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Exercise and the growth hormone-insulin-like growth factor axis
1The Medical Research Laboratories, Clinical Institute of Medicine, and Medical Department M (Diabetes and Endocrinology), Aarhus University Hospital, Aarhus, Denmark. jan@frystyk.dk
Medicine and Science in Sports and Exercise
|December 17, 2009
Summary
Exercise stimulates growth hormone (GH) and insulin-like growth factor I (IGF-I), crucial for muscle growth. This review explores whether local (paracrine/autocrine) IGF-I, not just circulating levels, drives exercise-induced muscle hypertrophy.
Area of Science:
- Exercise physiology
- Endocrinology
- Muscle biology
Background:
- Exercise stimulates pituitary growth hormone (GH) secretion, increasing within 15 minutes.
- GH and insulin-like growth factor I (IGF-I) are vital for skeletal muscle development, maintenance, and repair.
- The link between exercise-induced GH and muscle hypertrophy is logical but requires clarification regarding IGF-I's role.
Purpose of the Study:
- To review the role of free and bioactive IGF-I in understanding the relationship between exercise, muscle hypertrophy, and the GH/IGF-I axis.
- To discuss IGF-I assay methodologies and current literature on different IGF-I forms in exercising individuals.
Main Methods:
- Literature review focusing on exercise studies investigating GH and IGF-I.
- Analysis of studies measuring immunoreactive (total), free, and bioactive IGF-I.
- Examination of longitudinal exercise data correlating fitness changes with IGF-I levels.
Main Results:
- Longitudinal studies suggest exercise enhances muscle strength and performance without significant increases in circulating IGF-I.
- Evidence indicates that paracrine/autocrine IGF-I, rather than endocrine IGF-I, may primarily mediate exercise-induced skeletal muscle adaptations.
- Newer studies measuring free and bioactive IGF-I offer deeper insights than traditional total IGF-I measurements.
Conclusions:
- Paracrine/autocrine IGF-I is likely more critical than circulating IGF-I for exercise-induced muscle hypertrophy.
- Measuring free and bioactive IGF-I provides a more nuanced understanding of the GH/IGF-I axis in response to exercise.
- Further research into IGF-I assay methodologies is needed to fully elucidate its role in exercise adaptation.
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