Whether people are tall or short strongly influences their performance in sports and career. Research shows that height is also associated with the types of diseases that develop later in life. Genetic differences largely determine the height differences in a population. A new study shows that these genetic differences influence growth as early as the foetal stage but also that the environment in the uterus has an influence.
When parents tell their children to eat their vegetables if they want to become tall and strong, they are telling a partial truth. In reality, the partner each parent chooses primarily determines whether their child becomes tall or short. The largest combined study of twins all over the world shows that, although diet and environment play a role, the genetic baggage primarily determines our height.
“Comparing the heights of identical and fraternal sets of twins clearly shows that genes play a much greater role than environmental influences. Diet and disease only influence height slightly. A new study of twins that also incorporated their size at birth showed that genes influence growth at the foetal stage, although the environment in the uterus also plays a part,” explains a co-author, Thorkild I.A. Sørensen, Novo Nordisk Foundation Center for Basic Metabolic Research.
Twin studies are decisive
The new studies are based on the Collaborative Project of Development of Anthropometrical Measures in Twins (CODATwins), a global collaboration initiated and coordinated by Karri Silventoinen from Helsinki, Finland that uses nearly all the known twin studies globally in pooled analyses. Nevertheless, most twin studies have been carried out in Europe, the United States and Australia, with a few in Asia.
“This enables us to determine the differences in the influence of genes and environment with great statistical certainty across time, place, sex, age and other such factors as social background. The first pooled analysis included 40 twin studies of 143,390 pairs of twins born between 1886 and 1994.”
The new study used twin data from 28 of these, with 41,852 pairs of twins. However, it was not just the enormous size of the pooled analysis that made the results very certain but also the fact that the significance of the variables could be used in ways that had not been possible previously when using a single twin study.
“In family studies, researchers usually examine the differences between individual family members to see how similar they are. In addition, examining the differences between identical and fraternal pairs of twins enable researchers to determine the extent to which the differences between people result from differences in the environment or genetic differences. Pooling the data from many twin studies enables researchers to determine how environment and genes differently affect various population groups.”
Identical (monozygotic) twins are genetically identical; fraternal (dizygotic) twins have the same genetic differences as those between other siblings. The differences between identical twins should thus only result from environmental influences, whereas the differences in fraternal twins stem from both their genes and the environment. The new pooled analysis showed specifically that twins become 1.14–4.25 cm taller for each kilogram extra they weighed at birth and that they become 0.18–0.90 cm taller for each centimetre they are taller at birth.
“The numbers show what we already knew: the heavier and taller you are at birth, the taller you become as an adult. The new study showed that environmental influence is relatively modest at the foetal stage. In contrast, the genetic differences between people mean that the differences that end up determining adult height are already active at the foetal stage and determine the size of the newborn baby.”
Height does not inherently determine disease risk
The new study is a very tangible example of the research for which the new international CODATwins database can be used. For the first time, the database compiles individual data on height and weight at birth and on through adulthood from the many previous twin studies. The researchers emphasize, however, the importance of not drawing conclusions on the causal relationship between height and the risk of developing various diseases.
“The research shows an association between size at birth and adult height, but we cannot conclude that height itself determines the development of a specific disease. We think that there may be a pleiotropic effect: the same genes influence several factors in the body. Thus, the same genes that make people tall may also have a biological effect on conditions in tissue that increase the risk of developing cancer.”
The researchers can therefore not reject a possible genetic association between, for example, the higher growth rate of tall people and the risk of developing cancer. Using modern gene technology tools, researchers worldwide are currently mapping the genetic associations to determine whether specific genes influence both height and the risk of disease.
"In addition, although genes drive height, the environment continues to play a part. Adult height is thus also determined somewhat by the environment while growing up, including such factors as diet, diseases in childhood and apparently also the environment in the uterus. The average height of many populations has risen considerably in the past century, and this reflects improved life conditions. Although we have no control over many genetic conditions, we can still do some things to get slightly fitter and healthier children."
“Genetic and environmental influences on adult human height across birth cohorts from 1886 to 1994” was published in 2016 in eLife, and “Associations between birth size and later height from infancy through adulthood: an individual based pooled analysis of 28 twin cohorts participating in the CODATwins project” was published in 2018 in Early Human Development. Thorkild I.A. Sørensen, a co-author of both articles, is affiliated with the Novo Nordisk Foundation Center for Basic Metabolic Research. The studies also use data from the Danish Twin Registry at the University of Southern Denmark.