Journal Title (Medline/Pubmed accepted abbreviation): J. Clin. Endocrinol. Metab.
doi (if applicable): 10.1210/jc.2010-2264
Summary of Background and Research Design
Background:Growth hormone (GH) and insulin-like growth factor (IGF-1) are anabolic hormones that are upregulated with exercise. There is evidence that they are also positively correlated with mitochondrial function, including rate of β-oxidation (i.e. fat breakdown), and increased rate of recovery as measured by recovery of phosphocreatine (PCr) in muscle. PCr is a molecule that rapidly transfers phosphate to adenosine diphosphate (ADP) to create adenosine triphosphate (ATP), or energy. ATP reserves are low after high intensity exercise, and therefore PCr is also low. During the recovery period ATP and PCr are replenished.
Research question: Does GH or IGF-1 correlate with PCr recovery during the 2 hr post-exercise interval?
Subjects:Males (n=16) and females (n=21) age 38.6 ± 1.7 y
Treatments and protocol:PCr was measured using 31P magnetic resonance spectroscopy (MRS, similar to magnetic resonance imaging, MRI). Baseline measurements were acquired, and subjects were asked to perform squats at a constant load of 40% maximal voluntary contraction and a rate of 1 every 2 sec for 3 min. After a 5 min recovery period, MRS spectra were acquired and PCr recovery could be monitored.
GH was assessed by measuring basal (resting) GH and peak GH by artificially stimulating its release with arginine. Sermorelin acetate (GHRH 1-29) at 1 µg/kg body weight then arginine hydrochloride (0.5 g/kg or 30 g maximum) were administered. Blood samples were acquired at 0, 30, 45, 60, 90, and 120 min after administration of sermorelin. Testosterone was measured in male subjects and estradiol was assessed in all subjects.
Summary of research findings:
- Men vs. women and normal weight [body mass index (BMI) < 25 kg/m2] vs. obese subjects (BMI ≥ 30) responded similarly to exercise in regard to pH of blood, PCr depletion, the rate of mitochondrial phosphorylation to regenerate PCr (τPCr), and the initial rate of PCr recovery (ViPCr),
- ViPCr was positively correlated with IGF-I in both men and women (r = 0.33, p = 0.04).
- In men only, τPCr was negatively correlated with IGF-I (r = -0.54, p = 0.03).
- Also in men only, ViPCr was associated with peak stimulated GH (r = 0.52, p = 0.04) and the total amount of GH released over the 2 hr period (area under the curve, or AUC, of GH vs. time, r = 0.53, p = 0.04).
- No correlations were found between total testosterone, free testosterone, or estradiol in men or women.
- Interestingly, ViPCr nor τPCr were associated with any GH measurements or calculations in women.
- Frequency of physical activity (self-reported) was positively associated with ViPCr in men (p = 0.002) but not women.
Interpretation of findings/Key practice applications:
There was a clear correlation between the rate of PCr recovery and the hormonal response (GH and IGF-I) in men, but not in women. However, since GH and PCr were not measured at the same time, it is impossible to draw any cause-effect relationships from these data. This study provides a basis for future work regarding the relationship between hormone response and mitochondrial function as well as gender differences in the response to exercise.
As stated above, causation cannot be drawn between the parameters measured in this study since this was a cross-sectional study. Also, GH was stimulated artificially (with an arginine injection), not with exercise. It would be interesting to assess correlations in the rate of recovery and GH at the same time, after resistance training.