Our overall results suggest that aerobic exercise improves TC, HDL-C and TG with a trend for statistically significant reductions in LDL-C among men. Although the reductions in TG (9%) were high relative to the improvements for TC (2%) and HDL-C (3%), the small changes observed for the latter two might be important. For example, a reduction of 1% in TC has been shown to reduce the risk for coronary artery disease (CAD) by 2% [81
]. For HDL-C, as little as a 1% decrease in HDL-C has been associated with a 2–3% increase in the risk for coronary heart disease (CHD) [82
]. Assuming that the reverse is true, the approximate 3% increase observed in our meta-analysis should decrease CHD risk by 6–9%. For TG, previous research has shown that elevated levels are an independent risk factor for CHD [83
Lowering LDL-C is the primary target of lipid-lowering therapy [3
]. Although we did not find a statistically significant reduction in LDL-C as a result of aerobic exercise, there was a trend for statistical significance (p
= 0.08). The 2% reduction in LDL-C observed in our meta-analysis may also be important because it has been shown that a 1% reduction in LDL-C reduces the risk of major coronary events by approximately 2% [84
Although our results are encouraging, it appears that aerobic exercise alone might not be sufficient for bringing lipid and lipoprotein levels to recommended levels for many of those with less than optimal levels [3
]. Therefore, it would be plausible to suggest that in addition to aerobic exercise [85
], additional lifestyle (for example, prudent diet) and/or pharmacologic (for example, statins) interventions are necessary for optimizing lipid and lipoprotein levels in adult men.
The large amount of statistical heterogeneity observed in our results might partly be explained by the results of our subgroup and meta-regression analyses. For TC, greater decreases were found for unpublished versus published studies. These findings are antagonistic to the concept of publication bias where larger and more statistically significant reductions should be found among published versus unpublished studies. Greater decreases in TC were also found for studies in which some of the subjects consumed alcohol as well as studies in which none of the subjects were diabetic. For HDL-C, the statistically significant association between greater increases in HDL-C and initial body composition, especially percent body fat, suggests that those with poorer initial body composition profiles experience greater increases in HDL-C. In addition, the statistically significant association between greater increases in HDL-C with lower initial levels of HDL-C suggests that those with lower levels of HDL-C have the most to gain from an aerobic exercise program. Furthermore, the association and subgroup differences between changes in HDL-C with older age as well as initial maximum oxygen consumption suggests that older people and/or those with lower initial levels of maximum oxygen consumption experience the greatest increases in HDL-C. For LDL-C, some of the heterogeneity may be explained by the statistically significant association between higher initial levels of LDL-C and greater reductions in LDL-C. Finally, the heterogeneity observed for TG may be partly explained by the association between greater reductions in TG and higher initial levels of TG as well as decreases in percent body fat. Overall, it appears that some of the heterogeneity observed for changes in lipid and lipoprotein levels in adult men may be explained primarily by baseline lipid and lipoprotein levels as well as body composition.
Meta-analysis, like any type of review, is limited by the availability of data. For example, we were unable to conduct any type of multiple regression analysis because of missing data for different variables from different studies and our preference to include the maximum about of data possible for each analysis. Because of this approach, it is possible that some of the predictor (independent) variables (for example, initial body weight and initial percent body fat) were related to each other. In addition, our subgroup (ANOVA) analyses for changes in TC were limited to two outcomes for the “no” category for alcohol and two outcomes for the category of “some” for diabetes. Finally, our subgroup analyses for drugs that could affect lipids and lipoproteins, cigarette smoking, alcohol consumption, previous physical activity, and diabetes were partitioned according to “no” versus “some” as opposed to “no” versus “yes.”
The fact that we performed a large number of simple regression and ANOVA tests raises the possibility that one or more of our statistically significant findings could be due to nothing more than the play of chance. However, as pointed out by Rothman [86
], limiting observations may miss important findings.
Based on our current meta-analytic work, we would suggest that future studies dealing with the effects of aerobic exercise on lipids and lipoproteins in men include complete data on study characteristics (method of randomization, dropout information), subject characteristics (race/ethnicity, drugs that could affect lipids and lipoproteins, cigarette smoking, alcohol consumption, diet, previous physical activity habits), lipid assessment characteristics (instrumentation, timing of post-exercise blood draws), and training program characteristics (compliance to the exercise protocol). The timing of post-exercise blood draws, for example, is vital during lipid and lipoprotein assessment. In addition, future studies should conduct and report power analysis data to ensure that adequate sample sizes are included to answer their research question(s). Furthermore, a major research question that we believe needs to be addressed in future studies is the quantification beyond general recommendations regarding the dose-response relation between aerobic exercise and predicted changes in TC, HDL-C, LDL-C, and TG in men. Answering this research question is critical in determining treatment recommendations on a patient-level basis.
In conclusion, the results of our study suggest that aerobic exercise reduces TC and TG and increases HDL-C in adult men 18 years of age and older.