The current findings suggest that concurrent aerobic exercise and diet are associated with improvements in TC, TC
HDL-C, LDL-C, and TG, but not HDL-C in overweight and obese adults. Given the low baseline levels, the decreases observed for TG may be especially noteworthy. The lack of increase in HDL-C, despite an increase in aerobic capacity, may have been the result of decreases in total fat intake and weight loss. However, this would need to be tested in a large, well-designed, randomized controlled trial before any firm conclusions could be drawn.
Improvements remained robust when each study was deleted from the model once. In addition, results have been statistically significant since at least 2002. Furthermore, these findings appear to be more pronounced than those achieved with aerobic exercise or diet alone. For example, previous meta-analytic work addressing the independent effects of aerobic exercise on lipid and lipoprotein concentrations in overweight and obese adults reported statistically significant decreases in TG but not TC, HDL-C or LDL-C after sensitivity analyses were applied [36
]. Another meta-analysis that examined the effects of cholesterol-lowering diets on TC alone reported a statistically significant reduction of 5.7% [37
]. This compares to the approximate 8.2% reduction observed in the current investigation. However, this prior meta-analysis did not appear to be limited to overweight and obese adults and excluded trials aimed primarily at lowering body weight [37
The findings of the current investigation appear to be clinically important. Using data from previous research, the improvements in lipids and lipoproteins observed in the current meta-analysis would be equivalent to relative risk reductions in all-cause and coronary heart disease mortality of 5.7% and 8.0%, respectively, for TC and 2.8% and 5.0% for LDL-C [38
] while reductions in TC
HDL-C would equate to a an approximate 12.4% decrease in the relative risk of mortality from ischemic heart disease [39
]. For TG, the observed decreases would be equivalent to a relative risk reduction of 6.4% in coronary heart disease mortality [40
]. The decreases observed for LDL-C may be especially important given that LDL-C is currently the primary target of lipid-lowering therapy in adults [5
Although the overall results of the current meta-analysis suggest that a combined program of aerobic exercise and diet improves TC, TC
HDL-C, LDL, and TG, a moderate to large amount of heterogeneity and inconsistency was observed for all lipid and lipoprotein outcomes. Given these findings and despite the fact that a random-effects model that incorporates heterogeneity into the analysis was used, the generalization of results may not be appropriate [27
]. However, the use of such statistics to decide what comprises true heterogeneity and inconsistency is rather arbitrary in nature, and thus, should be viewed with caution [41
]. Another issue has to do with the fact that all prediction intervals for estimating the expected results of a new trial included zero for all lipid and lipoprotein outcomes. However, these values should not be confused with confidence intervals since prediction intervals are based on a random mean effect while confidence intervals are not [35
Meta-regression analyses resulted in several statistically significant associations, most notably, the association between decreases in bodyweight with reductions in TC, TC
HDL-C, LDL-C, and TG. However, while an important first step, all reported meta-regression analyses should be viewed with caution. For example, the investigative team was unable to conduct any type of multiple meta-regression analyses because of the small number of ESs as well as missing data for different variables from different studies, a common occurrence in meta-analysis. Consequently, the potential for confounding exists. In addition, because of the large number of statistical tests conducted, one or more of the statistically significant findings could have been due to chance. Given the former, the validity of the associations observed in the current meta-analysis would need to be tested in large, well-designed randomized controlled trials.
The significant changes observed for bodyweight and BMI as well as VO2max
suggest that a combined aerobic exercise and diet regimen results in improvements beyond those for lipid and lipoprotein concentrations in overweight and obese adults. This reinforces the investigative team's perspective that it is highly unlikely that any pharmacologic intervention will ever be developed that targets as many risk factors as a combined program of aerobic exercise and diet. In addition, aerobic exercise and diet may synergistically improve the effects pharmacologic therapies such as HMG-CoA reductase inhibitors (statins) [5
]. Finally, the significant reductions observed for kilocalories, total fat, saturated fat, and cholesterol suggest good adherence to the different diet regimens employed.
The small amount of heterogeneity and inconsistency as well as non-overlapping prediction intervals for BMI, kilocalories, and cholesterol intake provide greater credence and applicability of these outcomes with respect to the effects of aerobic exercise and diet in overweight and obese adults. For example, prediction intervals may be more relevant from a practical perspective since they provide an approximation of the expected treatment effect in a new trial [35
]. However, the representativeness of the results for all secondary outcome analyses may need to be interpreted with caution since they were only included if data for the primary outcomes (lipids and lipoproteins) were available.
The overall reporting and conduct of randomized controlled trials on this topic could be improved. First, information on study design characteristics, including allocation concealment, incomplete data (dropouts, reasons for dropping out, adverse events), and incomplete outcome reporting should be provided. For incomplete outcome reporting, the inclusion of the study identification protocol number would be especially helpful to readers. In addition, since part of the intervention includes exercise, only those participants not engaged in a regular exercise program prior to enrollment should be included since prior exercise may diminish the effects of the intervention. Furthermore, all of the studies used a per-protocol approach in the analysis of their data. Given the former, the investigative team suggests that future studies report data using both the per-protocol and intention-to-treat approach. Second, the reporting of selected participant characteristics could be improved. This includes data on race/ethnicity, medication use before and during the intervention, cigarette smoking, and alcohol consumption, as well as any changes in physical activity, void of the exercise intervention, which occurred during the study. Third, in order to examine dose-response effects, additional information regarding the aerobic exercise and diet intervention should be provided. For aerobic exercise, data on the duration and intensity of the intervention as well as compliance to the exercise protocol should be provided. In addition, the provision of data on energy expenditure is also recommended. With respect to the diet intervention, complete data should be provided on the intake of kilocalories, protein, total fat, saturated fat, transfat, and cholesterol. Fourth, additional lipid assessment data that includes the number of hours in which subjects refrained from exercise prior to lipid testing, season(s) in which lipids were assessed and whether assessment of LDL-C occurred using the direct or indirect method should be included. Fifth, since non-HDL-C has been shown to be a better predictor of cardiovascular morbidity and mortality than LDL-C [42
], the calculation of non-HDL-C from TC and HDL-C, including dispersion statistics is recommended. Given their potential association with changes in lipid and lipoprotein concentrations, data on percent body fat as well as lean body mass would also be helpful.
Finally, the primary purpose of this important meta-analysis was to focus on the combined
versus independent effects of aerobic exercise and diet on lipids and lipoproteins in overweight and obese adults. The rationale for this approach was based on the fact that both are recommended in tandem for improving the lipid-lipoprotein profile of adults [5
]. Given this study design, we were unable to determine the independent effects of each on the lipid-lipoprotein profile. Therefore, future meta-analytic research should examine the independent effects of aerobic exercise and diet for improving the lipid-lipoprotein profile of overweight and obese adults.
In conclusion, the overall results of this study suggest that a combined program of aerobic exercise and diet is associated with improvements in TC, LDL-C, TC
HDL-C and TG, but Not HDL-C, in overweight and obese adults. However, additional, well-designed randomized controlled trials on this topic are needed.