Abstract
Insulin-like growth factor-1 (IGF-1) is a trophic factor promoting growth and survival in various types of cells including muscle cells and neurons. In our recent study, young men showed comparable IGF-1 responses in continuous and intermittent exercise at lactate threshold, whereas stress responses were smaller in the intermittent exercise. It is yet unclear if the exercise format affects the IGF-1 response in older individuals. PURPOSE: To compare changes in the circulating level of IGF-1 in middle-aged and older men during continuous and intermittent cycling at lactate threshold. METHODS: Six men, middle-aged and older (57.8±11.6 years), randomly performed two cycling tests at lactate threshold load (108.8±27.3 W) and a control test with rest on separate days. The cycling tests comprised a 20-min continuous cycling (CC) and an intermittent cycling (IC) consisting of 20 repetitions of 1-min bouts separated by 30-sec rests. The control test was administered with a schedule identical to CC. During each session, blood samples were drawn via peripheral cannulation at rest, at 25, 50, and 100% of cycling time, and 10 min after cycling. A blood concentration of IGF-1 and those of lactate, cortisol, and catecholamines were measured at each time point. Two-way analyses of variance for repeated measures with post-hoc tests, if appropriate, were performed to mainly compare changes between CC and IC. RESULTS: Significant interactions were found in all the indices (p<.05). Changes in IGF-1 concentration were comparable between CC (102.0±29.3 to 116.0±30.2 ng/ml, p<.05) and IC (104.2±33.6 to 112.0±38.6 ng/ml, p<.05). In contrast, although concentrations of lactate and norepinephrine increased in both CC and IC (p<.05), the effect was greater for CC (p<.05). Furthermore, dopamine concentration increased only in CC (p<.05) while cortisol concentration decreased in IC (p<.05) but not in CC. CONCLUSIONS: As previously shown in young men, intermittent cycling at lactate threshold evoked an IGF-1 response comparable to its continuous counterpart in men middle-aged and older, but the two cycling formats induced different stress responses. These results suggest that moderate intermittent exercise has the potential to stimulate the IGF-1 pathway without considerable stress in older individuals. Supported by JSPS 25242065.
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