While most differences in body mass index (BMI) among siblings are determined by genetics, shared environmental factors contribute to about one-third of that variance, and can potentially be modified to reduce the risk of obesity, a recent study suggests.
The effects of genetics and common environmental factors were greater in children ages 5 to 11, but decreased in adolescents, for whom unique environmental factors were the main driver of BMI variation.
The study, “Using a sibling-adoption design to parse genetic and environmental influences on children’s body mass index (BMI),” was published in the journal PLOS One.
A combination of genetics and environmental factors is the main predisposing contributor to obesity in children. But studies pinpointing the separate effects of genetics and environment have provided diverse results.
Several explanations may account for such findings. First, the influence of genetics and the environment on BMI appears to change with age. Second, environmental factors can be shared by children in a household — such as access to physical activity opportunities, green space, and healthy foods, as well as parental support for healthy eating and exercise — or be unique if siblings are exposed to them at different levels.
Finally, most studies examining the impact of genetics and environment have looked at twins or genetically-related siblings that grow in the same environment, making it difficult to estimate the contribution of each factor alone.
To shed more light on the relative contribution of genetics and environmental factors on BMI variability in children and adolescents, a team led by researchers at the Prevention Science Institute at the University of Oregon investigated a large group of children, including biological siblings who grew in the same home or in different homes, and nonbiological siblings who lived in the same household.
The approach allowed the scientists to pinpoint the relative contribution of shared genetics, common environment, and unique environment in middle childhood and adolescence.
The analysis involved 711 children from 414 households, including 12 pairs of identical twins, 115 pairs of full siblings or fraternal twins, 192 pairs of half-siblings, and 260 pairs of nonbiological siblings.
Children had a mean age of 11.3 years, ranging from 5 to 18 years, and sibling pairs had a mean age difference of nearly 3.6 years.
Adopted children were adopted at birth, at a median age of 2 days. The sample also included their biological siblings, who remained with their birth parents.
Overall, mothers in the adoptive families had higher levels of education. These families also had higher incomes and more supportive parenting styles than families whose members were biologically related. These factors collectively lower the risk of childhood obesity. As such, home type (birth vs. adoptive) was taken into account in the statistical model.
In the total group, 15.5% of the children were obese, which is similar to the current obesity prevalence in the U.S., the researchers said.
Results showed that both the genetic relatedness of the siblings and shared environmental factors explained most of the variance in BMI in middle childhood, or in children ages 5 to 11. In these children, genetics explained 63% of BMI variance and common environmental factors explained 31%, with little contribution from unique environment.
In contrast, none of these factors seemed to significantly impact BMI in adolescents, “suggesting that unique environment is a main driver of variation in this group,” the researchers wrote.
Still, they said that caution is needed to interpret the results, as most statistical models were underpowered — meaning that more participants should have been included — to detect the diminishing effects of genetics and common environmental factors in older sibling pairs.
“This sibling-adoption design allowed us to estimate the contributions of genetic background and home environment to child BMI status without confounding genetic relatedness and home sharing between siblings,” the investigators wrote.
“These results support the importance of childhood intervention strategies aimed at modifying the family or contextual rearing environment to mitigate obesity risk in children,” they concluded.
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