In the United States, the transition from adolescence to young adulthood is accompanied by increases in time spent in sedentary behaviors, such as screen time [11
]. In this study, we identified important gene-by-sedentary behavior interactions on BMI change. In particular, we found evidence that genetic factors influencing BMI change were differentially expressed in individuals with high levels of screen time, compared to those individuals with low levels of screen time. These findings are important because the etiology of body mass change is complex with both genetic and environmental components, but with limited research that has tackled this complex relationship.
An increasing number of studies illustrate that genes as well as environmental factors are important in the etiology of BMI and BMI change. Many studies have examined the relationships between physical activity, genetic susceptibility, and obesity related measures [16
]. However, the relationship between sedentary behaviors, such as screen time, and genetic susceptibility has been largely unexplored [32
] and may be particularly crucial to understanding the increase in BMI with age as adolescents transition to young adulthood.
We found evidence that the variation in the genetic component of BMI change from adolescence to young adulthood was different among individuals with low versus high levels of baseline (adolescent) screen time. Among adolescents with low screen time (7.86 ± 3.75 hours/week), the additive influence from genes was larger than for the adolescents that reported high screen time (36.49 ± 24.01 hours/week). From these results we infer that behavioral factors can modify the genetic effects on BMI change. The genetic component of variance is larger for individuals with low versus high baseline (adolescent) screen time and the residual environmental component of variance is smaller for individuals with low versus high baseline (adolescent) screen time. Thus, for adolescents that often invest extensive time watching TV or videos, the time spent in sedentary pursuits has a substantial influence on body mass seemingly reducing the importance of genetic susceptibility. In conjunction with previous findings in young adult twins demonstrating that physical activity reduced the influence of genetic factors on the development of obesity [16
], our results further suggest that genetic susceptibility for BMI gain is also lowest in individuals that are extremely sedentary. Thus, behaviors (be it those individuals that were apparently extremely sedentary or in other studies those persons that show extreme levels of physical activity), demonstrate a modification of the underlying genetic susceptibility for BMI gain (in both cases demonstrating lower heritabilities than their comparison groups).
Our findings illustrate that there may be a complex interplay between genes and environment affecting body mass change between the period of adolescence and young adulthood. We know little about how changes in BMI are affected by individual susceptibility to environmental contexts and individual behaviors, such as screen time. The transitional period between adolescence and young adulthood is a particularly important time to investigate such relationships, as it is a major lifecycle period of risk for weight gain. From the genetics perspective, studying periods of rapid weight gain provides focus on the maximal expression of phenotypes of interest [33
]. From the environment perspective, it is well known that periods of rapid growth are also periods of heightened sensitivity to environmental inputs [34
]. Clearly more research is needed on the dynamic and complex relationship between genes and environment over lifecycle periods of risk for weight gain.
We acknowledge important limitations to our study. First, it is limited by the self-reported information on sedentary behaviors which were obtained in an interview. However, questionnaires are the most feasible methodology for assessing such behaviors in a large, population-based study and have been shown to have moderate-high reliability [37
]. Such questionnaires combining television viewing and computer game use fared best in terms of validity, but also showed television viewing to be under-reported [38
]. Second, we used self-reported weight for individuals who exceeded scale capacity (Wave II, n
=54; III, n
= 155). In our sample, even in cases where obese individuals underestimated their weight, their self-report weights still put them in the obese category. Even if they were within the upper limits of the scale, their BMI values would still be within the tails of the distribution. Self reported values have been shown to correctly classify a large proportion of the Add Health sample [10
Third, heritability estimates obtained from sib-based data sets could be inflated because such data do not account for either shared or common environmental factors or dominance effects. In addition, it is important to note that this is a quantitative genetic analysis and no specific gene or set of genes were investigated.
In summary, our analyses suggest that behavioral factors (i.e. sedentary behavior) can modify the additive genetic effects on BMI change between adolescence and early adulthood. The genetic component of variance was smaller in those individuals that reported extreme levels of sedentary behavior in comparison to those that were not as sedentary. Thus, for adolescents who spend a considerable amount of time watching TV or videos, the time spent in sedentary pursuits may have a substantial influence on the expression of their inherit underlying genetic susceptibility. Our results could improve current understanding of gene-environment interaction and BMI change. In our case, we observed different influences from genes based on level of screen time. Our findings reinforce the importance of obesogenic behaviors, like screen time, in public health efforts to reduce weight gain.