The overall results of this study suggest that short-term aerobic exercise does not improve TC, HDL-C, and LDL-C in children and adolescents (all 95% CIs crossed zero). However, there was a trend for statistically significant reductions in TG which were independent of changes in body weight and percent body fat. Similar results were found when we limited our analysis to studies in which all subjects had Type 1 diabetes [5
]. Reductions in TG did reach statistical significance when three studies were independently deleted from the model [3
] as well as when studies were limited to overweight and obese subjects [8
]. The antagonistic results for TG in relation to the Ferguson et al. study [8
] most likely reflect the differential weighting of this study across the two different types of analyses. In addition to our TG findings, there was also a trend for statistically significant increases in HDL-C in overweight and obese subjects. The statistically significant reductions in TG among overweight and obese children and adolescents may be especially important given the increasing prevalence of overweight and obesity among children, adolescents, and adults [29
], as well as the fact that elevated TG levels are an independent risk factor for coronary heart disease in adults [31
]. The former notwithstanding, our analysis was limited to only two studies in which all children and adolescents were overweight or obese [8
]. Given the above, it would appear plausible to suggest that a need exists for additional randomized controlled trials in this population.
We found several potentially important relationships in this study. These included (1) an increase in HDL-C with lower initial levels of HDL-C, (2) greater decreases in LDL-C with older age, and (3) greater decreases in LDL-C with higher levels of training intensity. While these results are interesting, they should be considered exploratory in nature. Furthermore, these findings, like any correlation that deviates from 1, could be a statistical (regression toward the mean) versus physiological phenomenon [32
With the exception of changes in TG, our findings for TC, HDL-C, and LDL-C were generally unimpressive. Overall, our findings are consistent with previous narrative reviews on this topic. For example, Linder and Durant concluded that intervention studies have failed to demonstrate that aerobic exercise alters lipid and lipoprotein levels in children and adolescents [33
] while Haskell concluded that most aerobic training studies in children and adolescents did not result in any changes in lipids and lipoproteins [34
]. In addition, Vaccaro and Mahon [35
] as well as Despres et al. [36
] concluded that because of conflicting information from intervention studies, no recommendation could be made regarding the feasibility of exercise for improving lipids and lipoproteins in children and adolescents. More recently, Tolfrey et al. concluded that the majority of intervention studies have suggested that regular aerobic exercise has little, if any, influence on the lipid and lipoprotein levels of children and adolescents [37
]. The strength of our study over previous narrative reviews was the use of a more quantitative approach (meta-analysis) in order to provide more definitive information regarding the magnitude of change in selected lipids and lipoproteins as a result of aerobic exercise in children and adolescents as well as an examination of potential associations between changes in lipids and lipoproteins and selected characteristics. The strength of our meta-analysis over the individual studies themselves provided the opportunity to increase estimates of treatment effectiveness as well as power for our primary endpoints [38
]. For example, since TG have high day-to-day variability, the added power of our meta-analysis may have uncovered the important relationship that we observed between aerobic exercise and changes in TG.
While our overall changes in lipids and lipoproteins among children and adolescents were generally modest, aerobic exercise should almost always be recommended because of the numerous other benefits that can be derived from such. For example, the fact that we found statistically significant decreases in percent body fat as well as increases in VO2max
in ml kg−1
lends support for the importance of aerobic exercise for the overall health of children and adolescents. In addition, participation in aerobic exercise improves overall cardiovascular risk and may prevent risk factor acquisition over time [39
]. More specifically, the benefits of aerobic exercise may be the result of its independent protective effect on event rates and inflammatory markers as well as its usefulness in the prevention of the multiple risk phenotype or metabolic syndrome and hypertension. These positive benefits should be obtainable by adhering to the recent physical activity guidelines for children and adolescents as set forth by the United States Department of Health and Human Services and the United States Department of Agriculture [40
]. This includes participation in at least 60 min of moderate intensity physical activity on most days of the week, preferably daily.
While the results of our meta-analysis provide important information, they need to be considered with respect to the following limitations. First, a common problem with all reviews, meta-analytic or otherwise, is missing data. For example, we were unable to conduct any type of multiple regression analysis because of missing data for different variables from different studies. Consequently, we chose to conduct a number of simple regression tests in order to maximize the amount of data that was available. However, as a result of this approach, we were unable to control for potential confounders such as the potential for differential changes in dietary behavior between exercise and control groups. In addition, we increased our risk for conducting a Type I error despite using a more stringent confidence level (99%).
Less than half the studies (46%) reported data on compliance of the subjects to the exercise protocol. Since this could affect changes in lipids and lipoproteins, it is strongly suggested that future studies include this information. In addition, only four studies reported that subjects refrained from exercise for at least 24 h prior to the assessment of lipids [3
]. Since this may affect lipid and lipoprotein levels, it would appear plausible to suggest that future studies have subjects refrain from exercise for at least 24 h prior to lipid assessment and that the number of hours that exercise is avoided be reported.
Ideally, many of the limitations of our current meta-analysis could be overcome by conducting an individual-patient data (IPD) versus summary means meta-analysis. However, this must be countered with the difficulty in obtaining IPD from investigators. For example, in our previous meta-analytic research dealing with the effects of exercise on bone mineral density in adults we were only able to obtain IPD from approximately 29 of 76 (38%) eligible studies [41
In conclusion, the results of our study suggest that aerobic exercise decreases TG in overweight and obese children and adolescents. In addition, decreases in LDL-C are associated with increased training intensity and older age while increases in HDL-C are associated with lower initial HDL-C. However, these associations need to be tested further in large, well-designed, randomized controlled trials.