Normal growth is regulated by hormones, but is also greatly influenced by genetic makeup and nutrition. Parental stature and growth patterns are usually indicative of the same in their offspring. Poor nutrition will negatively affect the growth process. This nutritional boost to growth occurs at conception, and must continue through embryonic and fetal development.
Newborn babies have high hGH levels, which continue through early infancy. Baseline plasma levels of hGH are normal, however, through childhood until puberty, when the resting plasma hGH level increases. Metabolically, hGH functions to increase the rate of protein synthesis in muscle, increase the rate of fat breakdown in fatty tissue, and decrease the rate of glucose use by tissues, resulting in an increase in glucose output by the liver. In the gastrointestinal tract, (GI) growth hormone increases absorption of calcium, an increase in metabolic rate, and a decrease in sodium and potassium excretion. The sodium and potassium are thought to be diverted to growing tissues. In essence, hGH frees up energy to build up tissues. hGH creates an increase in both cell sizes and numbers.
hGH is produced in the anterior portion of the pituitary gland under the control of hormonal signals in the hypothalamus. Two hypothalamic hormones regulate hGH; they are growth hormone-releasing hormone (GHRH) and growth hormone-inhibiting hormone (GHIH). When blood glucose levels fall, GHRH triggers the secretion of stored hGH. As blood glucose levels rise, GHRH release is turned off. Increases in blood protein levels trigger a similar response. As a result of this hypothalamic feedback loop, hGH levels fluctuate throughout the day. Normal plasma hGH levels are 1–3 ng/ml with peaks as high as 60 ng/ml. In addition, plasma glucose and amino acid availability for growth is also regulated by the hormones: adrenaline, glucagon, and insulin.
Most hGH is released at night. Peak spikes of hGH release occur around 10 p.m., midnight, and 2 a.m. The logic behind this night-time release is that most of hGH's effects are mediated by other hormones, including the somatomedins, IGH-I and IGH-II. As a result, the effects of hGH are spread out more evenly during the day. There is also evidence that GH secretion in humans follows a sexually dimorphic pattern, meaning that secretion patterns and levels of hormone are different in males and females.
Other fluctuations in growth occur naturally or because of illness. Growth slows in sick children, so that resources are channeled to heal. However, most children experience a catch-up acceleration of growth after a sick period. This growth can be as much as 400–above normal, but resumes normal levels once the child has caught up. Children given long treatments with steroids may experience hindered growth, as steroids stop growth.
Factors influencing hGH secretion include diet (nutrition) and stressors. Inhibition of hGH secretion occurs with high blood glucose levels, steroid use, and during REM sleep. HGH secretion increases with ingestion of a protein meal, deep sleep, low blood glucose levels, fasting, exercise, physical stress (such as infection or trauma), and psychological stress.
A second major growth spurt occurs at puberty with the coupled effect of sex hormones on growth. Puberty usually occurs earlier in girls (around the ages of age 10–12) than in boys (a few months later). During puberty, the epiphyseal ends of long bones begin to close, signaling the end of length growth. This closure is usually completed by the age of twenty.