Human Height - Determinants of Growth and Height

Determinants of Growth and Height

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 are 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. Malnutrition including chronic undernutrition and acute malnutrition is known to have caused stunted growth in various populations (WHO, 2012: Public Health Nutrition 2012;15:142-8.). This has been seen in North Korean, portions of African, certain historical European, and other populations. Countries such as Guatemala have rates of stunting as high as 82% in children under 5 (ENSMI 2008-2009).

Height measurements are by nature subject to statistical sampling errors even for a single individual. In a clinical situation, height measurements are seldom taken more often than once per office visit, which may mean sampling taking place a week to several months apart. The smooth 50th percentile male and female growth curves illustrated above are aggregate values from thousands of individuals sampled at ages from birth to age 20. In reality, a single individual's growth curve shows large upward and downward spikes. Partly due to actual differences in growth velocity, and partly due to small measurement errors. For example, a typical measurement error of plus or minus 0.5 cm may completely nullify 0.5 cm of actual growth resulting in either a "negative" 0.5 cm growth (due to overestimation in the previous visit combined with underestimation in the latter), up to a 1.5 cm growth (the first visit underestimating and the second visit overestimating) in the same elapsed time period between measurements. Note there is a discontinuity in the growth curves at age 2, which reflects the difference in recumbent length (with the child on his or her back), used in measuring infants and toddlers, and standing height typically measured from age 2 onwards.

Height, like other phenotypic traits, is determined by a combination of genetics and environmental factors. A child's height based on parental heights is subject to regression toward the mean, therefore extremely tall or short parents will likely have correspondingly taller or shorter offspring, but their offspring will also likely be closer to average height than the parents themselves. Genetic potential and a number of hormones, minus illness, is a basic determinant for height. Diet only influences growth in malnourished children who experience delayed development and short stature. There is no evidence that enriching a diet with (or avoiding) a particular food will alter the height one is otherwise destined to reach. Humans grow fastest (other than in the womb) as infants and toddlers, rapidly declining from a maximum at birth to roughly age 2, tapering to a slowly declining rate, and then during the pubertal growth spurt, a rapid rise to a second maximum (at around 11–12 years for female, and 13–14 years for male), followed by a steady decline to zero. On average, female growth speed trails off to zero at about 15 or 16 years, whereas the male curve continues for approximately 3 more years, going to zero at about 18–20. These are also critical periods where stressors such as malnutrition (or even severe child neglect) have the greatest effect.

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 a cumulative 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 her child's height. Studies in modern times have observed a gradual increase in height with maternal age, though these early studies suggest that trend is due to various socio-economic situations that select certain demographics as being more likely to have a first birth early in the mother's life. These same studies show that children born to a young mother are more likely to have below-average educational and behavioural development, again suggesting an ultimate cause of resources and family status rather than a purely biological explanation.

The precise relationship between genetics and environment is complex and uncertain. Human height is 60%–80% heritable, according to several twin studies and has been considered polygenic since the Mendelian-biometrician debate a hundred years ago. The only gene known to have an influence on human height is HMGA2. People who carry two copies of the "tall" allele of the HMGA2 gene are up to 1 cm taller than those who carry two copies of the "short" allele. A genome-wide association (GWA) study of more than 180,000 individuals has identified hundreds of genetic variants in at least 180 loci associated with adult human height.

The Nilotic peoples of Sudan such as the Shilluk and Dinka have been described as some of the tallest in the world. Dinka Ruweng males investigated by Roberts in 1953–54 were on average 1.813 m tall, and Shilluk males reached even 1.826 m. The Nilotic people are characterized as having long legs, narrow bodies and short trunks, an adaptation to hot weather. However, male Dinka and Shilluk refugees measured in 1995 in Southwestern Ethiopia were on average only 1.764 m and 1.726 m tall, respectively.

The Polynesian peoples of Samoa and Tonga have been described since European contact as being tall and muscular. Samoan males investigated by University of Hawaii–54 averaged 1.778 m tall, and Tongan males reached even 1.8034 m. The Samoan and Tongan people are characterized as having large, muscular physiques, an adaptation to long ocean voyages.