A person’s weight is not influenced by the genetic makeup of their siblings, according to a recent study looking at a large sample of American youngsters.
The study, “Testing for family influences on obesity: The role of genetic nurture,” was published in the journal Health Economics.
Many studies have documented a strong correlation between siblings’ health and their behaviors. In addition to sharing similar genetic and environmental backgrounds, siblings may acquire similar characteristics from their interactions among themselves, or between them and their parents.
One such effect is dubbed genetic nurture, also known as indirect genetic effects, which is defined as one individual being affected by another person’s genes, but without having inherited those genes. In other words, the nature of a parent or sibling can influence the way they nurture, and thereby influence a child’s nature.
In this study, researchers sought to test if genetic nurture among siblings has any influence on a person’s weight gain — that is, if a person’s weight is affected by the genes of a sibling.
Past studies found no role in body mass index (BMI) for genetic nurture between parents and children, but little is known about possible effects among siblings. Such effects would be conceivable considering that American siblings have a correlation in BMI of 0.53, the highest correlation when compared with smoking (0.355), drinking alcohol (0.350), frequency of TV watching (0.366), and frequency of exercise (0.209).
“A sibling may be a more powerful role model than parents for diet, physical activity, and weight,” the researchers wrote. “Siblings may spend more time playing with each other than with a parent and thus may have greater influence on each other’s physical activity.” They may also influence each other’s diet and appearance.
The researchers sought to investigate how much of a person’s BMI could be explained by the presence of BMI-related genetic variants in their brothers or sisters.
They used data from full siblings in the National Longitudinal Survey of Adolescent to Adult Health, a representative sample of American adolescents, 12–19 years old, who were followed into young adulthood. The most recent follow-up was conducted in 2008–2009 when the subjects were 25–34 years old.
This was the first study of its kind in the U.S., a country with a particularly “obesogenic” environment. According to recent estimates, 39.6% of U.S. adults (20 years and older) and 18.5% of youths (2–19 years old) are obese.
The main sample of siblings analyzed for this study included 2,546 observations on 914 unique individuals and 481 unique sibling pairs. In this sample, mean BMI was 26.74, and 26.6% of the subjects were obese.
For each individual, the scientists determined a polygenic score, which reflected the combined influence of 97 DNA regions known to be linked with BMI. But the analysis showed no evidence of a genetic nurture effect between siblings in weight.
This finding was consistent across two measures of weight — BMI and an indicator for obesity — and across numerous groups and different follow-up times.
“Thus, we find no evidence that the large, positive correlations in BMI between siblings are attributable to genetic nurture or peer effects between siblings,” the researchers said.
“The downside of this null result is that it also implies that successful weight reduction strategies may not have beneficial spillover effects within families, which, if they existed, would increase their cost‐effectiveness,” they concluded.