Over the last 30 years, the term Human Growth Hormone (hGH) has predominantly been associated with performance enhancement and illegal usage in professional athletics. The stigma associated with this term has roused tremendous amounts of discourse in the medical and sports community alike (Martinez, 2012). Today, new insights have prompted an inquiry into how sleep and exercise may affect the human growth hormone. Researchers have discovered that poor diet, inadequate sleep and lack of proper exercise deplete the body’s ability to produce hGH. Deep sleep (lack of rapid eye movement) and heightened activity at the sympathetic division of the nervous system (as affected by vigorous physical activity or stress) promote the secretion of hGH (Seo, Park, Chang, So, & Song, 2009). This paper examines existing research evidence on how sleep and physical activity affect the human growth hormone.
- Human growth hormone (hGH)
According to DEA (2013), the human growth hormone (hGH) is a polypeptide hormone that is produced and released by the anterior pituitary gland as a response to different stimuli, the most significant of which are exercise and sleep. It is essential for body growth. Secretion of hGH occurs throughout an individual’s lifetime. It increases in childhood and peaks during adolescence. It then declines throughout adulthood. Synthetic hGH was first developed and used by the FDA. In subsequent years, it has been abused by bodybuilders, athletes and aging adults due to its performance enhancing, muscle-building and fat-decreasing qualities. It has also been associated with the reversal of aging effects. Secretion of hGH follows a circadian pattern, occurring in 6-12 distinct pulses daily. The largest pulse occurs about one hour after night time sleep begins. The hGH is crucial to the regeneration of hormone responsible for muscle growth as well as the growth of other connective tissue. Before hGH can be utilized in the body, several requirements necessary for its secretion must be fulfilled. The secretory activity of somatrophs (which release hGH) is controlled by two hormones: Growth Hormone Releasing Hormone (GHRH) and Growth Hormone Inhibiting Hormone (GHIH). GHRH promotes hGH secretion while GHIH suppresses it. Other stimuli that cause the secretion of hGH include deep sleep and increased physical activity (exercise).
- Exercise effects on hGH secretion
According to Godfey (2004), acute physical exercise has been established as a physiological stimulus for hGH secretion. Physical exercise, whether resistance or aerobic results in increases in hGH secretion. However, whether acute or aerobic, there is a relationship between the intensity the exercise with the extent of hGH secretion.
2.1.1) Acute Aerobic Exercise
According to Wideman, Weltman, Hartman, Veldhuis and Weltman (2002), aerobic exercise leads to an increase in hGH secretion 15 minutes after the exercise begins. Peak values of hGH secretion are recorded towards the end of the exercise. Recent studies that utilize highly frequent sampling techniques indicate that aerobic exercise of the appropriate duration and intensity is a highly potent stimulus for hGH.
Research shows that women record higher hGH at rest, greater basal hGH secretion and less ordered hGH release. Women also show an anticipatory response to exercise as well as quicker attainment of peak hGH concentrations in aerobic exercises. While young men and men reach large increases in hGH concentrations during aerobic exercises, the rise from baseline is considerably higher in men than it is in women.
Figure 1: Response patterns for men and women during exercise
Exercise intensity is a main modifier of aerobic exercise-stimulated hGH secretion. The relationship between increased exercise intensity and hGH secretion is higher in women than in men. A duration of 30 minutes aerobic exercise and that of 10 minutes (both spread throughout the day) result in the same level off hGH release in a 24-hour period.
2.1.2) Acute resistance exercise
Resistance activity is mainly anaerobic activity. Studying acute high responses to resistance training exercise has been primarily hampered by the challenge involved in equating the level of intensity across individuals and studies. Keeping intravenous catheters attached throughout the exertion period and extracting blood samples un-intrusively has also been a major headache for researchers. To avoid these difficulties, many researchers have opted for pre-exercise sampling, interval sampling and post-exercise sampling. Regardless of the method used, hGH release patterns are similar. According to Ahtiainen (2006), secretion of hGH results from the resistance exercise stimulus among others. They document that non-weight-trained women have higher hGH secretion levels after bouts of acute resistance exercises than weight-trained women. Although resistance exercises stimulate hGH release, the response to these exercises is influenced mostly by the work-rest intervals, the load and the frequency. According to Yardley, Kenny, Perkins and Riddell (2013), resistance exercise produces the highest overall hGH secretion levels.
- Effects of sleep on hGH secretion
Sleep is a well known modulator of the functions of endocrine glands. The secretion of hGH is particularly reliant on sleep. In normal adults, the onset of sleep is associated with a heightened increase in hGH secretion. hGH secretion in sleep happens in slow-wave sleep (SWS) and is interfered with during awakenings. A study by Latta, Tasali, Leproult et al (2005) on the effect of sleep on hGH secretion in males and females indicated that there are major sex differences in the manner of nocturnal hGH secretion. GHG was shown to have been released before and after the onset of sleep in both sexes. In women release of pre-sleep hGH seemingly inhibited post-sleep hGH release while in men, there were no pre- and post-sleep hormone levels. Although sleep stimulates release of hGH, a study conducted by Brandenberger & Weibel (2004) by depriving subjects of sleep indicates that hGH secretion is not lowered in subjects who are sleep-deprived at night within a 24-hour duration. This is because their hGH secretion is significantly increased during the day.
- Best combination of sleep and exercise
Many dynamics exist in the determination of the combination of sleep times and type of exercises that can lead to the highest hGH secretion levels. This is because of the effects of factors such as age, gender, weight-training or lack of it, type of exercise (whether aerobic or resistance training) etc. However, according to Drews & Wilmore (2005), acute resistance exercises leads to higher hGH elevation per unit time that aerobic exercise. According to Tuckow, Alexander, Rarick at al (2006), nocturnal secretion dynamics of hGH change after a bout of resistance training. The results from their research suggest that resistance exercise changes the dynamics of release by attenuating the burst amplitude and mass and increasing burst frequency (although reducing mass per burst). Although the effect of diurnal resistance exercises on amounts of nocturnal hGH secretion is unresolved.
Human Growth Hormone (hGH) is a polypeptide hormone that is produced and released by the anterior pituitary gland as a response to different stimuli, the most significant of which are exercise and sleep. Different exercises result in different secretion dynamics that differ across gender and time durations. Sleep is also associated with heightened hGH released. Research suggests that nocturnal hGH release dynamics change after a bout of resistance exercises but the relationship in terms of mass remains unresolved.
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