Abstract and Introduction
Purpose: High levels of cardiorespiratory fitness (CRF) may attenuate the association between the excessive adiposity and the risks of cardiovascular and metabolic disease. The purpose of this study was to stratify children according to their body mass index (BMI) and adiposity (body fat percentage [BF%]) and to compare levels of CRF across subgroups.
Methods: This prospective cohort study comprises a cross-sectional and longitudinal analyses of data collected at baseline (n = 641) and 2 yr later (n = 579) on children (7.4–11.6 yr) attending public school in Denmark. Levels of CRF were measured using the Andersen test, whereas BF% was measured by dual-energy x-ray absorptiometry.
Results: There were 560 children (87.4%) classified as normal weight according to BMI at baseline, of which 46 (7.4%) were identified as having excessive BF%. These children had significantly lower CRF (mean [95% confidence interval]: -63.1 m [-100.2 to -25.9]) than children with normal BMI and normal BF%, and the effect of BF% on CRF was significantly worse in boys than girls. Overweight children with high BF% had significantly lower prospective (2 yr) CRF levels (-34.4 m [-58.0 to -10.7]) than children with normal BMI and BF%. However, children who improved their BMI and/or BF% classification during the 2-yr period achieved CRF levels (8.9 m [-30.2 to 47.9]), which were comparable with children with normal BMI and BF% at both measurement time points.
Conclusion: The CRF levels in children are affected by BMI and BF%, although BF% appears to play a greater role. This association between BF% and CRF is sex dependent, with CRF levels in boys being affected to a greater extent by BF%. Children identified as "normal weight" by BMI but presenting with excessive BF% had significantly lower CRF than "normal weight" children with low BF%.
Global estimates using objectively measured physical activity data in children (4–11 yr old) indicate they perform between 22 min (95% confidence interval [CI] = 19.9–24.1) and 45 min (95% CI = 39.6–50.4) of moderate-to-vigorous physical activity per day, which falls well below the recommended 60 min of daily moderate-to-vigorous physical activity. The low levels of physical activity in children correspond with a high prevalence of overweight and obesity[6,20,23] as well as low levels of cardiorespiratory fitness (CRF).[23,24] This is alarming considering both low CRF and high adiposity are associated with increased cardiovascular and metabolic disease risk.[4,8,9,11,15,16,18,27]
Although the association between low CRF and adiposity is well recognized,[21,25–27,29] the magnitude of this association remains equivocal. This is due in part to the differences in the techniques used to measure adiposity, which range from the direct measurement of body fat using dual-energy x-ray absorptiometry (DXA) to the adoption of a combination of anthropometric techniques (i.e., height and weight; waist circumference; skinfolds). Of these techniques, body mass index (BMI) or BMI z-score are most commonly used as the outcome measure. This is despite the growing number of studies reporting substantial variance in the adiposity of children within the given BMI categories, with a particular concern being the concealment of excessive adiposity in children categorized as "normal" by BMI (the so-called thin–fat phenotype). This is a concern because total body fat percentage (BF%) has been identified as a stronger predictor of composite and single cardiovascular risk factors than either BMI or waist circumference in children.
The purpose of the present study therefore was to stratify children according to BMI and DXA-derived adiposity and to identify differences in the children's CRF. We hypothesized that children identified as being of "normal weight" (BMI) and low BF% would have the highest CRF, whereas those with high BF%—irrespective of their BMI—would have the lowest CRF. Children were then tested 2 yr later to explore the effect of an increase (considered detrimental; increasing adiposity or shifting into the overweight/obese BMI category) or decrease (considered beneficial; decreasing adiposity or shifting into the normal weight BMI category) in weight status on their CRF. We hypothesized that children who demonstrated a beneficial shift in their weight status would demonstrate similar CRF to those children who were constantly "normal weight" and "low adiposity," but that these children would have significantly higher CRF than those who maintained a high adiposity at each time point.
Med Sci Sports Exerc. 2016;48(9):1708-1714. © 2016 American College of Sports Medicine