QUESTION: How does our body know when to stop growing so that we are not giants?
– Raechelle Stokes,
El Cajon
ANSWER: We would seem like giants to some populations of the past. Higher standards of living (better nutrition, less infectious disease) in many developed nations have led to significant increases in height with each generation. For example, in the past century, average height has increased about four inches in Japan and many European countries.
Intriguingly, Americans, who were the tallest in the world from Colonial times to post World War II, have been surpassed by the Dutch, Swedes, Norwegians, Danes, British and Germans, according to a study in Economics and Human Biology led by economist John Komlos.
Komlos argues that universal access to health care and greater social equality in Northern Europe, relative to the U.S., has lead to healthier and taller populations.
An immigration-related change in demographics is not the reason. When Komlos only compared non-Hispanic, non-Asian people who were born in the U.S., Americans were still shorter than their northern European counterparts.
Height is controlled by genetic programs that lead to the production of growth hormone and a cocktail of other hormones in our bodies. Exactly how environmental factors influence growth is not well understood, but scientists have a pretty good idea of the mechanisms through which hormones exert their influence on height.
Growth hormone is produced by the pituitary gland, a tiny organ near the base of the brain. In about 1 in 20,000 people, the pituitary gland produces too much growth hormone. If this happens in children before puberty, it can cause gigantism – excessive growth of the long bones in the limbs, as well as muscle and organ overgrowth.
Elongation of the bones in the arms and legs occurs at growth plates, regions near the ends of the bone consisting of cartilage. Stimulated by growth hormone, the cartilage cells reproduce and the cartilage is later converted to compact bone.
A variety of other hormones play a role in the proliferation and maturation of cartilage and the process by which it is removed and replaced with bone.
At puberty, the sex hormones (estrogen, testosterone) initially boost the release of growth hormone and lead to a growth spurt. Later, higher levels of sex hormones close the growth plates by causing the cartilage-producing cells to die and be replaced with bone.
Therefore after puberty, an excess of growth hormone does not lead to gigantism. However, it can cause acromegaly – growth of soft features, resulting in enlarged feet, hands and facial features.
Sherry Seethaler has a doctorate in science education and is a UCSD science writer and educator. Send your scientific questions to her at Quest, San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail sseethaler@ucsd.edu. Please include your name, city of residence and phone number.
