A major objective of this study was to examine how changes in physical activity over a 16-week period are associated with changes in adiposity in overweight Hispanic adolescents. The primary findings are that a short-term increase in objectively-measured total physical activity is significantly associated with a decrease in both total fat mass and percent body fat. Specifically, an increase of 28% of total physical activity, or 100 cpm, was associated with a decrease of 1.4 kg fat mass and 1% body fat after controlling for energy intake and standard covariates. To translate the accelerometry unit of counts per minute (cpm) into more physiologically relevant terms, prediction equations based on previous observations in normal weight adolescents (6
) were used to estimate that an increase of 100 cpm is broadly similar to an increase of 250 kcal of energy expenditure.
A secondary objective was to examine energy intake as a confounder of the relationship between changes in physical activity and adiposity. Recent studies in adolescents state that adjusting for energy intake may strengthen the observed relationships between objectively-measured physical activity and adiposity (6
). The current study supports these assertions, finding that accounting for changes in energy intake strengthened the relationship between changes in physical activity and adiposity. Change in energy intake was identified as a confounder in the relationship between total physical activity and percent body fat, because adjusting for change in energy intake increased the coefficient estimate by 25%, which was greater than the 10% change in the coefficient needed to identify it as a confounder. Change in energy intake was not identified as a confounder in the relationship between total physical activity and total fat mass, because adjusting for change in energy intake only changed the coefficient estimate by 8%. The 10% change in coefficient to define a confounder is a rather arbitrary rule of thumb and is not based on a statistical test (14
). Given that accounting for energy intake strengthened the relationship between changes in physical activity and adiposity, future studies attempting to quantify the magnitude of this relationship may consider whether to account for energy intake.
Accounting for the influence of energy intake had a modest effect on the relationship between physical activity and body fat. When the change in energy intake was accounted for, the decrease in fat mass that was associated with an increase of 100 cpm of total physical activity changed by 0.1 kg (1.3 kg to 1.4 kg), and the decrease in percent body fat changed by 0.2% (0.8% to 1.0%). The modest change observed when accounting for change in reported energy intake may be partly explained by the large variation in the change in energy intake from pre-test to post-test (from a decrease of −1409 kcal to an increase of 2048 kcal). This variation may be due to the self-report nature of the dietary measure. The variation could also be a reflection of a fasting/gorging cycle of eating that may be practiced by the adolescents in this sample. No studies describing the eating behaviors of overweight Hispanic adolescents were identified. Future studies that assess diet over a longer measurement period (7 to 14 days) may help to identify the dietary patterns of overweight Hispanic adolescents.
An increase in the percent of time spent in MVPA was not associated with a decrease in either percent body fat or total fat mass after accounting for total physical activity. These findings suggest that increases in total physical activity may be sufficient to result in improvements in body composition in overweight Hispanic adolescents. It should be noted that these findings do not suggest that increases in MVPA will not lead to reductions in fat mass, but MVPA may be difficult for and not well tolerated by this population. As such, it is important for future randomized trials to determine whether increases in total physical, independent of MVPA, can reduce adiposity in overweight Hispanic adolescents.
There is no clear consensus on whether the intensity level of physical activity influences changes in adiposity, especially in overweight adolescents. Some longitudinal studies that report a relationship between objectively-measured MVPA and body fat have accounted for total physical activity (6
), while others have not (35
). Therefore it remains unclear whether the reductions in body fat observed in these studies were associated with MVPA independent of total physical activity. In a large cohort of 12-year olds, time spent in MVPA was negatively associated with fat mass after adjusting for total physical activity (22
), suggesting that the physiological effects of MVPA may be importantly related to adiposity. Future longitudinal studies examining the relationship between MVPA and adiposity should control for total physical activity to determine whether decreases in adiposity are related specifically to an increase in MVPA or whether the effects are due simply to an increase in total physical activity. This has important public health implications given that the barriers to increasing MVPA may be different than the barriers to increasing total daily physical activity, especially in overweight youth (43
It is also possible that a significant relationship between changes in percent of time spent in MVPA and adiposity was not detected because of the cut-points used to designate time spent in MVPA. There is no agreement on which cut-points should be used in different populations, and using cut-points derived from different prediction equations can yield markedly different results (13
). The prediction equations, also, introduce residual error into the measurement, and thus counts per minute may be a more valid measure of activity than intensity level (8
). It is unlikely, though, that the MVPA cut-point used in the current study contributed to the failure to detect a relationship between changes in MVPA and adiposity independent of total activity, because when cut-points from a different prediction equation were used to designate MVPA (8
), the results were replicated.
In a prospective four year study with older adolescents, Ekelund et al. (6
) also reported that change in total activity, not MVPA, was related to change in adiposity, though they only reported these results in normal weight participants. The authors postulate that a failure to detect an association in overweight adolescents may be due to the lack of change in activity in this group. In the current study, we also reported a lack of overall change, but there was a nicely distributed variability in change in physical activity at the individual level. One reason for the apparently discrepant findings may be that the distribution of change in activity was differently dispersed in Ekelund et al., making it difficult to identify a relationship. It is difficult to compare results from these studies, though, due to differences in the populations studied, the length of the follow-up period, and the different covariates, such as dietary intake, included in the analyses.
Some longitudinal studies using body mass index to measure adiposity have failed to find a relationship with physical activity (35
). One strength of the current study was the ability to examine the relationship with objective measures of physical activity and adiposity, e.g., activity by accelerometry (29
) and total fat mass and percent body fat measured by DEXA (10
). Another strength of the current study is the homogeneity of the demographic characteristics (e.g., ethnicity, age, tanner stage, overweight status) of the sample of the participants. This homogeneity helps to reduce the effects of maturation bias, though this also restricts the generalizability of the findings to those who are similar to the study participants.
Our findings are supported by cross-sectional studies that have found a significant association between physical activity and adiposity (30
). Coupled with cross-sectional and longitudinal studies, the current analysis lends support to a causal inverse association between changes in physical activity and adiposity, but caution should be used when interpreting these findings as definitive evidence of causation.
A potential limitation to this study is the fact that accelerometry is not a perfect measure of physical activity, because it is worn on the hip and does not accurately detect bicycling or upper body movement, such as weight lifting. Similarly, the accelerometer cannot be worn during water-based activities, such as swimming. A recent large cohort study reported that swimming is not a common activity among Hispanic adolescents (1
), so this limitation may not be of concern in the current sample. The definition of valid accelerometry data used in the current study, 2 days and 6 hours, is another potential limitation, though the average measurement period was approximately 70 hours. The self-report nature of the dietary data may also be a limitation, especially given the overweight status of the sample and the likelihood of overweight participants to underreport their dietary intake (25
). Another possible limitation of the study is the fact that dietary intake was not examined as either an effect modifier or a mediator of the relationship between physical activity and adiposity. The authors acknowledge the probability that energy intake may act as a mediator and/or effect modifier, but that investigation is outside the scope of the current study.
To our knowledge, this is the first study to examine the relationship between short-term changes in objectively-measured activity by accelerometry and adiposity and to examine the influence of energy intake on this relationship in overweight Hispanic adolescents. In summary, after accounting for changes in energy intake an increase of 28% of total physical, or roughly 250 kcal, was associated with a modest, yet significant, decrease in 1.4 kg of fat mass and 1% percent body fat in overweight Hispanic adolescents over 16-weeks, while there was no association between change in time spent in MVPA and adiposity independent of total physical activity.