Determinants of growth and height
An example of humans’ growth velocity under optimal conditions (Courtesy: Richard Steckel)
The study of height is known as auxology. Growth has long been recognized as a measure of the health of individuals, hence part of the reasoning for the use of growth charts. For individuals, as indicators of health problems, growth trends are tracked for significant deviations and growth is also monitored for significant deficiency from genetic expectations. Genetics is a major factor in determining the height of individuals, though it is far less influential in regard to populations. Average height is increasingly used as a measure of the health and wellness (standard of living and quality of life) of populations. Attributed as a significant reason for the trend of increasing height in parts of Europe is the egalitarian populations where proper medical care and adequate nutrition are relatively equally distributed. Changes in diet (nutrition) and a general rise in quality of health care and standard of living are the cited factors in the Asian populations. Average height in the United States has remained essentially stagnant since the 1950s even as the racial and ethnic background of residents has shifted. Severe malnutrition is known to cause stunted growth in North Korean, portions of African, certain historical European, and other populations. Diet (in addition to needed nutrients; such things as junk food and attendant health problems such as obesity), exercise, fitness, pollution exposure, sleep patterns, climate (see Allen’s rule and Bergmann’s Rule for example), and even happiness (psychological well-being) are other factors that can affect growth and final height.
Sir Francis Galton’s (1889) data showing the relationship between offspring height (928 individuals) as a function of mean parent height (205 sets of parents). The correlation was 0.57.
Height is, like other phenotypic traits, determined by a combination of genetics and environmental factors. Genetic potential plus nutrition minus stressors is a basic formula. Genetically speaking, the heights of mother and son and of father and daughter correlate, suggesting that a short mother will more likely bear a shorter son, and tall fathers will have tall daughters.[62] Humans grow fastest (other than in the womb) as infants and toddlers (birth to roughly age 2) and then during the pubertal growth spurt. A slower steady growth velocity occurs throughout childhood between these periods; and some slow, steady, declining growth after the pubertal growth spurt levels off is common. These are also critical periods where stressors such as malnutrition (or even severe child neglect) have the greatest effect. Conversely, if conditions are optimal then growth potential is maximized; and also there is catch-up growth – which can be significant – for those experiencing poor conditions when those conditions improve.[citation needed]
Moreover, the health of a mother throughout her life, especially during her critical periods, and of course during pregnancy, has a role. A healthier child and adult develops a body that is better able to provide optimal prenatal conditions. The pregnant mother’s health is important as gestation is itself a critical period for an embryo/fetus, though some problems affecting height during this period are resolved by catch-up growth assuming childhood conditions are good. Thus, there is an accumulative generation effect such that nutrition and health over generations influences the height of descendants to varying degrees.
The age of the mother also has some influence on the her child’s height. Although 2 Esdras 5:53 recorded that “Those born in the strength of youth” were taller than “those born during the time of old age, when the womb is failing”, studies in modern times have observed a gradual increase in height with maternal age.[63][64][65]
The precise relationship between genetics and environment is complex and uncertain. Human height is 60%-80% heritable, according to several twin studies[66] and has been considered polygenic since the Mendelian-biometrician debate a hundred years ago.[67] The only gene so far attributed with normal height variation is HMGA2. This is only one of many, as each copy of the allele concerned confers an additional 0.4 cm (0.16 in) accounting for just 0.3% of population variance.[66]
The Nilotic peoples of Sudan such as the Dinka have been described as the tallest in the world, with the males in some communities having average heights of 1.9 m (6 ft 3 in) and females at 1.8 m (5 ft 11 in).[68] The Dinka are characterized as having long legs, narrow bodies and short trunks, an adaptation to hot weather.[69] However, a 1995 study casts doubt on the claim of extraordinary height in Dinka, which after studying the average height of Dinka males in one location, listed the actual number as 1.76 m (5 ft 9+1⁄2 in).[70] Males in the Dinaric Alps have an average height of 1.85 m (6 ft 1 in).[16]
Process of growth
Main pathways in endocrine regulation of growth.
Growth in stature, determined by its various factors, results from the lengthening of bones via cellular divisions chiefly regulated by somatotropin (human growth hormone (hGH)) secreted by the anterior pituitary gland. Somatotropin also stimulates the release of another growth inducing hormone insulin-like growth factor 1 (IGF-1) mainly by the liver. Both hormones operate on most tissues of the body, have many other functions, and continue to be secreted throughout life; with peak levels coinciding with peak growth velocity, and gradually subsiding with age after adolescence. The bulk of secretion occurs in bursts (especially for adolescents) with the largest during sleep.
The majority of linear growth occurs as growth of cartilage at the epiphysis (ends) of the long bones which gradually ossify to form hard bone. The legs compose approximately half of adult human height, and leg length is a somewhat sexually dimorphic trait. Some of this growth occurs after the growth spurt of the long bones has ceased or slowed. The majority of growth during growth spurts is of the long bones. Additionally, the variation in height between populations and across time is largely due to changes in leg length. The remainder of height consists of the cranium. Height is sexually dimorphic and statistically it is more or less normally distributed, but with heavy tails.
Height abnormalities
Most intra-population variance of height is genetic. Short stature and tall stature are usually not a health concern. If the degree of deviation from normal is significant, hereditary short stature is known as familial short stature and tall stature is known as familial tall stature. Confirmation that exceptional height is normal for a respective person can be ascertained from comparing stature of family members and analyzing growth trends for abrupt changes, among others. There are, however, various diseases and disorders that cause growth abnormalities. Most notably, extreme height may be pathological, such as gigantism (very rare) resulting from childhood hyperpituitarism, and dwarfism which has various causes. Rarely, no cause can be found for extreme height; very short persons may be termed as having idiopathic short stature. The Food and Drug Administration (FDA) in 2003 approved hGH treatment for those 2.25 standard deviations below the population mean (approximately the lowest 1.2% of the population). An even rarer occurrence, or at least less used term and recognized “problem”, is idiopathic tall stature.
If not enough growth hormone is produced and/or secreted by the pituitary gland, then a patient with growth hormone deficiency can undergo treatment. This treatment involves the injection of pure growth hormone into thick tissue to promote growth.
Role of an individual’s height
Certain studies have shown that height is a factor in overall health while some suggest tallness is associated with better cardio-vascular health and shortness with overall better-than-average health and longevity.[71] Being excessively tall can cause various medical problems, including cardiovascular issues, due to the increased load on the heart to supply the body with blood, and issues resulting from the increased time it takes the brain to communicate with the extremities. For example, Robert Wadlow, the tallest man known to verifiable history, developed walking difficulties as his height continued to increase throughout his life. In many of the pictures of the later portion of his life, Wadlow can be seen gripping something for support. Late in his life he was forced to wear braces on his legs and to walk with a cane, and he died after developing an infection in his legs because he was unable to feel the irritation and cutting caused by his leg braces (it is important to note that he died in 1940, before the widespread use of modern antibiotics). Height extremes of either excessive tallness or shortness can cause social exclusion and discrimination for both men and women (heightism).
Epidemiological studies have also demonstrated a positive correlation between height and intelligence. The reasons for this association appear to include that height serves as a biomarker of nutritional status or general mental and physical health during development, that common genetic factors may influence both height and intelligence, and that both height and intelligence are affected by adverse early environmental exposures.[dubious – discuss]
A study done on men in Sweden has shown that there exists in this country a strong correlation between subnormal stature and suicide.[72]
This can also sometimes be translated over into the corporate world.[citation needed]
Historically this assumption has not always reflected reality; for instance Napoleon was not much taller than 1.5 m (4 ft 11 in) according to sources (though Napoleon’s height is subject to great debate, and he may have been as tall as 1.67 m (5 ft 5+1⁄2 in), see Napoleon’s height for further information). Ignatius Loyola, founder of the Jesuit order was 1.5 m (4 ft 11 in). Both Lenin and Stalin were of below average height. A modern example would be Deng Xiaoping of China who undertook massive reforms to the Chinese economy in the 1980s and was reported to have only been 1.55 m (5 ft 1 in).