg/min) completed all measures for the study. HIGH significantly decreased all ghrelin isoforms compared to CON (all; p <0.05). Females had a reduction of TG and DAG (p<0.05), while both males and females had a reduction of AG in HIGH compared to CON (p<0.05). OG had significantly decreased TG and DAG in HIGH compared to CON (p<0.05), and both LG and OG had suppressed AG in HIGH compared to CON (p<0.05). There was a significant inverse, relationship between AG/DAG and lactate (p<0.05). Appetite was significantly suppressed in HIGH compared to CON (p<0.05), and all forms of ghrelin were positively associated with appetite (p<0.05). HIGH had a higher %FMD than CON, and MOD had a higher %FMD compared to CON (both; p<0.0001). No ghrelin isoform was significantly related to FMD (all; p>0.05). Discussion: High intensity exercise significantly lowers plasma ghrelin levels. All isoforms of ghrelin may be associated with perception of appetite; however more work is needed to determine if the strength of such relationship differs by isoform. Our findings also suggest lactate may be involved in exercise-induced ghrelin suppression. Isocaloric acute exercise of moderate and high intensity both improved FMD to a similar extent. Although all ghrelin isoforms were suppressed following high intensity exercise, changes in FMD were not associated with changes in ghrelin levels regardless of obesity status.