According to a new twin study, BMI differences in early childhood and during puberty are more strongly influenced by environmental factors, but this influence disappears in late adolescence and onset of adulthood, when genetic factors explain most of the variation. Further, BMI differences among adolescents of different ethnicities subjected to different environmental factors associated with obesity were also influenced by genetic factors.
Childhood obesity is a significant public health problem across the globe. In 2011-2012, more than 30% of children in the United Sates were classified as obese or overweight, and childhood obesity is an increasing issue among several developing countries. Earlier family and twin research has suggested that obesity is influenced by both environmental and genetic factors. However, it is unclear how environmental and genetic influences of body mass index (BMI, index of weight-for-height used to classify obesity and overweight) differ from infancy to early adulthood, and the ways they vary with sex and geographic areas with varying levels of obesogenic environments (i.e. environments tending to cause weight gain).
To find the answer to these questions, a group of researchers conducted a study using data from the CODATwins (Collaborative project of Development of Anthropometrical measures in Twins). Using 45 twin cohorts from 20 countries, they examined 383,092 BMI measurements from 87,782 twin pairs between the ages of 6 months to 19.5 years old. These cohorts were divided into three geographical regions: Europe, Australia and North America, and East Asia. The researchers aimed to determine the amount of BMI variation contributed by environmental and genetic factors and how it changes from infancy to early adulthood and how it differs based on sex and age. They further explored how these estimates compare in different geographical areas characterized by low (East Asia), moderate (Europe), and high (North America and Australia) degree of obesogenic environment.
The findings, recently published in the American Journal of Clinical Nutrition, showed that heritability had a weaker influence on BMI in mid-childhood compared to late adolescence and beginning of adulthood. BMI differences in early childhood and throughout puberty were more strongly influenced by environmental factors shared by twins, but this influence disappeared in late adolescence and before the start of adulthood (after 15 years of age) when genetic factors explained most of the differences. According to the researchers, the influence of genetic factors in BMI increased in strength from mid-childhood to the beginning of adulthood possibly because children had the opportunity to behave based on their genetic background due to increasing independence from their parents.
The study further showed that the contribution of genetics on BMI differences was partially based on sex, even during infancy. However, their impact became more significant during and after puberty (i.e. at 13 years of age or older), which may be explained by increasing differences between boys and girls with age in terms of body makeup. Apart from that, the genetic makeup of BMI between girls and boys showed relatively insignificant differences.
Moreover, when the researchers compared geographical regions, they found the greatest differences in BMI in Australia and North America (higher level of obesogenic environment) and lowest in Europe and East Asia (lower level of obesogenic environment). In other words, a greater obesogenic environment was linked to greater BMI. Across the different regions, however, the relative amount of environmental and genetic differences of BMI was almost identical. The results also indicate that genetic factors have a significant impact on BMI differences in adolescents in wealthy societies, regardless of different environmental exposures and ethnicities.
The overall findings suggest that environmental factors shared by twins influence BMI in early childhood, but play a much weaker role in late adolescence. Additionally, in populations of different ethnicities subjected to varying environmental factors associated with obesity, BMI variation in adolescence is significantly affected by genetic factors.
Written By: Nigar Celep, BASc