Endurance training amplifies the pulsatile release of growth hormone: effects of training intensity

Growth Hormone: Myths and Misconceptions

Growth hormone is one of the many dangerous and illegal ergogenic aids currently used by athletes. In those who suffer from a growth hormone deficiency, supplementation does produce positive results: Muscle volume increases while adipose tissue volume is significantly reduced. Growth hormone supplementation can also lead to strength increases in the deficient population (2, 6, 13) as well as in the elderly population (16, 18, 25). In healthy young men, growth hormone supplementation has been shown to increase fat-free mass and to decrease fat mass. However, these changes are not accompanied by strength gains (5, 7, 23, 24). This finding, coupled with the numerous side effects associated with the drug, presents a strong case for athletes to abandon its use as an ergogenic aid.

Growth hormone and aging: Regulation, signal transduction and replacement therapy

It is widely accepted that during the aging process a number of alterations occur at the molecular, cellular, and tissue levels, ranging from an alteration in receptor signal transduction and gene expression to structural and morphological changes in various tissues. There is abundant empirical and scientific evidence to support the hypothesis that many of these aging processes are closely related to a decline in hormone concentrations and/or hormone action, but the etiology of these deficiencies remains elusive. GH and insulinlike growth factor-I (IGF-I) are two potent anabolic hormones that decrease with age and appear to contribute to the loss of tissue function that is associated with normal aging. In this review, age-related changes in the regulation of these hormones are detailed as well as relatively new information on mechanisms of tissue resistance to GH. Finally, the potential use of GH as a therapeutic intervention to delay physiological changes associated with age is discussed.

Blood hormones as markers of training stress and overtraining

An imbalance between the overall strain experienced during exercise training and the athlete's tolerance of such effort may induce overreaching or overtraining syndrome. Overtraining syndrome is characterised by diminished sport-specific physical performance, accelerated fatiguability and subjective symptoms of stress. Overtraining is feared by athletes yet there is a lack of objective parameters suitable for its diagnosis and prevention. In addition to the determination of substrates (e.g. lactate, ammonia and urea) and enzymes (e.g. creatine kinase), the possibilities for monitoring of training by measuring hormonal levels in blood are currently being investigated. Endogenous hormones are essential for physiological reactions and adaptations during physical work and influence the recovery phase after exercise by modulating anabolic and catabolic processes. Testosterone and cortisol are playing a significant role in metabolism of protein as well as carbohydrate metabolism. Both are competitive agonists at the receptor level of muscular cells. The testosterone/cortisol ratio is used as an indication of the anabolic/catabolic balance. This ratio decreases in relation to the intensity and duration of physical exercise, as well as during periods of intense training or repetitive competition, and can be reversed by regenerative measures. Correlations have been noted with the training-induced changes of strength. However, it seems more likely that the testosterone/cortisol ratio indicates the actual physiological strain in training, rather than overtraining syndrome. The sympatho-adrenergic system might be involved in the pathogenesis of overtraining. Overtraining appears as a disturbed autonomic regulation, which in its parasympathicotonic form shows a diminished maximal secretion of catecholamines, combined with an impaired full mobilisation of anaerobic lactic reserves. This is supposed to lead to decreased maximal blood lactate levels and maximal performance. Free plasma adrenaline (epinephrine) and noradrenaline (norepinephrine) may provide additional information for the monitoring of endurance training. While prolonged aerobic exercise conducted at intensities below the individual anaerobic threshold lead to a moderate rise of sympathetic activity, workloads exceeding this threshold are characterised by a disproportionate increase in the levels of catecholamines. In addition, psychological stress during competitive events is characterised by a higher catecholamines to lactate ratio in comparison with training exercise sessions. Thus, the frequency of training sessions with higher anaerobic lactic demands or of competition, should be carefully limited in order to prevent overtraining syndrome. In the state of overtraining syndrome and overreaching, respectively, an intraindividually decreased maximum rise of pituitary hormones (corticotrophin, growth hormone), cortisol and insulin has been found after a standardised exhaustive exercise test performed with an intensity of 10% above the individual anaerobic threshold.(ABSTRACT TRUNCATED AT 400 WORDS)

Altered endogenous growth hormone secretory kinetics and diurnal GH-binding protein profiles in adults with chronic liver disease

OBJECTIVE

Increased serum GH concentrations and GH responses to a variety of stimuli have been reported in patients with chronic liver disease (CLD). We investigated the pulsatile pattern of endogenous GH release and GH-binding protein (GHBP) and insulin-like growth factor-I (IGF-I) diurnal profiles in adults with cirrhosis, in comparison with healthy, matched control subjects.

DESIGN

Case-control, cross-sectional.

PATIENTS

Seven patients with biopsy proven cirrhosis, and sex, age, height, weight and oestrogen status matched controls.

MEASUREMENTS

Serum immunoreactive GH concentrations in samples collected at 20-minute intervals for 24 hours were analysed using a multi-parameter deconvolution method to simultaneously resolve endogenous GH secretory and disappearance rates. Diurnal patterns of GHBP (specific immunoprecipitation method) and serum IGF-I (RIA after acid-ethanol extraction) were assessed.

RESULTS

The mean daily GH secretion rate in patients with CLD was increased (210 +/- 93 vs 100 +/- 55 mU/I/day; P = 0.025), and GH disappearance half-time was prolonged (43 +/- 10 vs 24 +/- 9 min; P = 0.006) compared to controls. Detectable GH secretory bursts were more frequent in patients with CLD (10 +/- 1 vs 6 +/- 3/day; P = 0.038), but of similar mean mass (21 +/- 10 vs 17 +/- 8 mU/I) compared to controls. In patients with CLD, mean serum GHBP was slightly lower (63 +/- 36 vs 71 +/- 14% pooled control; P > 0.1). Fasting serum IGF-I concentrations (after size-exclusion HPLC) were lower in the patients with CLD (13 +/- 5 vs 21 +/- 2 nmol/l; P < 0.0001). Multiple regression analysis showed that GH secretion rate was increased in patients with CLD with higher Child's classifications (R2 = 0.86; P = 0.002) and with lower serum IGF-I concentrations measured after HPLC (R2 = 0.11; P = 0.044).

CONCLUSIONS

Adults with chronic liver disease have (1) increased total daily GH secretion rates, which appear to be influenced by disease severity and diminished serum IGF-I-mediated negative feedback; (2) markedly impaired endogenous GH clearance, possibly reflecting changes in hepatic GH-receptor status; and (3) GHBP levels which do not correlate with GH kinetics or serum IGF-I concentrations.

Muscle function and origin of pain in fibromyalgia

It may be concluded that both peripheral and central mechanisms may operate in the pathophysiology of both impaired muscle function and pain in FM. These mechanisms may in part be attributable to physical deconditioning and disuse of muscle secondary to the characteristic pain and fatigue so often seen in FM. Most likely the initiation of this condition is multifactorial and the combination of peripheral and central factors that constitute a vicious circle may perpetuate the condition into a chronic state.

Serum insulin-like growth factor I in healthy older men in relation to physical activity

OBJECTIVE

The serum level of insulin-like growth factor-I (IGF-I) is lower in old men than in young men. It has been theorized that this change may result in part from the lower level of physical activity in the elderly. The objective of the present study was to test this theory.

DESIGN

Cross-sectional study.

SETTING

Community-dwelling healthy men.

PARTICIPANTS

Twenty-nine young men aged 21-29 and 120 older men aged 58-98. A wide range of habitual physical activity was assured in the older men by recruiting them both in senior centers and in fitness centers.

MEASUREMENTS

Serum IGF-I level was measured by radioimmunoassay. Physical activity was estimated as the sum of moderate, hard, and very hard activities in leisure and occupation during the previous week as determined by the interviewer-administered Seven Day Physical Activity Recall questionnaire.

RESULTS

IGF-I was significantly higher (363.7 +/- 16 ng/mL, avg +/- SEM) in the young men than in the senior center older men (219.6 +/- 5 ng/mL) or in the fitness center older men (168.3 +/- 11 ng/mL) (P < 0.01). The level of IGF-I in the senior center older men was higher than that in the fitness center older men (P = 0.05). The physical activity of the fitness center older men (128.5 +/- 15 Kcal/kg/wk, avg +/- SE) was statistically similar to that of the young men (101.3 +/- 19 Kcal/kg/wk). The physical activity levels of both of these groups were significantly higher than that of the senior center older men (33.6 +/- 3 Kcal/kg/wk) (P < 0.010). Within none of the three groups was there a significant correlation between IGF-I and physical activity. When the two groups of older men were combined, there was, in fact, an inverse correlation of IGF-I and physical activity (P = 0.001).

CONCLUSION

The data do not support the hypothesis that lack of physical activity is responsible for the decline in serum IGF-I with advancing age.

The effects of walking at the anaerobic threshold level on vertebral bone loss in postmenopausal women

The purpose of this study was to determine the optimal intensity of exercise necessary to prevent the postmenopausal bone loss on the basis of anaerobic threshold (AT). Thirty-three postmenopausal women were randomized to control (group C: n = 12) or two exercise groups (group H and group M). All women performed a treadmill exercise test, and the AT was measured by expired gas analysis. The exercise regimen consisted mainly of walking at a speed that kept the exercise heart rate above the AT (group H: n = 12) or below the AT (group M: n = 9). Exercise was performed for 30 minutes, three times a week for 7 months. The bone mineral density (BMD) of the lumbar vertebrae was measured using dual energy X-ray absorptiometry. The BMD level in group C decreased by 1.7 +/- 2.7%, but there was a significant increase of 1.1 +/- 2.9% in group H. In group M there was a decrease of 1.0 +/- 3.1% which did not differ from group C. In group C, serum osteocalcin and urinary hydroxyproline excretion were significantly increased, but no changes were seen in either of the exercise groups. Urinary calcium significantly decreased in the exercise groups. We conclude that short-term (7 months) exercise with intensity above the AT is safe and effective in preventing postmenopausal bone loss.