Aging and physical fitness are more important than obesity in determining exercise-induced generation of GH

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.

Dynamic Tests in Pituitary Endocrinology: Pitfalls in Interpretation during Aging

Aging and age-related diseases represent hot topics of current research. Progressive damage in morphology and function of cells and tissue characterizes the normal process of aging that is influenced by both genetic and environmental factors. The ability of each individual to adapt to these stressors defines the type of aging and the onset of age-related diseases (i.e., metabolic syndrome, inflammatory disorders, cancer, and neurodegenerative diseases). The endocrine system plays a critical role in this process because of its complex relationships with brain, immune system, and skeletal muscle; thus, alterations in hormonal networks occur during aging to maintain homeostasis, with consequent under- or overactivity of specific hypothalamic-pituitary-peripheral hormone axes. On the other hand, the increase in life expectancy has led to increasing incidence of age-related diseases, including endocrine disorders, which may prompt assessment of endocrine function in aging individuals. In this context, there is growing awareness that natural changes of endocrine physiology and physiopathology occurring with increasing age may necessitate age-driven diagnostic cutoffs requiring validation in the elderly. This review aims to analyze the available literature on the hormone response to the most important dynamic tests currently used in the clinical practice for the screening of anterior pituitary-related diseases to underline pitfalls in interpretation during aging.

Could Age, Sex and Physical Fitness Affect Blood Glucose Responses to Exercise in Type 1 Diabetes?

Closed-loop systems for patients with type 1 diabetes are progressing rapidly. Despite these advances, current systems may struggle in dealing with the acute stress of exercise. Algorithms to predict exercise-induced blood glucose changes in current systems are mostly derived from data involving relatively young, fit males. Little is known about the magnitude of confounding variables such as sex, age, and fitness level-underlying, uncontrollable factors that might influence blood glucose control during exercise. Sex-related differences in hormonal responses to physical exercise exist in studies involving individuals without diabetes, and result in altered fuel metabolism during exercise. Increasing age is associated with attenuated catecholamine responses and lower carbohydrate oxidation during activity. Furthermore, higher fitness levels can alter hormonal and fuel selection responses to exercise. Compounding the limited research on these factors in the metabolic response to exercise in type 1 diabetes is a limited understanding of how these variables affect blood glucose levels during different types, timing and intensities of activity in individuals with type 1 diabetes (T1D). Thus, there is currently insufficient information to model a closed-loop system that can predict them accurately and consistently prevent hypoglycemia. Further, studies involving both sexes, along with a range of ages and fitness levels, are needed to create a closed-loop system that will be more precise in regulating blood glucose during exercise in a wide variety of individuals with T1D.

Waist circumference vs body mass index in association with cardiorespiratory fitness in healthy men and women: a cross sectional analysis of 403 subjects

Objective

Body mass index (BMI) is more commonly used than waist circumference as a measure of adiposity in clinical and research settings. The purpose of this study was to compare the associations of BMI and waist circumference with cardiorespiratory fitness.

Methods

In a cross-sectional study of 403 healthy men and women aged 50 ± 8.8 years, BMI and waist circumference were measured. Cardiorespiratory fitness was assessed from estimated maximal O₂ uptake (VO₂max), as calculated from a maximal fitness test.

Results

Mean BMI (kg/m²) was 27.8 ± 3.7 and 25.5 ± 4.6; and mean waist circumference (cm) 94.1 ± 9.7 and 84.3 ± 10.4 for men and women, respectively. Both men and women reported an average of 2.5 hours of weekly sports related physical activity, and 18% were current smokers. Correlation coefficients between both BMI and waist circumference, and VO₂max were statistically significant in men (r = −0.280 and r = −0.377, respectively, p > 0.05 for both) and in women (r = −0.514 and r = −0.491, respectively, p > 0.05 for both). In women, the contribution of BMI to the level of VO₂max in a regression model was greater, while in men waist circumference contributed more to the final model. In these models, age, hours of training per week, and weekly caloric expenditure in sport activity, significantly associated with VO₂max, while smoking did not.

Conclusion

The differences observed between the sexes in the associations of BMI and waist circumference with VO₂max support the clinical use of both obesity measures for assessment of cardiorespiratory fitness.

Hormone replacement therapy and physical function in healthy older men. Time to talk hormones?

Improving physical function and mobility in a continuously expanding elderly population emerges as a high priority of medicine today. Muscle mass, strength/power, and maximal exercise capacity are major determinants of physical function, and all decline with aging. This contributes to the incidence of frailty and disability observed in older men. Furthermore, it facilitates the accumulation of body fat and development of insulin resistance. Muscle adaptation to exercise is strongly influenced by anabolic endocrine hormones and local load-sensitive autocrine/paracrine growth factors. GH, IGF-I, and testosterone (T) are directly involved in muscle adaptation to exercise because they promote muscle protein synthesis, whereas T and locally expressed IGF-I have been reported to activate muscle stem cells. Although exercise programs improve physical function, in the long-term most older men fail to comply. The GH/IGF-I axis and T levels decline markedly with aging, whereas accumulating evidence supports their indispensable role in maintaining physical function integrity. Several studies have reported that the administration of T improves lean body mass and maximal voluntary strength in healthy older men. On the other hand, most studies have shown that administration of GH alone failed to improve muscle strength despite amelioration of the detrimental somatic changes of aging. Both GH and T are anabolic agents that promote muscle protein synthesis and hypertrophy but work through separate mechanisms, and the combined administration of GH and T, albeit in only a few studies, has resulted in greater efficacy than either hormone alone. Although it is clear that this combined approach is effective, this review concludes that further studies are needed to assess the long-term efficacy and safety of combined hormone replacement therapy in older men before the medical rationale of prescribing hormone replacement therapy for combating the sarcopenia of aging can be established.

Growth hormone doping: a review

The use of growth hormone (GH) as a performance enhancing substance was first promoted in lay publications, long before scientists fully acknowledged its benefits. It is thought athletes currently use GH to enhance their athletic performance and to accelerate the healing of sporting injuries. Over recent years, a number of high profile athletes have admitted to using GH. To date, there is only limited and weak evidence for its beneficial effects on performance. Nevertheless the “hype” around its effectiveness and the lack of a foolproof detection methodology that will detect its abuse longer than 24 hours after the last injection has encouraged its widespread use. This article reviews the current evidence of the ergogenic effects of GH along with the risks associated with its use. The review also examines methodologies, both currently available and in development for detecting its abuse.

Growth hormone and lactate responses induced by maximal isometric voluntary contractions and whole-body vibrations in healthy subjects

BACKGROUND

In contrast with maximal voluntary resistance exercise, which is allegedly considered a potent GH stimulus in young subjects, evaluation of GH response to whole-body vibrations (WBV) has yielded conflicting results.

METHODS

The acute effects of WBV alone (test A), maximal voluntary isometric contractions (MVC) (test B), and combination of WBV and MVC (test C) on serum GH and blood lactate (LA) levels were studied in 9 healthy adult males. Muscle soreness was assessed 24 and 48 h after exercise by a visual analogue scale.

RESULTS

GH responses were significantly higher after tests B and C than after test A (GH peaks: 18.8 ± 9.5 ng/ml or 20.8 ± 13.7 ng/ml, respectively, vs 4.3 ± 3.5 ng/ml; p<0.05), with no difference between tests B and C. LA concentrations significantly increased after tests A, B, and C, being significantly higher after tests B and C than after test A (LA peaks: 2.0 ± 0.5 mmol/l or 6.7 ± 2.3 mmol/l, respectively, vs 7.6 ± 0.9 mmol/l; p<0.05). Peak LA values were significantly correlated to GH peaks in the 3 tests (r=0.48; p<0.05). Muscle soreness was significantly higher 24-48 h after tests B and C than after test A, no significant differences being present between tests B and C.

CONCLUSIONS

WBV stimulates GH secretion and LA production, with no additive effect when combined with repeated isometric voluntary contractions. Optimization of protocols based on WBV seems important to maximize the positive effects of this intervention on the somatotropic function.

Growth hormone, IGF-I and insulin and their abuse in sport

There is widespread anecdotal evidence that growth hormone (GH) is used by athletes for its anabolic and lipolytic properties. Although there is little evidence that GH improves performance in young healthy adults, randomized controlled studies carried out so far are inadequately designed to demonstrate this, not least because GH is often abused in combination with anabolic steroids and insulin. Some of the anabolic actions of GH are mediated through the generation of insulin-like growth factor-I (IGF-I), and it is believed that this is also being abused. Athletes are exposing themselves to potential harm by self-administering large doses of GH, IGF-I and insulin. The effects of excess GH are exemplified by acromegaly. IGF-I may mediate and cause some of these changes, but in addition, IGF-I may lead to profound hypoglycaemia, as indeed can insulin. Although GH is on the World Anti-doping Agency list of banned substances, the detection of abuse with GH is challenging. Two approaches have been developed to detect GH abuse. The first is based on an assessment of the effect of exogenous recombinant human GH on pituitary GH isoforms and the second is based on the measurement of markers of GH action. As a result, GH abuse can be detected with reasonable sensitivity and specificity. Testing for IGF-I and insulin is in its infancy, but the measurement of markers of GH action may also detect IGF-I usage, while urine mass spectroscopy has begun to identify the use of insulin analogues.

The growth hormone/insulin-like growth factor-I axis in exercise and sport

The syndrome of adult GH deficiency and the effects of GH replacement therapy provide a useful model with which to study the effects of the GH/IGF-I axis on exercise physiology. Measures of exercise performance including maximal oxygen uptake and ventilatory threshold are impaired in adult GH deficiency and improved by GH replacement, probably through some combination of increased oxygen delivery to exercising muscle, increased fatty acid availability with glycogen sparing, increased muscle strength, improved body composition, and improved thermoregulation. In normal subjects, in addition to the long-term effects of GH/IGF-I status, there is evidence that the acute GH response to exercise is important in regulating substrate metabolism after exercise. Administration of supraphysiological doses of GH to athletes increases fatty acid availability and reduces oxidative protein loss, particularly during exercise, and increases lean body mass. Despite a lack of evidence that these metabolic effects translate to improved performance, GH abuse by athletes is widespread. Tests to detect GH abuse have been developed based on measurement in serum of 1) indirect markers of GH action, and 2) the relative proportions of the two major naturally occurring isoforms (20 and 22kDa) of GH. There is evidence that exercise performance and strength are improved by administration of GH and testosterone in combination to elderly subjects. The potential benefits of GH in these situations must be weighed against potential adverse effects.

Malnutrition during lactation changes growth hormone mRNA expression in offspring at weaning and in adulthood

Mothers' nutrition during lactation programs growth in their offspring. We studied the contribution of the growth hormone (GH) for this programming, evaluating GH mRNA expression. Lactating dams were grouped as follows: C, control diet with 23% protein; PR, 8% protein-restricted diet; and ER, energy-restricted diet, receiving the control diet in restricted quantities of the PR group's ingestion. Some pups were killed at weaning; the others received the control diet until they were sacrificed as adults. Pituitary GH mRNA was analyzed by Northern blot analysis. At weaning, the ER and PR animals had lower GH mRNA levels (-29% and -18%, respectively) and lower length as well as body weight. Ninety-day-old PR offspring showed a lower body length (-5%), whereas ER offspring showed a higher one (+5%); however, at 180 days, the lengths were not different. Both 90- and 180-day-old animals showed body weight differences against control animals, with PR offspring showing a lower (-10%) and ER offspring showing a higher (+12%) body weight. GH mRNA was higher in ER offspring at 90 and 180 days (+19% and +22%, respectively); it was lower in PR offspring at 90 and 180 days (-19% and -17%, respectively). Thus, we showed a direct relation between GH mRNA expression and length as well as body weight. We suggest that malnutrition during lactation may program GH mRNA expression patterns in adulthood and that these changes could be responsible for differences in growth patterns.

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.

The exercise-induced growth hormone response in athletes

Human growth hormone (hGH) is secreted in a pulsatile fashion, generally following a circadian rhythm. A number of physiological stimuli can initiate hGH secretion, the most powerful, non-pharmacological of which are sleep and exercise. hGH has many varied roles throughout life, from growth itself, including the turnover of muscle, bone and collagen, to the regulation of selective aspects of metabolic function including increased fat metabolism and the maintenance of a healthier body composition in later life. The exercise-induced growth hormone response (EIGR) is well recognised and although the exact mechanisms remain elusive, a number of candidates have been implicated. These include neural input, direct stimulation by catecholamines, lactate and or nitric oxide, and changes in acid-base balance. Of these, the best candidates appear to be afferent stimulation, nitric oxide and lactate. Resistance training results in a significant EIGR. Evidence suggests that load and frequency are determining factors in the regulation of hGH secretion. Despite the significant EIGR induced by resistance training, much of the stimulus for protein synthesis has been attributed to insulin-like growth factor-1 with modest contributions from the hGH-GH receptor interaction on the cell membrane. The EIGR to endurance exercise is associated with the intensity, duration, frequency and mode of endurance exercise. A number of studies have suggested an intensity 'threshold' exists for EIGR. An exercise intensity above lactate threshold and for a minimum of 10 minutes appears to elicit the greatest stimulus to the secretion of hGH. Exercise training above the lactate threshold may amplify the pulsatile release of hGH at rest, increasing 24-hour hGH secretion. The impact of chronic exercise training on the EIGR remains equivocal. Recent evidence suggests that endurance training results in decreased resting hGH and a blunted EIGR, which may be linked to an increased tissue sensitivity to hGH. While the potential ergogenic effects of exogenous GH administration are attractive to some athletes, the abuse of GH has been associated with a number of pathologies. Identification of a training programme that will optimise the EIGR may present a viable alternative. Ageing is often associated with a progressive decrease in the volume and, especially, the intensity of exercise. A growing body of evidence suggests that higher intensity exercise is effective in eliciting beneficial health, well-being and training outcomes. In a great many cases, the impact of some of the deleterious effects of ageing could be reduced if exercise focused on promoting the EIGR. This review examines the current knowledge and proposed mechanisms for the EIGR, the physiological consequences of endurance, strength and power training on the EIGR and its potential effects in elderly populations, including the aged athlete.

The role of the growth hormone-insulin-like growth factor axis in glucose homeostasis

Homeostatic mechanisms normally maintain the plasma glucose concentration within narrow limits despite major fluctuations in supply and demand. There is increasing evidence that the growth hormone (GH)-insulin-like growth factor (IGF) axis may play an important role in glucose metabolism. GH has potent effects on intermediary metabolism, some of which antagonize the actions of insulin. In contrast, IGF-I has insulin-like actions, which are, in the case of glucose metabolism, opposite to those of GH. There is often deranged glucose metabolism in situations where GH is deficient or in excess. The clinical administration of GH or IGF-I results in altered glucose metabolism and changes in insulin resistance. Despite these observations, the precise role of GH and IGF-I and their interactions with insulin in controlling normal glucose homeostasis are unknown. In diabetes, GH secretion is abnormally increased as a result of reduced portal insulin resulting in impaired hepatic IGF-I generation. Evidence suggests that this may contribute to the development of diabetic microvascular complications. IGF-I 'replacement' in diabetes is under investigation and new methods of delivering IGF-I as a complex with IGFBP-3 offer exciting new prospects.