Synergy of L-arginine and GHRP-2 stimulation of growth hormone in men and women: modulation by exercise

Growth Hormone Response to L-Arginine Alone and Combined with Different Doses of Growth Hormone-Releasing Hormone: A Systematic Review and Meta-Analysis

Background

Arginine (ARG) can modulate growth hormone (GH) release by suppressing its endogenous inhibitory regulator, somatostatin. ARG also induces the release of the GH-releasing hormone (GHRH). This study aims to review the effects of L-arginine supplementation alone and combined with GHRH in different doses on GH secretion.

Methods

In this systematic review and meta-analysis, an electronic literature search was conducted on Medline database (PubMed), Scopus, and Web of Science databases. All eligible studies were randomized clinical trials that reported the effects of ARG supplementation alone or with GHRH on GH levels. Mean difference (MD) and 95% confidence intervals (CI) were computed as the effect size.

Results

Meta-analyses showed significant effects of ARG alone on GH release (MD = 10.07, 95% CI: 7.87, 12.28). Moreover, the response of GH was greater with ARG in combination with GHRH (MD = 24.96, 95% CI: 17.51, 32.42). There was no significant difference between the patients and healthy individuals and between oral and injection use of ARG. The systematic review revealed the important role of ARG in combination with other amino acids on GH secretion in patients with GH deficiency.

Conclusion

This study revealed that in GH-deficient individuals, high doses of ARG supplementation in combination with GHRH and/or other amino acids might have potential therapeutic effects on increasing GH concentrations. These findings propose that ARG supplementation can be considered as a potential stimulator in management of GH deficiency.

Supplements with purported effects on muscle mass and strength

PURPOSE

Several supplements are purported to promote muscle hypertrophy and strength gains in healthy subjects, or to prevent muscle wasting in atrophying situations (e.g., ageing or disuse periods). However, their effectiveness remains unclear.

METHODS

This review summarizes the available evidence on the beneficial impacts of several popular supplements on muscle mass or strength.

RESULTS

Among the supplements tested, nitrate and caffeine returned sufficient evidence supporting their acute beneficial effects on muscle strength, whereas the long-term consumption of creatine, protein and polyunsaturated fatty acids seems to consistently increase or preserve muscle mass and strength (evidence level A). On the other hand, mixed or unclear evidence was found for several popular supplements including branched-chain amino acids, adenosine triphosphate, citrulline, β-Hydroxy-β-methylbutyrate, minerals, most vitamins, phosphatidic acid or arginine (evidence level B), weak or scarce evidence was found for conjugated linoleic acid, glutamine, resveratrol, tribulus terrestris or ursolic acid (evidence level C), and no evidence was found for other supplements such as ornithine or α-ketoglutarate (evidence D). Of note, although most supplements appear to be safe when consumed at typical doses, some adverse events have been reported for some of them (e.g., caffeine, vitamins, α-ketoglutarate, tribulus terrestris, arginine) after large intakes, and there is insufficient evidence to determine the safety of many frequently used supplements (e.g., ornithine, conjugated linoleic acid, ursolic acid).

CONCLUSION

 In summary, despite their popularity, there is little evidence supporting the use of most supplements, and some of them have been even proven ineffective or potentially associated with adverse effects.

The effects of branch-chain amino acids on fatigue in the athletes

Research has shown that injection of a mixture of amino acids into humans led to stimulation of protein synthesis about 30 min after injection, and the synthesis rate remained at a high level for up to 90 min. Various theories have been proposed to justify this effect, such as increasing the availability of amino acids in the body to increase their transmission to muscle, and this increase in transmission can stimulate the synthesis of protein in muscle cells. A total of 30 volunteers with a history of 2–3 years in the field of Athletics were participated in this study. The experimental group (15 subjects) used BCAAs supplement during 6-week athletics training. Control group (15 subjects) participated in athletics training for 6 weeks without any supplementation. Blood was taken before and after 6 weeks supplementary. The findings of the t-test showed that basal levels of lactate and ammonium had not changed significantly after 42 days of supplementation. The results of this study showed that BCAAs supplementation has no effect on the reduction of lactate and ammonium indices as indicators of fatigue.

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.

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.

Hormonal response to L-arginine supplementation in physically active individuals

Background

Nutritional supplements based on the amino acid L-arginine have been hypothesized to improve exercise performance by increasing levels of insulin and growth hormone (GH). Changes of these parameters in response to L-arginine supplementation may clarify the mechanisms underlying its putative physiological effects on physical performance.

Objective

The aim of the study was to evaluate the effect of L-arginine supplementation on serum insulin, GH, Growth Factor Insulin-like (IGF-1), and cortisol in response to exercise. Exercise performance was also evaluated.

Design

Fifteen trained runners were divided into groups supplemented with 6 g of L-arginine (ARG) or placebo (PLA). Blood samples were collected before supplementation (T0), immediately after the first exercise session (T1), after the second exercise session (T2), and after 20 min of rest (T3). The exercise consisted of two bouts of 5 km time-trial running test.

Results

There was a significant increase in serum GH (T0: 3.28±0.95 vs. 3.21±0.5 ng/mL; T1: 4.35±0.23 vs. 4.17±0.13 ng/mL; T2: 4.22±0.25 vs. 4.17±0.09 ng/mL; T3: 4.14±0.29 vs. 4.13±0.18 ng/mL) and cortisol (T0: 198.71±53.77 vs. 207.57±69.51 nmol/L; T1: 458.16±116.12 vs. 433.26±101.77 nmol/L; T2: 454.61±125.21 vs. 431.88±74.82 nmol/L; T3: 311.14±102.91 vs. 362.26±110.42 nmol/L) after T1, T2, and T3, with no significant difference between the ARG and PLA groups, respectively. There was also no significant difference observed in the variables of IGF-1, insulin, and total running time between the ARG and PLA groups.

Conclusions

The supplementation of L-arginine did not appear to stimulate the production of insulin, GH, and IGF-1 and, thus, provided no benefit in hormonal response or exercise performance in trained runners.

The acute effects of a low and high dose of oral l-arginine supplementation in young active males at rest

l-arginine (2-amino-5-guanidinovaleric acid) is a conditionally essential amino acid. Intravenous (IV) administration of l-arginine invokes a large metabolic (nitrate/nitrite (NOx)) and hormonal (growth hormone (GH), insulin-like growth factor 1 (IGF-1), and insulin) response; however, research examining oral l-arginine supplementation is conflicting, potentially owing to dose. The purpose of this study was examine a low and high dose of oral l-arginine on blood l-arginine, NOx, GH, IGF-1, and insulin response. Fourteen physically active males (age: 25 ± 5 years; weight: 78.0 ± 8.5 kg; height: 179.4 ± 4.7 cm) volunteered to be in a randomized, double-blind, repeated-measures study. Following an overnight fast, an IV catheter was placed in a forearm vein and a resting blood sample was drawn at ∼0800 hours. Each subject was then provided 1 of 3 treatment conditions (placebo, low (0.075 g·kg–1 of body mass), or high (0.15 g·kg–1 of body mass of l-arginine)). Blood samples were drawn at 30, 60, 90, 120, and 180 min after consumption. l-arginine plasma concentrations significantly increased (p < 0.001) to a similar level at any time point in both the low- and high-dose conditions; there was no change over time in the placebo condition. There was no significant difference between conditions for NOx, GH, IGF-1, or insulin. Based on these findings, a low dose of l-arginine was just as effective at increasing plasma l-arginine concentrations as a high dose; however, neither dose was able to promote a significant increase in NOx, GH, IGF-1, or insulin at rest.

Somatostatin and its receptors contribute in a tissue-specific manner to the sex-dependent metabolic (fed/fasting) control of growth hormone axis in mice

Somatostatin (SST) inhibits growth hormone (GH) secretion and regulates multiple processes by signaling through its receptors sst1-5. Differential expression of SST/ssts may contribute to sex-specific GH pattern and fasting-induced GH rise. To further delineate the tissue-specific roles of SST and sst1-5 in these processes, their expression patterns were evaluated in hypothalamus, pituitary, and stomach of male and female mice under fed/fasted conditions in the presence (wild type) or absence (SST-knockout) of endogenous SST. Under fed conditions, hypothalamic/stomach SST/ssts expression did not differ between sexes, whereas male pituitary expressed more SST and sst2A/2B/3/5A/5TMD2/5TMD1 and less sst1, and male pituitary cell cultures were more responsive to SST inhibitory actions on GH release compared with females. This suggests that local pituitary SST/ssts can contribute to the sexually dimorphic pattern of GH release. Fasting (48 h) reduced stomach sst2A/B and hypothalamic SST/sst2A expression in both sexes, whereas it caused a generalized downregulation of pituitary sst subtypes in male and of sst2A only in females. Thus, fasting can reduce SST sensitivity across tissues and SST input to the pituitary, thereby jointly contributing to enhance GH release. In SST-knockout mice, lack of SST differentially altered sst subtype expression levels in both sexes, supporting an important role for SST in sex-dependent control of GH axis. Evaluation of SST, IGF-I, and glucocorticoid effects on hypothalamic and pituitary cell cultures revealed that these hormones could directly account for alterations in sst2/5 expression in the physiological states examined. Taken together, these results indicate that changes in SST output and sensitivity can contribute critically to precisely define, in a tissue-dependent manner, the sex-specific metabolic regulation of the GH axis.

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.

Supplements and the endocrine system in athletes

In the world of athletes' nutrition, there are many ethical concerns, because there is the suspicion that in practice, large doses of supplements in athletes are not taken for nutritional purposes. It is beyond the scope of this article to highlight the possible roles of supplements or methods of supplementation in the improvement of athletic performance in elite athletes. Instead, the author briefly reviews some of the substances taken by athletes, with particular attention to their mechanisms of action and the pathways involved. Very often, the effects of many supplements are hormone-related, or supplements influence hormone secretion. Examples of possible links between "supplements or ergogenic compounds" and the endocrine/metabolic system are addressed.

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.

Combining growth hormone releasing hormone-arginine and synacthen testing diminishes the cortisol response

OBJECTIVES

The GHRH/arginine test and short synacthen test (SST) have been validated as safe alternatives to the insulin tolerance test for the assessment of the GH reserve and hypothalamic-pituitary-adrenal axis integrity, respectively. However, these two tests are usually performed separately. The objective was to see whether the synacthen and GHRH/arginine tests could be combined to save time and blood samples and minimize inconvenience to patients.

PATIENTS/METHODS

Twenty-four consecutive patients with adult onset pituitary disease requiring pituitary function testing were randomized to receive sequentially and in random order a SST, a GHRH/arginine test, and a combined SST and GHRH/arginine test on three different visits separated by at least 1 wk.

RESULTS

There was no difference in basal cortisol or ACTH values for the SST done alone or during the combined test. However, when GHRH/arginine was given with synacthen, patients had a lower peak cortisol response with a mean difference of 116 nmol/liter (95% confidence interval, 52.54 to 179.37; P < 0.001), and one patient with a normal response on the SST had a subnormal cortisol response in the combined test. Similar lower peak cortisol responses were observed in males and females with combined test. The difference between the peak cortisol responses showed no significant correlation with age (r = 0.123; P = 0.58) or with the body mass index (r = -0.376; P = 0.09). There was no difference in GH measurements between the GHRH/arginine test done alone or in combination with the SST.

CONCLUSIONS

Combining the SST and GHRH/arginine test results in a lower cortisol response to synacthen. For this reason, the combined test cannot be recommended to assess the integrity of cortisol and GH reserve using current diagnostic criteria.

Low-dose L-arginine administration increases microperfusion of hindlimb muscle without affecting blood pressure in rats

The objective of this work was to evaluate the influence of exogenous L-arginine on the capillary blood flow of peripheral tissues of normotensive subjects. Rats were anesthetized with sodium pentobarbital, and the blood flow of femoral, dorsal, and ventral skin and gastrocnemius and soleus muscle was measured by laser Doppler flow and microsphere methods to compare the blood flow before and after the L-arginine infusion. L-arginine lowered the mean blood pressure in a dose-dependent manner, but a statistically significant reduction in mean blood pressure was detected only at a high dose of 500 mg/kg of body weight. The significant blood flow increment was detected after the L-arginine infusion at doses of 50 and 150 mg/kg without causing hypotension. Nicardipine, a calcium channel blocker, also increased the skin blood flow, but the blood flow increment and blood pressure fall were comparable. A significant increment in microperfusion was detected in gastrocnemius, soleus muscle, and ventral skin compared with control group by the microsphere method. No adverse effects were observed during L-arginine and microsphere infusion. The present work indicates that l-arginine infusion increases muscle capillary blood flow in rats that are not performing exercise. Supplementation with l-arginine might provide additional blood flow at rest and during exercise and result in the improvement of muscle performance and exercise capacity.

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.

Oral arginine attenuates the growth hormone response to resistance exercise

This study investigated the combined effect of resistance exercise and arginine ingestion on spontaneous growth hormone (GH) release. Eight healthy male subjects were studied randomly on four separate occasions [placebo, arginine (Arg), placebo + exercise (Ex), arginine + exercise (Arg+Ex)]. Subjects had blood sampled every 10 min for 3.5 h. After baseline sampling (30 min), subjects ingested a 7-g dose of arginine or placebo (blinded, randomly assigned). On the exercise days, the subject performed 3 sets of 9 exercises, 10 repetitions at 80% one repetition maximum. Resting GH concentrations were similar on each study day. Integrated GH area under the curve was significantly higher on the Ex day (508.7 ± 169.6 min·ng/ml; P < 0.05) than on any of the other study days. Arg+Ex (260.5 ± 76.8 min·ng/ml) resulted in a greater response than the placebo day but not significantly greater than the Arg day. The GH half-life and half duration were not influenced by the stimulus administered. The GH secretory burst mass was larger, but not significantly, on the Arg, Ex, and Arg+Ex day than the placebo day. Endogenous GH production rate (Ex > Arg+Ex > Arg > placebo) was greater on the Ex and Arg+Ex day than on the placebo day ( P < 0.05) but there were no differences between the Ex and Arg+Ex day. Oral arginine alone (7 g) stimulated GH release, but a greater GH response was seen with exercise alone. The combined effect of arginine before exercise attenuates the GH response. Autonegative feedback possibly causes a refractory period such that when the two stimuli are presented there will be suppression of the somatotrope.

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.

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.

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.

Use of amino acids as growth hormone-releasing agents by athletes

Specific amino acids, such as arginine, lysine and ornithine, can stimulate growth hormone (GH) release when infused intravenously or administered orally. Many individuals consume amino acids before strength training workouts, believing this practice accentuates the exercise-induced GH release, thereby promoting greater gains in muscle mass and strength. The GH response to amino acid administration has a high degree of interindividual variability and may be altered by training status, sex, age, and diet. Although parenteral administration consistently leads to increased circulating GH concentration, oral doses that are great enough to induce significant GH release are likely to cause stomach discomfort and diarrhea. During exercise, intensity is a major determinant of GH release. Although one study showed that arginine infusion can heighten the GH response to exercise, no studies found that pre-exercise oral amino acid supplementation augments GH release. Further, no appropriately conducted scientific studies found that oral supplementation with amino acids, which are capable of inducing GH release, before strength training increases muscle mass and strength to a greater extent than strength training alone. The use of specific amino acids to stimulate GH release by athletes is not recommended.

Increased pulsatility, process irregularity, and nocturnal trough concentrations of growth hormone in amenorrheic compared to eumenorrheic athletes

Amenorrheic athletes exhibit a spectrum of neuroendocrine disturbances, including alterations in the GH-insulin-like growth factor I (IGF-I) axis. Whether these changes are due to exercise or amenorrhea is incompletely characterized. The present study investigates spontaneous (overnight) and exercise-stimulated GH secretion and associated IGF-binding proteins (IGFBPs) in amenorrheic (AA; n = 5), and eumenorrheic athletes ( n = 5) matched for age, percent body fat (dual energy x-ray absorptiometry), training history, and maximal oxygen consumption. Each volunteer participated in two hospital admissions consisting of a 50-min submaximal exercise bout (70% maximal oxygen consumption) and an 8-h nocturnal sampling period. Deconvolution analysis of serum GH concentration time series revealed increases in the half-life of GH (60%) and the number of secretory bursts (85%) as well as a decrease in their half-duration (50%) and the mass of GH secreted per pulse (300%) in the AA cohort. Time occupancy at elevated trough GH concentrations was significantly increased, and GH pulsatility (approximate entropy) was more irregular in the AA group. During exercise, AA exhibited a reversal of the normal relationship between IGF-I and GH, and a 4- to 5-fold blunting of stimulated peak and integrated GH secretion. Fasting levels of plasma IGF-I, IGFBP-3, and IGFBP-1 appeared to be unaffected by menstrual status. In ensemble, this phenotype of GH release in amenorrheic athletes suggests disrupted neuroregulation of episodic GH secretion, possibly reflecting decreased somatostinergic inhibition basally, and reduced GHRH output in response to exercise compared with eumenorrheic athletes. Accordingly, we postulate that the amenorrheic state, beyond the exercise experience per se, alters the neuroendocrine control of GH output in amenorrheic athletes.

Synergy of L-arginine and growth hormone (GH)-releasing peptide-2 on GH release: influence of gender

We test the hypotheses that 1) growth hormone (GH)-releasing peptide-2 (G) synergizes with L-arginine (A), a compound putatively achieving selective somatostatin withdrawal and 2) gender modulates this synergy on GH secretion. To these ends, 18 young healthy volunteers (9 men and 9 early follicular phase women) each received separate morning intravenous infusions of saline (S) or A (30 g over 30 min) or G (1 microg/kg) or both, in randomly assigned order. Blood was sampled at 10-min intervals for later chemiluminescence assay of serum GH concentrations. Analysis of covariance revealed that the preinjection (basal) serum GH concentrations significantly determined secretagogue responsiveness and that sex (P = 0.02) and stimulus type (P < 0.001) determined the slope of this relationship. Nested ANOVA applied to log-transformed measures of GH release showed that gender determines 1) basal rates of GH secretion, 2) the magnitude of the GH secretory response to A, 3) the rapidity of attaining the GH maximum, and 4) the magnitude or fold (but not absolute) elevation in GH secretion above preinjection basal, as driven by the combination of A and G. In contrast, the emergence of the G and A synergy is sex independent. We conclude that gender modulates key facets of basal and A/G-stimulated GH secretion in young adults.