The aim of this study was to examine whether adiposity level is associated with subsequent PA level in childhood (the reverse causation hypothesis). This study found that, in continuous data analysis, BF% at age 8 was negatively associated with IW-MVPA at age 11 in both boys and girls. Categorical data analysis also showed that boys and girls with high BF% at age 8 were more likely to have low PA levels at age 11 than those with lower BF% age 8. These findings are consistent with those of five adult cohort studies (3
), where obesity was a significant predictor of PA level later in life. However, the current study results are inconsistent with Sallis et al.
), who reported no association between skinfold thickness category at baseline and total activity accelerometer counts measured for one day at a 20-month follow-up among 732 children who were fourth graders at baseline.
In additional analysis, we examined the association between early adiposity and later PA using other PA indicators. BF% at age 8 was significantly negatively associated with total activity (the daily sum of ≥ 100 accelerometer counts per minutes) at age 11 in both boys and girls (P < 0.05). When time spent in MVPA (Time MVPA) was used as a PA indicator, the negative association between Time MVPA at age 11 and BF% at age 8 was significant among girls (P < 0.05) and suggestive among boys (P < 0.10). Considering that there was a significant negative association between BF% at age 8 and IW-MVPA at age 11 among boys, these results may imply that boys with low BF% are more likely to engage in higher intensity PA than those with high BF%. From an energy expenditure perspective, IW-MVPA was expected to better represent PA level than Time MVPA in establishing a relationship between adiposity and PA. PA intensity may be critical particularly for boys. However, researchers should be cautious because the use of IW-MVPA may amplify measurement error, which is derived from the difference of relative intensities between individuals when absolute intensity is given.
The current study findings indicate that adiposity status may be a determinant of PA behavior in childhood. Godin et al.
) suggested a theoretical model of the reverse causation hypothesis based on Ajzen’s Theory of Planned Behavior (TPB) (1
). According to this model, adiposity may impact PA behaviors by influencing cognition such as intention (motivation) and perceived behavioral control (ease or difficulty in engaging in the behavior, e.g., social barriers). Although we assumed that obesity-related psychological, societal, and physical functioning may negatively influence PA participation, we were not able to examine whether these are mediating factors in an association between early adiposity and subsequent PA because those potentially mediating variables were not assessed in the Iowa Bone Development Study.
The study results suggest at least two significant points in terms of public health implications. First, this study suggests that a new perspective is needed to best develop intervention strategies to promote PA and to prevent obesity in children. This study suggests weight status-specific intervention strategies for PA promotion, since overweight children may have different barriers to PA participation. Given the childhood obesity epidemic, it would be critical to identify specific barriers for overweight children and develop strategies to overcome these barriers. Second, the study results support PA promotion interventions from an early age, before excess fat is accumulated. Once excess fat is accumulated in early childhood possibly due to a low level of PA, it may lead to low PA participation. In turn, lack of PA may exacerbate fat accumulation. This assumption is supported by Valerio’s study (27
) showing that overweight and obese children 7 years of age had a higher BMI increase at three-year follow-up than normal-weight children. PA interventions from an early age are recommended to prevent excess fat accumulation throughout childhood and later in life.
Several limitations of this study should be acknowledged. We only included data from those who completed all three examinations. Loss to follow-up may have caused selection bias; but, IW-MVPA and BF% levels at age 8 were not significantly different between those who completed all three examinations and those who did not. The study sample was not randomly selected from Iowa Fluoride Study participants; which could also have led to selection bias. Therefore, an association between early adiposity and subsequent PA observed in this sample may not represent that in the general child population, and caution should be taken in generalizing the results. In the participant cohort, approximately 95% were white, which is a lower risk population for childhood obesity than the Hispanic or African American population. However, homogeneity of ethnicity and living-environment can be an advantage because unknown confounders are less likely to exist. Genetic predisposition was not considered. This observational study cannot eliminate error introduced by residual and unmeasured confounding factors.
Nonetheless, to our knowledge, this study is the first prospective cohort study in a fairly large childhood sample to explicitly examine the reverse causation hypothesis. The use of objective and accurate measures for both PA and adiposity helped reduce measurement error and increase internal validity. Examinations at three time points allowed accounting for the preceding changes in adiposity and PA between the first two examinations.
In conclusion, this study showed that children with low adiposity were more likely to be active at three-year follow-up than their counterparts with high adiposity. Adiposity may be a determinant of PA behavior in childhood. Regarding future research, more evidence should be accumulated to support the reverse causation hypothesis in childhood. Research is required to understand the mechanism underlying the effect of adiposity status on PA behaviors. It would be valuable to test the hypothesis that obesity-related psychological, societal, and physical functioning are mediating factors in an association between early adiposity and subsequent PA, using existing datasets containing PA, adiposity, and related psycho-societal measurement data.