The primary purpose of this study was to use the meta-analytic approach to examine the effects of aerobic exercise on HDL2
-C in adults. Our findings support the efficacy of aerobic exercise for increasing HDL2
-C in adults. While there were statistically significant decreases in body composition (body weight, BMI, percent body fat), none of these changes were significantly correlated with changes in HDL2
-C. Thus, it appears that increases in HDL2
-C as a result of aerobic exercise occur independent of changes in body weight. These findings are important because of the general belief that HDL2
-C provides greater protection against CHD than HDL3
]. In addition, the fact that we found statistically significant increases in HDL2
-C but not HDL-C suggests that cardioprotective benefits can occur independent of changes in HDL-C. This is noteworthy because in the traditional clinical setting, the assessment of HDL-C is readily available while the assessment of HDL-C subfractions is not [2
]. Consequently, the lack of an increase in HDL-C in the clinical setting may not be indicative of a lack of cardioprotection within this lipoprotein group.
The statistically significant increase in maximum oxygen consumption in ml/kg/min observed in our study suggests that the aerobic exercise programs were successful at improving cardiorespiratory fitness. Alternatively, the lack of a statistically significant change in HDL3-C suggests that aerobic exercise does not significantly change the level of this HDL-C subfraction.
An unexpected finding of this study was that increases in HDL2-C were greater in studies conducted in countries other than the United States. However, whether this reflects a true difference or is the result of some other potentially confounding factor(s) is not known. Furthermore, since we collapsed all countries other than the United States into one category because of sample size issues, caution is warranted in the interpretation of this finding.
The lack of a statistically significant relationship between increases in HDL2
-C and any of the training program characteristics is not surprising given the fact that the vast majority of studies appeared to follow the general guidelines for exercise prescription as recommended by the American College of Sports Medicine [41
]. Given such, it would seem appropriate to suggest that adherence to these guidelines would bring about the increases in HDL2
-C observed in our study. Briefly, this includes aerobic activities such as walking, jogging, cycling, or swimming, performed 3–5 days per week for 20–60 min at an intensity equivalent to 55–90% of maximum heart rate [41
]. Training at a lower versus higher intensity level may be preferable because of the potential for increased compliance as well as a reduced risk for injury [41
Meta-analysis, like any type of review, is limited by the available evidence. With the former in mind, it is important that our results be viewed with regard to the following issues. First, it is important to not try and generalize our findings beyond the characteristics of the included studies. For example, it would probably be inappropriate to assume that the findings of our investigation in adults would be similar in children and adolescents. Second, the fact that we conducted multiple tests, for example, simple regression, because of missing data for missing variables, may have increased our risk for a Type I error. However, Rothman has clearly pointed out that while adjustment for multiple comparisons assumes that “chance” is the first-order explanation for one's observations, such an assumption is antagonistic to the basic principle of research in which nature follows regular laws that can be studied through observations [38
]. Furthermore, limiting the number of tests conducted in order to reduce the risk for a Type I error limits one from exploring potentially important findings.
Missing data is a common occurrence with any meta-analytic investigation. For our investigation, less than half the studies reported data on race/ethnicity (26%), alcohol consumption (42%), and compliance to the exercise protocol (11%). Since these could influence lipid and lipoprotein outcomes, it is suggested that future studies report, and editors publish, complete information for these variables. In addition to missing data, only one study each reported that all subjects were either hyperlipidemic [12
], had cardiovascular disease [5
], or were overweight [22
]. Since subjects with these conditions may have the most to gain in relation to increasing HDL2
-C levels as a result of aerobic exercise, it would seem plausible to suggest that future studies limit inclusion of subjects to those with one or more of these conditions. Finally, there was a lack of complete reporting regarding the specific method used for the determination of HDL2
-C. Since this could potentially alter the outcome, it is suggested that future studies provide more complete information for this. In conclusion, the results of our study suggest that aerobic exercise increases HDL2
-C in adults.