Gender governs the relationship between exercise intensity and growth hormone release in young adults

The multiple roles of GH in neural ageing and injury

Advanced age is typically associated with a decrease in cognitive function including impairment in the formation and retention of new memories. The hippocampus is critical for learning and memory, especially spatial learning, and is particularly affected by ageing. With advanced age, multiple neural components can be detrimentally affected including a reduction in the number of neural stem and precursor cells, a decrease in the formation of adult born neurons (neurogenesis), and deficits in neural circuitry, all of which ultimately contribute to impaired cognitive function. Importantly, physical exercise has been shown to ameliorate many of these impairments and is able to improve learning and memory. Relevantly, growth hormone (GH) is an important protein hormone that decreases with ageing and increases following physical exercise. Originally described due to its role in longitudinal growth, GH has now been identified to play several additional key roles, especially in relation to the brain. Indeed, the regular decrease in GH levels following puberty is one of the most well documented components of neuroendocrine ageing. Growth hormone deficiency (GHD) has been described to have adverse effects on brain function, which can be ameliorated via GH replacement therapy. Physical exercise has been shown to increase circulating GH levels. Furthermore, we recently demonstrated the increase in exercise-mediated GH is critical for improved cognitive function in the aged mouse. Here we examine the multiple roles that GH plays, particularly in the aged brain and following trauma, irradiation and stroke, and how increasing GH levels can ameliorate deficits in cognition.

Time-of-Day Effects on Anaerobic Power and Concentration of Selected Hormones in Blind Men

Knowledge of the circadian rhythm of the blind person and diurnal changes in anaerobic power and hormones concentration can create the possibility of individualising physical training. The aim of the study was to examine the time-of-day effects on anaerobic performance and the concentration of selected hormones. The measurements were performed at two different times of the day (10:00 a.m., 10:00 p.m.) in blind men at the age of 20–25 years old. The experiment group was chosen by using repeated hormonal tests four times a day so that each selected patient had a sleep/wake cycle even of 24 h. Anaerobic peak power and total work were tested in an anaerobic sprint test, and the concentration of growth hormone, testosterone, cortisol, and melatonin was determined. In blind men, the hormonal response was not driven by the photoperiod as in the control group. In the blind group, at 10:00 p.m., anaerobic peak power and total work results were significantly higher than at 10:00 a.m. and negatively correlated with melatonin levels. No such correlation was found in the control group.

Interplay between hormones and exercise on hippocampal plasticity across the lifespan

The hippocampus is a brain structure known to play a central role in cognitive function (namely learning and memory) as well as mood regulation and affective behaviors due in part to its ability to undergo structural and functional changes in response to intrinsic and extrinsic stimuli. While structural changes are achieved through modulation of hippocampal neurogenesis as well as alterations in dendritic morphology and spine remodeling, functional (i.e., synaptic) changes can be noted through the strengthening (i.e., long-term potentiation) or weakening (i.e., long-term depression) of the synapses. While age, hormone homeostasis, and levels of physical activity are some of the factors known to module these forms of hippocampal plasticity, the exact mechanisms through which these factors interact with each other at a given moment in time are not completely understood. It is well known that hormonal levels vary throughout the lifespan of an individual and it is also known that physical exercise can impact hormonal homeostasis. Thus, it is reasonable to speculate that hormone modulation might be one of the various mechanisms through which physical exercise differently impacts hippocampal plasticity throughout distinct periods of an individual's life. The present review summarizes the potential relationship between physical exercise and different types of hormones (namely sex, metabolic, and stress hormones) and how this relationship may mediate the effects of physical activity during three distinct life periods, adolescence, adulthood, and senescence. Overall, the vast majority of studies support a beneficial role of exercise in maintaining hippocampal hormonal levels and consequently, hippocampal plasticity, cognition, and mood regulation.

Pilot study: an acute bout of high intensity interval exercise increases 12.5 h GH secretion

The purpose of this study was to test the hypothesis that high‐intensity interval exercise (HIE) significantly increases growth hormone (GH) secretion to a greater extent than moderate‐intensity continuous exercise (MOD) in young women. Five young, sedentary women (mean ± SD; age: 22.6±1.3 years; BMI: 27.4±3.1 kg/m²) were tested during the early follicular phase of their menstrual cycle on three occasions. For each visit, participants reported to the laboratory at 1700 h, exercised from 1730–1800 h, and remained in the laboratory until 0700 h the following morning. The exercise component consisted of either 30‐min of moderate‐intensity continuous cycling at 50% of measured peak power (MOD), four 30‐s “all‐out” sprints with 4.5 min of active recovery (HIE), or a time‐matched sedentary control using a randomized, cross‐over design. The overnight GH secretory profile of each trial was determined from 10‐min sampling of venous blood from 1730–0600 h, using deconvolution analysis. Deconvolution GH parameters were log transformed prior to statistical analyses. Calculated GH AUC (0–120 min) was significantly greater in HIE than CON (P = 0.04), but HIE was not different from MOD. Total GH secretory rate (ng/mL/12.5 h) was significantly greater in the HIE than the CON (P = 0.05), but MOD was not different from CON or HIE. Nocturnal GH secretion (ng/mL/7.5 h) was not different between the three trials. For these women, in this pilot study, a single bout of HIE was sufficient to increase 12.5 h pulsatile GH secretion. It remains to be determined if regular HIE may contribute to increased daily GH secretion.

Vigorous Intervals and Hypoglycemia in Type 1 Diabetes: A Randomized Cross Over Trial

Adding vigorous-intensity intervals (VII) to moderate-intensity exercise prevents immediate declines in blood glucose in type 1 diabetes (T1D) however the intensity required to minimize post-exercise hypoglycemia is unknown. To examine this question, ten sedentary T1D individuals completed four treadmill exercise sessions: a control session of 45 minutes of walking at 45–55% of heart rate reserve (HRR) and three sessions consisting of 60 seconds (VII) at 70%, 80%, or 90% of HRR every 4 minutes during exercise at 45–55% of HRR. We used continuous glucose monitoring (CGM) to measure time ≤3.9 mmol/L, glucose variability, hypoglycemia frequency and area under the curve (AUC) for hypoglycemia and hyperglycemia for 12 hours post-exercise. We also examined growth hormone and cortisol responses during and following exercise. In the 12 hours post-exercise, the percentage of time ≤3.9 mmol/L, glucose variability, and AUC for hypoglycemia and hyperglycemia were similar across conditions. The frequency of hypoglycemic events was highest after the 90% intervals compared to the control arm (12 vs 3 events, p = 0.03). There was a trend towards elevated growth hormone with increasing exercise intensity but cortisol levels were similar across conditions. Adding VII to moderate intensity exercise may increase hypoglycemia risk at higher intensities.

Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance

Elite athletic competitions have separate male and female events due to men's physical advantages in strength, speed, and endurance so that a protected female category with objective entry criteria is required. Prior to puberty, there is no sex difference in circulating testosterone concentrations or athletic performance, but from puberty onward a clear sex difference in athletic performance emerges as circulating testosterone concentrations rise in men because testes produce 30 times more testosterone than before puberty with circulating testosterone exceeding 15-fold that of women at any age. There is a wide sex difference in circulating testosterone concentrations and a reproducible dose-response relationship between circulating testosterone and muscle mass and strength as well as circulating hemoglobin in both men and women. These dichotomies largely account for the sex differences in muscle mass and strength and circulating hemoglobin levels that result in at least an 8% to 12% ergogenic advantage in men. Suppression of elevated circulating testosterone of hyperandrogenic athletes results in negative effects on performance, which are reversed when suppression ceases. Based on the nonoverlapping, bimodal distribution of circulating testosterone concentration (measured by liquid chromatography-mass spectrometry)-and making an allowance for women with mild hyperandrogenism, notably women with polycystic ovary syndrome (who are overrepresented in elite athletics)-the appropriate eligibility criterion for female athletic events should be a circulating testosterone of <5.0 nmol/L. This would include all women other than those with untreated hyperandrogenic disorders of sexual development and noncompliant male-to-female transgender as well as testosterone-treated female-to-male transgender or androgen dopers.

The Wingate anaerobic test cannot be used for the evaluation of growth hormone secretion in children with short stature

PURPOSE

To assess the growth hormone (GH) response to the Wingate anaerobic test (WAnT) among children with short stature and suspected GH deficiency. We hypothesized that the GH response to the WAnT would be similar to the GH response to a commonly used pharmacologic provocation test.

METHODS

Ten children (6 males and 4 females, age range 9.0-14.9 years) participated in the study. Each participant performed 2 tests: a standard all-out WAnT, cycling for 30 s against constant resistance, and a standardized pharmacologic test (clonidine or glucagon). Blood samples for GH were collected before and 10, 30, 45, and 60 min after the beginning of exercise. In addition, we collected pre- and post-exercise blood lactate levels.

RESULTS

There was a significant increase in GH levels after the WAnT, yet in 9 of 10 participants, this increase was below the threshold for GH sufficiency. Peak GH after the WAnT was significantly lower compared to the pharmacologic GH provocation tests (with 9 of 10 demonstrating GH-sufficient response).

CONCLUSION

The traditional WAnT cannot be used as a GH provocation test. Further research is needed to develop anaerobic exercise protocols sufficient to promote GH secretion.

Why should people with type 1 diabetes exercise regularly?

Plethoric evidence reminds of the protective effects of exercise against a number of health risks, across all ages, in the general population. The benefits of exercise for individuals with type 2 diabetes are indisputable. An in-depth understanding of energy metabolism has reasonably entailed exercise as a cornerstone in the lifestyle of almost all subjects with type 1 diabetes. Nevertheless, individuals with type 1 diabetes often fail in accomplishing exercise guidelines and they are less active than their peer without diabetes. Two major obstacles are feared by people with type 1 diabetes who wish to exercise regularly: management of blood glucose control and hypoglycemia. Nowadays, strategies, including glucose monitoring technology and insulin pump therapy, have significantly contributed to the participation in regular physical activity, and even in competitive sports, for people with type 1 diabetes. Novel modalities of training, like different intensity, interspersed exercise, are as well promising. The beneficial potential of exercise in type 1 diabetes is multi-faceted, and it has to be fully exploited because it goes beyond the insulin-mimetic action, possibly through immunomodulation.

Rigorous Running Increases Growth Hormone and Insulin-Like Growth Factor-I Without Altering Ghrelin

It has been suggested that ghrelin may play a role in growth hormone (GH) responses to exercise. The present study was designed to determine whether ghrelin, GH, insulin-like growth factor-I (IGF-I), and IGF-binding protein-3 (IGFBP-3) were altered by a progressively intense running protocol. Six well-trained male volunteers completed a progressively intense intermittent exercise trial on a treadmill that included four exercise intensities: 60%, 75%, 90%, and 100% of Vo2max. Blood samples were collected before exercise, after each exercise intensity, and at 15 and 30 mins following the exercise protocol. Subjects also completed a separate control trial at the same time of day that excluded exercise. GH changed significantly over time, and GH area under the curve (AUC) was significantly higher in the exercise trial than the control trial. Area under the curve IGF-I levels for the exercise trial were significantly higher than the control trial. There was no difference in the ghrelin and IGFBP-3 responses to the exercise and control trials. Pearson correlation coefficients revealed significant relationships between ghrelin and both IGF-I and IGFBP-3; however, no relationship between ghrelin and GH was found. In conclusion, intense running produces increases in total IGF-I concentrations, which differs from findings in previous studies using less rigorous running protocols and less frequent blood sampling regimens. Moreover, running exercise that produces substantial increases in GH does not affect peripheral ghrelin levels; however, significant relationships between ghrelin and both IGF-I and IGFBP-3 exist during intense intermittent running and recovery, which warrants further investigation.

Netnography of Female Use of the Synthetic Growth Hormone CJC-1295: Pulses and Potions

BACKGROUND

Communal online folk pharmacology fuels the drive for short cuts in attaining muscle enhancement, fat loss, and youthful skin.

OBJECTIVES

The study used "netnography" to explore female use of CJC-1295, a synthetic growth hormone analogue from the perspectives contained in Internet forum activity.

METHODS

A systematic Internet search was conducted using variation of the term "CJC-1295"; and combined with "forum." Ninety-six hits related to bodybuilding websites where CJC-1295 was mentioned. Following application of exclusion criteria to confine to female use and evidence of forum activity, 9 sites remained. These were searched internally for reference to CJC-1295. Twenty-three discussion threads relating to female use of CJC-1295 formed the end data set, and analyzed using the Empirical Phenomenological Psychological method.

RESULTS

Forum users appeared well versed and experienced in the poly use of performance and image drug supplementation. Choice to use CJC-1295 centered on weight loss, muscle enhancement, youthful skin, improved sleep, and injury healing. Concerns were described relating to female consequences of use given gender variations in growth hormone pulses affecting estimation of dosage, cycling, and long-term consequences.

CONCLUSIONS

Public health interventions should consider female self-medicating use of synthetic growth hormone within a repertoire of product supplementation, and related adverse health consequences.

Effect of gender on the GH-IGF-I response to anaerobic exercise in young adults

Exercise-associated effects on the growth hormone-insulin-like growth factor-I (GH-IGF-I) axis were studied, mainly after aerobic exercise. We determined the gender effect on the GH-IGF-I axis response to a standard all-out Wingate anaerobic test (WAnT) in healthy active young adult men and women (men = 12 and women = 16; age range: 24-34 years). Blood samples for GH and IGF-I, key elements of the GH-IGF-I axis, were collected before and 20, 30, 40, and 60 minutes after the beginning of exercise. In addition, we collected postexercise blood lactate levels. Postexercise lactate levels were higher among men; however, this difference did not reach statistical significance (13.8 ± 1.3 vs. 11.1 ± 1.0 mmol·L, respectively; p = 0.1). The WAnT was associated with a significant increase in GH in both genders. However, GH peak was greater among women (10.8 ± 1.8 vs. 5.6 ± 1.4 ng·ml, in women and men, respectively; p < 0.01). In addition, postexercise GH peak occurred significantly earlier in female (20 minutes) compared with male participants (40 minutes). Exercise was associated with a significant increase in IGF only among men (from 166.8 ± 8.4 to 186.9 ± 9.3; p < 0.02); however, no significant between-gender effect was found. In summary, supramaximal anaerobic exercise was associated with a greater and earlier postexercise GH peak in women compared with men. All together, the results suggest anaerobic exercise-related anabolic-type hormonal response.

Acute resistance exercise stimulates sex-specific dimeric immunoreactive growth hormone responses

PURPOSE

We sought to determine if an acute heavy resistance exercise test (AHRET) would elicit sex-specific responses in circulating growth hormone (GH), with untreated serum and serum treated with a reducing agent to break disulfide-bindings between GH dimers.

METHODS

19 untrained participants (nine men and ten women) participated in an acute heavy resistance exercise test using the back squat. Blood samples were drawn before exercise (Pre), immediate post (IP), +15 min (+15), and +30 min (+30) afterwards. Serum samples were chemically reduced using glutathione (GSH). ELISAs were then used to compare immunoreactive GH concentrations in reduced (+GSH) and non-reduced (-GSH) samples. Data were analyzed using a three-way (2 sex × 2 treatment × 4 time) mixed methods ANOVA, with significance set at p ≤ 0.05.

RESULTS

GSH reduction resulted in increased immunoreactive GH concentrations when compared to non-reduced samples at Pre (1.68 ± 0.33 μg/L vs 1.25 ± 0.25 μg/L), IP (7.69 ± 1.08 μg/L vs 5.76 ± 0.80 μg/L), +15 min (4.39 ± 0.58 μg/L vs 3.24 ± 0.43 μg/L), and +30 min (2.35 ± 0.49 μg/L vs 1.45 ± 0.23 μg/L). Also, women demonstrated greater GH responses compared to men, and this was not affected by reduction.

CONCLUSIONS

Heavy resistance exercise increases immunoreactive GH dimer concentrations in men and women, with larger increases in women and more sustained response in men. The physiological significance of a sexually dimorphic GH response adds to the growing literature on aggregate GH and may be explained by differences in sex hormones and the structure of the GH cell network.

Silvestre DH, Cavalcanti-de-Albuquerque JPA, Louzada RA, Carvalho DP, Werneck-de-Castro JP. Thyroid hormone and estrogen regulate exercise-induced growth hormone release

Growth hormone (GH) regulates whole body metabolism, and physical exercise is the most potent stimulus to induce its secretion in humans. The mechanisms underlying GH secretion after exercise remain to be defined. The aim of this study was to elucidate the role of estrogen and pituitary type 1 deiodinase (D1) activation on exercise-induced GH secretion. Ten days after bilateral ovariectomy, animals were submitted to 20 min of treadmill exercise at 75% of maximum aerobic capacity and tissues were harvested immediately or 30 min after exercise. Non-exercised animals were used as controls. A significant increase in D1 activity occurred immediately after exercise (~60%) in sham-operated animals and GH was higher (~6-fold) 30 min after exercise. Estrogen deficient rats exhibited basal levels of GH and D1 activity comparable to those found in control rats. However, after exercise both D1 activity and serum GH levels were blunted compared to sedentary rats. To understand the potential cause-effect of D1 activation in exercise-induced GH release, we pharmacologically blocked D1 activity by propylthiouracil (PTU) injection into intact rats and submitted them to the acute exercise session. D1 inhibition blocked exercise-induced GH secretion, although basal levels were unaltered. In conclusion, estrogen deficiency impairs the induction of thyroid hormone activating enzyme D1 in the pituitary, and GH release by acute exercise. Also, acute D1 activation is essential for exercise-induced GH response.

Exercise-Induced growth hormone during acute sleep deprivation

The effect of acute (24‐h) sleep deprivation on exercise‐induced growth hormone (GH) and insulin‐like growth factor‐1 (IGF‐1) was examined. Ten men (20.6 ± 1.4 years) completed two randomized 24‐h sessions including a brief, high‐intensity exercise bout following either a night of sleep (SLEEP) or (24‐h) sleep deprivation (SLD). Anaerobic performance (mean power [MP], peak power [PP], minimum power [MinP], time to peak power [TTPP], fatigue index, [FI]) and total work per sprint [TWPS]) was determined from four maximal 30‐sec Wingate sprints on a cycle ergometer. Self‐reported sleep 7 days prior to each session was similar between SLEEP and SLD sessions (7.92 ± 0.33 vs. 7.98 ± 0.39 h, P =0.656, respectively) and during the actual SLEEP session in the lab, the total amount of sleep was similar to the 7 days leading up to the lab session (7.72 ± 0.14 h vs. 7.92 ± 0.33 h, respectively) (P =0.166). No differences existed in MP, PP, MinP, TTPP, FI, TWPS, resting GH concentrations, time to reach exercise‐induced peak GH concentration (TTP), or free IGF‐1 between sessions. GH area under the curve (AUC) (825.0 ± 199.8 vs. 2212.9 ± 441.9 μg/L*min, P <0.01), exercise‐induced peak GH concentration (17.8 ± 3.7 vs. 39.6 ± 7.1 μg/L, P <0.01) and ΔGH (peak GH – resting GH) (17.2 ± 3.7 vs. 38.2 ± 7.3 μg/L, P <0.01) were significantly lower during the SLEEP versus SLD session. Our results indicate that the exercise‐induced GH response was significantly augmented in sleep‐deprived individuals.

Human growth hormone release is heavily influenced by sleep and exercise. Our study shows that sleep deprivation dramatically augments the exercise‐induced human growth hormone response.

Oral contraception enhances growth hormone responsiveness to hyper- and hypoglycaemia

AIMS

Plasma glucose levels influence growth hormone concentrations. Oral contraceptives are known to affect circulating growth hormone levels and glucose metabolism. While growth hormone plays an important role in hypoglycaemia counter-regulation, it has been shown that oral contraceptives increase growth hormone concentrations. In this context, we tested if serum growth hormone concentrations display a differential response on glycaemic variations in healthy women using oral contraceptives and those not using contraceptives.

METHODS

Fifteen healthy women with oral contraceptive treatment and 10 without participated in a stepwise hyper- and hypoglycaemic glucose clamp procedure. Serum growth hormone concentrations were measured at euglycaemic baseline and subsequently at plasma glucose plateaus of 8.8, 6.8, 4.8 and 2.8 mmol/l.

RESULTS

Growth hormone values were significantly higher in women using oral contraceptives throughout the experiments (P = 0.001). Hyperglycaemia decreased growth hormone concentrations in women using oral contraceptives (P = 0.009), but not in those who were not using oral contraceptives (P = 0.241). Hypoglycaemia significantly elevated growth hormone concentrations in women using oral contraceptives (P = 0.009), but not in those not using oral contraceptives (P = 0.094). Maximum growth hormone values were reached at the end of the hypoglycaemic plateau, with significantly higher concentrations in the group using oral contraceptives than in the group not using oral contraceptives (P = 0.016).

CONCLUSION

Healthy women on oral contraceptive treatment display an increased responsiveness of growth hormone to hypoglycaemic, as well as hyperglycaemic conditions and generally higher serum growth hormone concentrations than women without oral contraceptives. Given the known boosting effects of growth hormone on hypoglycaemic hormonal counter-regulation, oral contraceptives may thus be a pharmacological candidate contributing to combat hypoglycaemia unawareness in women with diabetes in the future.

Greater growth hormone and insulin response in women than in men during repeated bouts of sprint exercise

AIM

In a previous study, sprint training has been shown to increase muscle cross-sectional area in women but not in men [Eur J Appl Physiol Occup Physiol 74 (1996) 375]. We hypothesized that sprint exercise induces a different hormonal response in women than in men. Such a difference may contribute to explaining the observed gender difference in training response.

METHOD

Metabolic and hormonal response to three 30-s sprints with 20-min rest between the sprints was studied in 18 physically active men and women.

RESULTS

Accumulation of blood lactate [interaction term gender (g) x time (t): P = 0.022], and plasma ammonia (g x t: P < 0.001) after sprint exercise was greater in men. Serum insulin increased after sprint exercise more so in women than in men (g x t: P = 0.020), while plasma glucose increased in men, but not in women (g x t: P < 0.001). Serum growth hormone (GH) increased in both women and men reaching similar peak levels, but with different time courses. In women the peak serum GH level was observed after sprint 1, whereas in men the peak was observed after sprint 3 (g x t; P < 0.001). Serum testosterone tended to decrease in men and increase in women (g x t: P = 0.065). Serum cortisol increased approx. 10-15% after sprint exercise, independent of gender (time: P = 0.005).

CONCLUSION

Women elicited a greater response of serum GH and insulin to sprint exercise. This may contribute to explaining the earlier observed muscle hypertrophy in women in response to sprint training.

Effects of exercise training intensity on nocturnal growth hormone secretion in obese adults with the metabolic syndrome

CONTEXT

Abdominal adiposity is associated with reduced spontaneous GH secretion, and an increased incidence of the metabolic syndrome, type 2 diabetes, and cardiovascular disease. Exercise training increases GH secretion, induces abdominal visceral fat loss, and has been shown to improve the cardiometabolic risk factor profile. However, little is known about the effects of endurance training intensity on spontaneous GH release in obese individuals.

OBJECTIVE

Our objective was to examine the effects of 16 wk endurance training on spontaneous 12-h overnight GH secretion in adults with the metabolic syndrome.

DESIGN AND SETTING

This randomized, controlled exercise intervention was conducted at the University of Virginia.

PARTICIPANTS

A total of 34 adults with the metabolic syndrome (mean +/- sem: age: 49.1 +/- 1.8 yr) participated.

INTERVENTION

Participants were randomized to one of three groups for 16 wk: no exercise training (control), low-intensity exercise training, or high-intensity training.

MAIN OUTCOME MEASURE

Change in nocturnal integrated GH area under the curve (AUC) was calculated.

RESULTS

Both exercise training conditions augmented within-group nocturnal GH AUC pretrain to post-training (low-intensity exercise training approximately (upward arrow) 49%, P < 0.05; and high-intensity training approximately (upward arrow) 65%, P < 0.01), and these changes were also greater than the changes in the control group (P < 0.01). The change in nocturnal GH AUC was inversely associated with the change in fat mass across the entire sample (r = -0.34; P = 0.051; n=34) but was not significantly associated with the change in abdominal visceral fat (r = 0.02; P = 0.920; n = 34).

CONCLUSIONS

Sixteen wk of supervised exercise training in adults with the metabolic syndrome increases spontaneous nocturnal GH secretion independent of exercise training intensity.

Age-related changes in body composition of bovine growth hormone transgenic mice

GH has a significant impact on body composition due to distinct anabolic and catabolic effects on lean and fat mass, respectively. Several studies have assessed body composition in mice expressing a GH transgene. Whereas all studies report enhanced growth of transgenic mice as compared with littermate controls, there are inconsistencies in terms of the relative proportion of lean mass to fat mass in these animals. The purpose of this study was to characterize the accumulation of adipose and lean mass with age and according to gender in a bovine (b) GH transgenic mouse line. Weight and body composition measurements were assessed in male and female bGH mice with corresponding littermate controls in the C57BL/6J genetic background. Body composition measurements began at 6 wk and continued through 1 yr of age. At the conclusion of the study, tissue weights were determined and triglyceride content was quantified in liver and kidney. Although body weights for bGH mice were significantly greater than their corresponding littermate controls at all time points, body composition measurements revealed an unexpected transition midway through analyses. That is, younger bGH mice had relatively more fat mass than nontransgenic littermates, whereas bGH mice became significantly leaner than controls by 4 months in males and 6 months in females. These results reveal the importance in timing and gender when conducting studies related to body composition or lean and fat tissue in GH transgenic mice or in other genetically manipulated mouse strains in which body composition may be impacted.

Effects of continuous versus intermittent exercise, obesity, and gender on growth hormone secretion

CONTEXT

Obesity attenuates spontaneous GH secretion and the GH response to exercise. Obese individuals often have low fitness levels, limiting their ability to complete a typical 30-min bout of continuous exercise. An alternative regimen in obese subjects may be shorter bouts of exercise interspersed throughout the day.

OBJECTIVE

The objective of the study was to examine whether intermittent and continuous exercise interventions evoke similar patterns of 24-h GH secretion and whether responses are attenuated in obese subjects or affected by gender.

DESIGN

This was a repeated-measures design in which each subject served as their own control.

SETTING

This study was conducted at the University of Virginia General Clinical Research Center.

SUBJECTS

Subjects were healthy nonobese (n = 15) and obese (n = 14) young adults.

INTERVENTIONS

Subjects were studied over 24 h at the General Clinical Research Center on three occasions: control, one 30-min bout of exercise, and three 10-min bouts of exercise.

MAIN OUTCOME MEASURES

Twenty-four hour GH secretion was measured.

RESULTS

Compared with unstimulated 24-h GH secretion, both intermittent and continuous exercise, at constant exercise intensity, resulted in severalfold elevation of 24-h integrated serum GH concentrations in young adults. Basal and pulsatile modes of GH secretion were attenuated both at rest and during exercise in obese subjects.

CONCLUSIONS

The present data suggest that continuous and intermittent exercise training should be comparably effective in increasing 24-h GH secretion.

Growth hormone isoforms release in response to physiological and pharmacological stimuli

Ten healthy subjects used to performing regular physical activity and eight subjects affected by idiopathic isolated GH deficiency (GHD) were enrolled; 22- and 20-kDa GH secretion and its biological activity were evaluated in response to pharmacological stimuli such as arginine, L-dopa or glucagon in GHD children, while the hormonal response to exercise was studied according to Bruce protocol in healthy subjects. We found a significant increase in 22- and 20-kDa GH level in healthy subjects after monitored physical exercise (MPE; basal 0.28+/-0.12 vs 7.37+/-2.08 ng/ml and basal 0.076+/-0.04 vs 0.18+/-0.05 ng/ml, respectively). Furthermore, the 22-kDa/20-kDa ratio significantly increased in children who had undergone MPE and the GH bioactivity basal mean value also increased significantly after exercise (basal 2.86+/-0.76 vs 7.64+/-1.9 ng/ml). The mean value of 22-kDa GH in GHD patients increased significantly following GH pharmacological stimulation (2.78+/-0.63 ng/ml) when compared with mean basal (0.20+/-0.11 ng/ml) value. In the GHD group the basal concentration of 20-kDa GH significantly increased following GH pharmacological stimulation (0.34+/-0.11 vs 0.72+/-0.2 ng/ml); the 22-kDa/20-kDa ratio significantly increased too. Likewise, GH bioactivity in children with GHD increased significantly after pharmacological stimulation test (basal 2.53+/-0.56 vs 7.33+/-1.26 ng/ml). Both GH isoform concentrations and their biological activity are significantly increased in healthy subjects after submaximal exercise protocol and in GHD children after pharmacological stimuli.

Growth hormone, arginine and exercise

PURPOSE OF REVIEW

To describe the effect of an acute bout of exercise on growth hormone responses and to discuss the effect of L-arginine supplementation on growth hormone responses.

RECENT FINDINGS

Recent studies have shown that resting growth hormone responses increase with oral ingestion of L-arginine and the dose range is 5-9 g of arginine. Within this range there is a dose-dependent increase and higher doses are not well tolerated. Most studies using oral arginine have shown that arginine alone increases the resting growth hormone levels at least 100%, while exercise can increase growth hormone levels by 300-500%. The combination of oral arginine plus exercise attenuates the growth hormone response, however, and only increases growth hormone levels by around 200% compared to resting levels.

SUMMARY

Exercise is a very potent stimulator of growth hormone release and there is considerable research documenting the dramatic growth hormone rise. At rest oral L-arginine ingestion will enhance the growth hormone response and the combination of arginine plus exercise increases growth hormone, but this increase may be less than seen with exercise alone. This diminished response is seen in both in both younger and older individuals.

The effect of exercise type on immunofunctional and traditional growth hormone

The purpose of this study was to compare the growth hormone (GH) response, including the immunfunctional (IF) GH response, between an acute bout of aerobic and resistance exercise in the same subjects. Ten cross-trained males (24.3 +/- 1.2 years) performed both 30 min of continuous cycling at 70% of VO(2max), and intermittent free weight squatting at 70% of 1-RM, in a randomly assigned crossover design, separated by at least 1 week. Blood samples were collected at 10-min intervals for 2 h (30 min rest, 30 min exercise, 60 min recovery) and analyzed for total human and IF GH. After adjusting for the amount of work performed per minute of exercise, integrated GH AUC was significantly greater during the resistance session than the aerobic session as measured by both the total and IF GH assays (P = 0.008 and P = 0.014, respectively). Peak GH concentrations were significantly greater during the resistance session than the aerobic session (P = 0.05). A similar overall GH pattern was observed in response to both types of exercise, with peak values occurring at the end of exercise, regardless of the GH assay used. These data demonstrate that in young, cross-trained males, intermittent resistance exercise elicits a greater response of GH, including IF GH, compared to a continuous aerobic session, when controlling for the work performed per minute, intersubject variability, relative exercise intensity and session duration.

Plasma ghrelin responses to acute sculling exercises in elite male rowers

The regulatory effect of ghrelin on growth hormone (GH) is limited in describing ghrelin response to acute submaximal exercise intensities in elite athletes. We investigated the effects of a single sculling exercise performed above and below the individual anaerobic threshold (IAT) on total ghrelin concentration in highly trained male rowers. Nine elite male rowers (20.1 +/- 3.7 years; 190.0 +/- 5.2 cm; 89.6 +/- 4.6 kg; %body fat: 9.9 +/- 2.5%) volunteered for this study. Single scull rowing was performed below and above IAT using a mean of 5 bpm above and below the heart rate of the IAT during graded exercise test. Ghrelin, leptin, GH, insulin, and glucose were measured before, immediately after, and after 30 min of recovery. Plasma ghrelin concentration did not increase significantly in either exercise but was approaching significance after 30 min of recovery (P = 0.051) when the constant load sculling was performed at the intensity above the IAT. There were no changes in plasma leptin levels. GH increased significantly immediately after exercise and remained elevated during the 30 min of recovery in both exercise conditions, while insulin decreased significantly immediately after exercise and remained significantly lower after the 30 min of recovery in both exercise intensities. Baseline ghrelin was not correlated with the body composition, physical performance, or blood biochemical data. There was no significant relationship between plasma ghrelin and other blood variables immediately after the 30 min of recovery in both exercise tests and changes in ghrelin were not related to blood biochemical variables after the exercise tests. The acute constant load sculling exercise above or below IAT that increased GH concentrations did not significantly increase the circulating plasma ghrelin levels.

The impact of sex and exercise duration on growth hormone secretion

Previous research clearly indicates a linear relationship between exercise intensity and growth hormone (GH) release and that this relationship is influenced by sex. The present study examined the GH response to increasing exercise duration in young men and women. Fifteen healthy subjects (8 men and 7 women) completed three randomly assigned exercise sessions (30, 60, and 120 min) at 70% of peak oxygen consumption. Blood samples were collected every 10 min beginning 30 min before exercise, for a total of 240 min. Total integrated GH concentration (IGHC) increased with increasing exercise duration for men and women (601, 1,394, and 2,360 microg/l.4 h; 659, 1,009 and 1,243 microg/l.4 h for 30, 60, and 120 min of exercise, respectively). Regression analysis revealed that IGHC (logarithmically transformed) was significantly influenced by exercise duration (logarithmically transformed) (120 min > 60 min > 30 min) and that a significant sex-dependent effect was present even after adjustments for fitness level and percent body fat (men > women). The slope of the regression line was greater for men than for women (1.003 vs. 0.612; P = 0.013), but the average height of the regression line was greater for women (7.287 vs. 6.595; P < 0.001). Although GH secretory pulse half-duration was greater in women (P = 0.001), and GH half-life was greater in men (P = 0.001), they were not affected by exercise duration. The total mass of GH secreted during exercise increased with exercise duration (P < 0.001) but was not affected by sex (P = 0.137). Results from the present investigation indicate that when exercise intensity is constant, exercise duration significantly increases IGHC and that this relationship is sex dependent.

Growth hormone and muscle function responses to skeletal muscle ischemia

We examined the effects of ischemia (ISC) alone and with low-intensity exercise (ISC+EX) on growth hormone (GH) and muscle function responses. Nine men (22 +/- 0.7 yr) completed 3 study days: an ISC day (thigh cuff inflated five times, 5 min on, 3 min off), an ISC+EX day [knee extension at 20% maximal voluntary contraction (MVC) with ISC], and a control day. MVCs and submaximal contraction tasks (15 and 30% MVC) were performed before and following the perturbations. Surface electromyogram signals were collected from thigh muscles and analyzed for median frequency and root mean square alterations. Blood samples were collected every 10 min (190 min total) and analyzed for GH concentrations. Peak GH concentrations and GH area under the curve were highest (P < 0.01) on the ISC+EX day (7.5 microg/l and 432 microg.l(-1).min(-1), respectively) compared with the ISC (0.9 microg/l and 76.4 microg.l(-1).min(-1)), and CON (1.1 microg/l and 83.8 microg.l(-1).min(-1)) days. A greater GH pulse amplitude, mass/pulse, and production rate were also observed on the ISC+EX day (P < 0.05). Following the intervention, force production decreased on the ISC and ISC+EX days by 16.1 and 55.8%, respectively, and did not return to baseline values within 5 min of recovery. During the submaximal contractions, median frequency shifted to lower frequencies for most of the muscles examined, and root mean square electromyogram was consistently elevated for ISC+EX day. In conclusion, ISC coupled with resistance exercise acutely increases GH levels and reduces MVC, whereas ISC alone decreases force capacity, without alterations in GH levels.

The effect of increased lipid intake on hormonal responses during aerobic exercise in endurance-trained men

In view of the growing health problem associated with obesity, clarification of the regulation of energy homeostasis is important. Peripheral signals, such as ghrelin and leptin, have been shown to influence energy homeostasis. Nutrients and physical exercise, in turn, influence hormone levels. Data on the hormonal response to physical exercise (standardized negative energy balance) after high-fat (HF) or low-fat (LF) diet with identical carbohydrate intake are currently not available. The aim of the study was to investigate whether a short-term dietary intervention with HF and LF affects ghrelin and leptin levels and their modulators, GH, insulin and cortisol, before and during aerobic exercise. Eleven healthy, endurance-trained male athletes (W(max) 365 +/- 29 W) were investigated twice in a randomized crossover design following two types of diet: 1. LF - 0.5 g fat/kg body weight (BW) per day for 2.5 days; 2. HF - 0.5 g fat/kg BW per day for 1 day followed by 3.5 g fat/kg BW per day for 1.5 days. After a standardized carbohydrate snack in the morning, metabolites and hormones (GH, ghrelin, leptin, insulin and cortisol) were measured before and at regular intervals throughout a 3-h aerobic exercise test on a cycloergometer at 50% of W(max). Diet did not significantly affect GH and cortisol concentrations during exercise but resulted in a significant increase in ghrelin and decrease in leptin concentrations after LF compared with HF diet (area under the curve (AUC) ghrelin LF vs HF: P < 0.03; AUC leptin LF vs HF: P < 0.02, Wilcoxon rank test). These data suggest that acute negative energy balance induced by exercise elicits a hormonal response with opposite changes of ghrelin and leptin. In addition, the hormonal response is modulated by the preceding intake of fat.

Growth hormone response to graded exercise intensities is attenuated and the gender difference abolished in older adults

We investigated the joint impact of age, gender, and exercise intensity on growth hormone (GH) secretion. At a university center, nine young men, eight young women, seven older men, and six older women were each tested on six randomly ordered occasions [control (C) and 5 exercise conditions (Ex)]. Serum GH concentrations were measured by immunochemiluminometry [10-min samples: 0700-0900 (baseline); 0900-1300 (C or Ex + recovery)]. Integrated GH concentrations (IGHC) were calculated by trapezoidal reconstruction, and GH secretion was modeled by deconvolution analyses. Subjects exercised from 0900 to 0930 at graded intensities [standardized to individual lactate threshold (LT)] of 25 and 75% of the difference between rest and LT, LT, and 25 and 75% of the difference between LT and peak oxygen consumption. Data were analyzed via mixed-effects ANOVA for repeated measures with post hoc contrasts. We found that 1) Ex elevated IGHC above C in all four cohorts, 2) 1.75 LT Ex resulted in maximal IGHC, 3) IGHC differed by gender in young (women > men) but not older adults, 4) older adults secreted 50% less GH during graded exercise, 5) Ex selectively augmented the mass of GH secreted per burst, and 6) higher Ex + recovery IGHC in young women was due to higher baseline IGHC, rather than greater stimulated GH secretion. We conclude that young women manifest a greater absolute and incremental IGHC response to exercise than postmenopausal women and men of any age. Age diminishes the GH response to exercise and abolishes the young-adult gender difference. Attenuation of GH responses to all exercise intensities in older adults has implications for exercise prescription because higher exercise intensities may be required to stimulate GH release in older adults.

Short-term administration of supraphysiological recombinant human growth hormone (GH) does not increase maximum endurance exercise capacity in healthy, active young men and women with normal GH-insulin-like growth factor I axes

CONTEXT

Despite the fact that the use of GH as a doping agent in sports is widespread, little is known about its short-term effects.

OBJECTIVE

The objective was to study the effects of GH on exercise capacity.

DESIGN

A double-blind, placebo-controlled study was used, with a treatment period of 28 d.

SETTING

Subjects from general community studied ambulatory at a university hospital.

PARTICIPANTS

Thirty healthy active young normal volunteers (15 women and 15 men) were recruited by local announcement, and all completed the study.

INTERVENTION

All subjects were randomized to receive a low GH dose (0.033 mg/kg.d or 0.1 IU/kg.d), a high GH dose (0.067 mg/kg.d or 0.2 IU/kg.d), or placebo.

MAIN OUTCOME MEASURES

Power output and oxygen uptake on bicycle exercise were the main outcome measures.

RESULTS

We found no effect of the low or high dosages of GH on maximum oxygen uptake during exercise (mean +/- se for placebo, 45.2 +/- 1.6 to 45.2 +/- 2.1 ml/kg.min; GH low dose, 42.8 +/- 1.6 to 42.8 +/- 1.6 ml/kg.min; GH high dose, 44.8 +/- 3.4 to 44.8 +/- 2.2 ml/kg.min; not significant by two-way ANOVA). Neither was there any effect on maximum achieved power output during exercise or on blood pressure, heart rate, or the electrocardiographic ST level at rest or during exercise. GH significantly increased total body weight (P = 0.028), an effect predominantly ascribed to fluid retention (increased extracellular water volume), whereas muscle mass (as indicated by intracellular water volume) did not change. However, changes in the latter correlated to changes in physical performance, possibly due to different training efforts.

CONCLUSION

Administration of supraphysiological recombinant human GH during a period of 4 wk does not improve power output or oxygen uptake.

Growth hormone responses to repeated bouts of aerobic exercise with different recovery intervals in cyclists

To characterise the specific GH responses to repeated bouts of standardised aerobic exercise in amateur competitive cyclists, 6 volunteers (mean age +/- SE: 28.7 +/- 2.3 yr, range: 18-35 yr) performed two consecutive 30-min cycling sessions at 80% of individual maximal oxygen uptake on three occasions with different time interval between bouts: 2 h (EXP A), 4 h (EXP B) and 6 h (EXP C). Serum GH concentration was determined in blood samples collected at 15-min intervals during exercise and following 1 h of recovery. In EXP A and EXP B, peak GH concentration in response to the second bout was significantly lower (p < 0.01) than that of the first bout, but in EXP C no difference was detected between bouts. Similarly, the average integrated GH concentration (AUC), determined during the exercise period and in the following 1 h of recovery in the course of the second bout, was significantly lower than that observed during the first bout only in EXP A (p < 0.05) and EXP B (p < 0.01) and not in EXP C, so that the second bout AUC of EXP C was significantly higher than that of EXP A (p < 0.01) and EXP B (p < 0.01). It was concluded that GH responses to subsequent bouts of aerobic exercise are dependent on the time interval between the exercise sessions.

Influence of body mass index and gender on growth hormone (GH) responses to GH-releasing hormone plus arginine and insulin tolerance tests

The aim of this study is to assess whether gender and body mass index (BMI) should be considered in developing thresholds to define GH deficiency, using GH responses to GHRH + arginine (ARG) stimulation and insulin tolerance test (ITT). Thirty-nine healthy subjects (19 males, 20 females; ages 21-50 yr) underwent GHRH + ARG, and another 27 subjects (19 males, 8 females; ages 20-49 yr) underwent ITT. Peak GH response was significantly higher (P = 0.005) after GHRH + ARG than with ITT, and this difference could not be explained by age, gender, or BMI. Peak GH response was negatively correlated with BMI in both tests (GHRH + ARG, r = -0.76; and ITT, r = -0.65). Peak GH response to GHRH + ARG was higher in females than males (P = 0.004; ratio = 2.4), but it was attenuated after eliminating the influence of BMI (P = 0.13; ratio = 1.6). No significant gender differences were found in peak GH responses to ITT, which could be due to the smaller number of female subjects studied. GH response to GHRH + ARG and ITT stimulation is sensitive to BMI differences and less so to gender differences. A higher BMI is associated with a depressed GH response to both stimulation tests. BMI should therefore be considered as a factor when defining the diagnostic cut-off points in the assessment of GH deficiency, whereas whether gender should be likewise used is inconclusive from this study.

Gender differences in growth hormone response to exercise before and after rhGH administration and the effect of rhGH on the hormone profile of fit normal adults

OBJECTIVES

Exercise is a potent physiological stimulus of GH secretion. We hypothesized that exogenous recombinant human growth hormone (rhGH) administration through an increase in GH and IGF-I levels would blunt the GH response to exercise. The aim of the study was to examine and compare the impact of rhGH on the exercise-induced GH response in healthy normal men and women.

DESIGN AND MEASUREMENTS

Sixty-nine subjects (36 men, 33 women) were randomized to receive low-dose rhGH (0.1 U/kg/day), high dose rhGH (0.2 U/kg/day), or placebo. Subjects were matched for age (24 +/- 3.1), and body mass index (BMI). rhGH was given as a single subcutaneous (s.c.) injection for the first 28 days. All subjects exercised to exhaustion (maximal oxygen consumption--VO2max) before rhGH treatment (Test 1), and on day 28 (Test 2). GH was measured before exercise (time 0), immediately after exercise (time 0') and at 15, 30, 60, 90 and 120 min postexercise. Baseline IGF-I levels were measured before exercise on days 0 and 28.

RESULTS

Baseline IGF-I levels showed no gender differences (42.3 women vs. 38.8 nmol/l men) but basal GH values were higher in women (9.9 vs. 1.8 mU/l, P < 0.001). The areas under the GH response curve, for Test 1 were similar in men and women. Peak GH values were higher in women than men (37.9 vs. 23.5 mU/l, but this did not quite reach statistical significance (P = 0.055). In men, administration of rhGH resulted in a significant increase in IGF-I levels over the basal state in both the LD and HD groups (P < 0.0001). In women, the increase in lGF-I levels reached significance only in the HD group (P < 0.0001). On day 28, GH secretion in response to exercise was calculated from the areas under the GH response curve correcting for an exogenous rhGH component (delta AUC). In men, the delta AUC, for Test 2 were similar in all three groups. In women, the delta AUC was higher in the placebo group, than in the HD group (P < 0.02). Free T4 levels decreased significantly in men, and free T3 increased in both men and women, in HD group after the rhGH administration. TSH levels were suppressed only in women. No changes in sex hormones were found in men or women in any of the treatment groups. Conclusions In terms of IGF-I, men are more responsive to rhGH treatment than women. In addition, as men, but not women, were able to overcome the negative feedback control of the elevated IGF-I levels, it seems that exercise may be a more robust stimulus to GH release in men compared to women.

Physical activity and exercise in women's health

Regular physical activity provides health benefits, including the reduction in risks of coronary heart disease, hypertension, type 2 diabetes mellitus, obesity, colon cancer, and premature mortality. Despite this information, most women are physically inactive. Research findings shed light on the gender differences in physiological responses to physical activity. Patterns and predictors of physical activity vary significantly by gender. Further study is needed of the benefits, barriers, and personally meaningful outcomes of physical activity for women, specifically including the frequently unspoken correlates of urinary incontinence, depression and mood disorders, and obesity.

Immuno-endocrine and metabolic responses to long distance ski racing in world-class male and female cross-country skiers

The purpose of this study was to characterize the extent of immune, endocrine, substrate and metabolic changes during a long-distance cross-country ski race in extremely well-trained athletes and evaluate if the blood perturbations would indicate signs of health risk. Ten male (M) and six female (F) national team skiers were investigated as they followed their usual routines of race preparations. Blood samples were drawn before and immediately after a World Cup 50-km M and 30-km F ski race with a mean finish time of 142 and 104 min, respectively. Hemoglobin, electrolytes, and C-reactive protein remained unchanged for both M and F. Serum testosterone remained unchanged in M, but doubled in F. Significant increases were observed in concentrations of granulocytes (F: 5 x, M: 5 x), natural killer cells (F: 2 x, M: 1.5 x), adrenaline (F: 12 x, M:10 x), noradrenaline (F: 7 x, M:5 x), growth hormone (F: 30 x, M: 2 x), cortisol (F: 1.5 x, M:2 x), glucose (F: 2 x, M:1.5 x), creatine kinase (F: 2 x, M:2 x), uric acid (F: 1.5 x, M: 1.5 x) and non-organic phosphate (F:2 x, M:2 x), while insulin concentration decreased (F: 0.5x, M: 0.8 x). Free fatty acid (FFA) concentration increased (F:2 x, M: 3 x). In conclusion, we observed substantial changes in several immuno-endocrine, substrate and metabolic measurements after long distance cross-country ski racing and suggest that some of these marked changes may reflect the large amount of muscle mass involved during skiing.

Reproducibility of the growth hormone response to sprint exercise

OBJECTIVE

There is a large inter-individual variation in the growth hormone (GH) response to exercise, but within-individual variation is unknown. The purpose of this study was to determine the reproducibility of the GH response to a single 30 s sprint on a cycle ergometer.

DESIGN

Eleven non-obese male volunteers completed two trials separated by at least seven days during which they completed a single all-out 30 s sprint on a cycle ergometer followed by 60 min of rest. Blood samples were taken at rest and at regular intervals post-exercise.

RESULTS

No differences were found in mean power output during the sprint, or in peak blood lactate concentrations or lowest measured pH following the sprints. Re-test correlation was significant for both peak GH concentrations (r=0.97, P<0.05) and GH AUC (r=0.97, P<0.05). Within-subject error (change in mean+/-typical error) of the peak GH concentrations and GH area under the curve (AUC) was 4.3+/-3.4 microg l(-1) and 2.9+/-54.3 min microg l(-1), respectively. Within-subject percentage error (percentage change in mean+/-typical percentage error) for peak GH concentration and GH AUC was 33.5+/-26.7% and 1.1+/-20.0%, respectively.

CONCLUSION

Growth hormone AUC is a reproducible measure of the GH response to sprint exercise.

Growth hormone responses to sub-maximal and sprint exercise

Exercise is a potent stimulus for growth hormone (GH) release and a single bout of exercise can result in marked elevations in circulating GH concentrations. The magnitude of the GH response to exercise will vary according to the type, intensity and duration of exercise as well as factors such as the age, gender, body composition and fitness status of the individual performing the exercise. However, the mechanisms regulating GH release in response to exercise are not fully understood. This review considers the GH responses to sub-maximal and sprint exercise and discusses the factors that might affect GH release along with the mechanisms that have been proposed to regulate exercise-induced GH release.

Mechanisms underlying the neuroendocrine response to physical exercise

Exercise initiates a coordinated series of physiological responses, including hypothalamic-pituitary-adrenal (HPA) axis and sympathetic nervous system activation, that, in combination, lead to the appropriate selection and utilization of metabolic substrates. Physical activity acts as a powerful stimulus for the hypothalamic-pituitary axis, leading to the liberation of several neuroendocrine hormones. The nature of this stimulation varies according to the kind of exercise (intensity, duration, aerobic, strength) and subject characteristics (gender, previous training), as well as depending on the time of the day and meal ingestion. As a whole, the neuroendocrine responses to exercise represent an accurate regulator of fuels (glucose, free fatty acids) homeostasis in a special situation characterized by a drastic increase of the energy requirements at muscle level. In this article the current knowledge about this topic is reviewed.

Neuroendocrine control of GH release during acute aerobic exercise

GH secretion declines with aging and is decreased in conditions such as obesity. Several physiologic factors alter pulsatile GH secretion, including age, gender, body composition, regional distribution of fat and in particular abdominal visceral fat, sleep, nutrition, exercise and serum concentrations of gonadal steroids, insulin and IGF-I. Acute aerobic exercise is a powerful stimulus to GH release. Available studies suggest that intensity and duration of acute exercise, fitness, and training state may all influence, in part, the GH response to exercise. Intensity of exercise plays a key role in GH response to exercise. In the present paper we will discuss the GH response during acute aerobic exercise with a focus on exercise intensity and GH release. We will also provide an overview of the neuroendocrine control of exercise-induced GH release. Finally, information related to the effects of aging and gender on the GH response to exercise will be provided.

GH responses to a near-maximal exercise training session on-the-field in cyclists

Acute plasma GH response to prolonged (1 h) near-maximal exercise was studied in 7 elite cyclists (6 males, 1 female; mean age +/- SE: 24.9 +/- 1.4 yr) during a routine training session on an uphill track (length: 22.0 km, average slope: 4.39%) and during a recovery (REC) period of 60 min from the end of exercise. The training session entailed a warming-up (WARM) phase of about 20 min at 63% of individual maximal heart rate (HRmax) followed by a high intensity exercise (HIE) phase of about 60 min at 90-92% of HRmax. GH resting values averaged 0.2 +/- 0.06 ng/ml; average GH concentration attained a maximal value (21.5 +/- 3.3 ng/ml, range: 11.0-38.2 ng/ml) between 20 and 40 min of HIE and significantly decreased thereafter (p=0.01), although exercise intensity was unchanged in the following period (p=0.14). After WARM, GH concentrations were significantly lower than peak values (p=0.05). During REC, GH levels steadily decreased, attaining a value of 2.6 +/- 0.8 ng/ml 60 min after the end of exercise. It was concluded that during prolonged and sustained exercise on-the-field in cyclists, GH value determined at the end of the bout may not correspond to the maximal value, which can be observed after 20 to 40 min of near-maximal exercise.

Growth hormone release during acute and chronic aerobic and resistance exercise: recent findings

Exercise is a potent physiological stimulus for growth hormone (GH) secretion, and both aerobic and resistance exercise result in significant, acute increases in GH secretion. Contrary to previous suggestions that exercise-induced GH release requires that a "threshold" intensity be attained, recent research from our laboratory has shown that regardless of age or gender, there is a linear relationship between the magnitude of the acute increase in GH release and exercise intensity. The magnitude of GH release is greater in young women than in young men and is reduced by 4-7-fold in older individuals compared with younger individuals. Following the increase in GH secretion associated with a bout of aerobic exercise, GH release transiently decreases. As a result, 24-hour integrated GH concentrations are not usually elevated by a single bout of exercise. However, repeated bouts of aerobic exercise within a 24-hour period result in increased 24-hour integrated GH concentrations. Because the GH response to acute resistance exercise is dependent on the work-rest interval and the load and frequency of the resistance exercise used, the ability to equate intensity across different resistance exercise protocols is desirable. This has proved to be a difficult task. Problems with maintaining patent intravenous catheters have resulted in a lack of studies investigating alterations in acute and 24-hour GH pulsatile secretion in response to resistance exercise. However, research using varied resistance protocols and sampling techniques has reported acute increases in GH release similar to those observed with aerobic exercise. In young women, chronic aerobic training at an intensity greater than the lactate threshold resulted in a 2-fold increase in 24-hour GH release. The time line of adaptation and the mechanism(s) by which this training effect occurs are still elusive. Unfortunately, there are few studies investigating the effects of chronic resistance training on 24-hour GH release. The decrease in GH secretion observed in individuals who are older or have obesity is associated with many deleterious health effects, although a cause and effect relationship has not been established. While exercise interventions may not restore GH secretion to levels observed in young, healthy individuals, exercise is a robust stimulus of GH secretion. The combination of exercise and administration of oral GH secretagogues may result in greater GH secretion than exercise alone in individuals who are older or have obesity. Whether such interventions would result in favourable clinical outcomes remains to be established.

Claims for the anabolic effects of growth hormone: a case of the emperor's new clothes?

This review examines the evidence that growth hormone has metabolic effects in adult human beings. The conclusion is that growth hormone does indeed have powerful effects on fat and carbohydrate metabolism, and in particular promotes the metabolic use of adipose tissue triacylglycerol. However, there is no proof that net protein retention is promoted in adults, except possibly of connective tissue. The overexaggeration of the effects of growth hormone in muscle building is effectively promoting its abuse and thereby encouraging athletes and elderly men to expose themselves to increased risk of disease for little benefit.

Effect of water polo practice on cytokines, growth mediators, and leukocytes in girls

PURPOSE

The effects of exercise on growth and development are mediated through a complex interaction between the endocrine, immune, and nervous systems. Very little is known about how these systems respond to exercise in children or adolescents. Moreover, there are few studies that have examined growth factors, inflammatory cytokines, and leukocyte responses to "real-life" or field exercise solely in girls. Thus, the goal of the present study was to determine the acute exercise-induced alterations in the growth hormone --> insulin-like growth factor-I axis, inflammatory cytokines, and certain aspects of immune function in a group of adolescent girls after a typical water polo practice.

METHODS

Ten, healthy, high-school female subjects, 14-16 yr old, performed a single, typical, 1.5-h water polo practice session. Blood was sampled before and after the session.

RESULTS

The exercise resulted in an increase in HR (from 82 +/- 2 to 161 +/- 5 beats.min(-1) at 30 min, P < 1.4.10(-6) ), as well as in circulating lactate levels (375 +/- 66%, P < 0.0005). Significant increases where noted in circulating IL-6 (396 +/- 162%, P < 0.005) and IL-1ra (71 +/- 20%, P < 0.015). A substantial increase in the level of IGFBP-1 (1344 +/- 344%, P < 0.001) was also observed. Interestingly, TNF-alpha levels decreased after the exercise (-10.4 +/- 3.8%, P < 0.04) as did insulin (55 +/- 12%, P < 0.005). The exercise led to significant increases in granulocytes, monocytes, and lymphocytes. The exercise significantly influenced adhesion molecules (such as CD62L and CD54), which has not been previously studied in adolescent girls.

CONCLUSIONS

These data demonstrate that an intense "real-life" exercise bout in adolescent females leads to profound increases in inflammatory cytokines and reductions in anabolic mediators with substantial alterations in white blood cell subpopulations and adhesion molecules. The role of these frequent, almost daily immune and cytokine changes on growth and development have yet to be determined.