Review ArticleEffects of aerobic exercise on lipids and lipoproteins in adults with type 2 diabetes: A meta-analysis of randomized-controlled trials
Introduction
Cardiovascular disease (CVD), the number one cause of mortality in the USA,1 is almost twice as common in individuals with diabetes.2 A significant risk factor for cardiovascular disease is less than optimal lipid and lipoprotein levels.3 Aerobic exercise, a low-cost therapeutic lifestyle change that is available to most of the general public, has been recommended for improving lipid and lipoprotein levels in adults, including those with diabetes.3 Unfortunately, previous randomized-controlled trials investigating the effects of aerobic exercise on lipids and lipoproteins in adults with Type 2 diabetes, the most common form of diabetes, have led to less than overwhelming results.4, 5, 6, 7, 8, 9, 10 For example, none of the results for total cholesterol from the aforementioned randomized-controlled trials were reported by the authors as statistically significant, whereas 29% of high-density lipoprotein cholesterol (HDL-C) outcomes, 14% of low-density lipoprotein cholesterol (LDL-C) outcomes, 29% of total cholesterol (TC)/HDL-C outcomes, and 29% of triglyceride (TG) outcomes were reported as statistically significant. One of the possible reasons for the lack of statistically significant findings may have to do with the small sample sizes included in these studies. Meta-analysis is a quantitative approach for pooling the results of studies in an attempt to arrive at an overall conclusion regarding a body of evidence. It is especially useful when the number of studies is small, the number of subjects that can be enrolled in the studies is small, or both.11 This is the case with the randomized-controlled trials that have examined the effects of aerobic exercise on lipids and lipoproteins in adults with Type 2 diabetes. In addition, we, as well as others12 are not aware of any meta-analytic research that has focused solely on the effects of aerobic exercise on lipids and lipoproteins when limited to adults with Type 2 diabetes. Thus, given: (1) the increased risk for CVD in adults with Type 2 diabetes; (2) the increased risk for CVD in individuals with less than optimal lipid and lipoprotein levels; (3) the less than overwhelming results of randomized controlled trials dealing with the effects of aerobic exercise on lipids and lipoproteins in adults with Type 2 diabetes; and (4) the absence of any meta-analytic work that has specifically focused on the effects of aerobic exercise on lipids and lipoproteins in adults with Type 2 diabetes, the primary purpose of this study was to use the meta-analytic approach to examine the effects of aerobic exercise on lipids and lipoproteins in adults with Type 2 diabetes.
Section snippets
Data sources
Studies for this meta-analysis were obtained from: (1) computerized literature searches (MEDLINE, EMBASE, Sport Discuss, Current Contents, Dissertation Abstracts International); (2) cross-referencing from review articles as well as original trials; (3) hand searching selected journals; and (4) expert review of our reference list (Dr. William Haskell, personal communication). Keywords used in our computerized literature searches included exercise, cholesterol, diabetes, physical activity,
Study characteristics
Of the 3750 citations reviewed, 10 met our inclusion criteria.4, 5, 6, 7, 8, 9, 10, 26, 27, 28 However, we were unable to obtain lipid data from three studies.26, 27, 28 Thus, a total of seven studies were included in our final analysis (Table 1).4, 5, 6, 7, 8, 9, 10 The primary reason for exclusion of the other studies was that all participants in the studies did not have Type 2 diabetes. All of the included studies were parallel group trials and seemed to use an analysis-by-protocol approach
Interpretation of research findings
The purpose of this study was to use the meta-analytic approach to examine the effects of aerobic exercise on lipids and lipoproteins in adults with Type 2 diabetes. Although changes in the direction of benefit were found for TC, HDL-C, LDL-C, TC/HDL-C and TG, only decreases in LDL-C were statistically significant when results were pooled. However, reductions in LDL-C remained statistically significant only when the study by Ligtenberg et al.6 was deleted from the model. Similar to the findings
Conclusion
Our overall results suggest that aerobic exercise lowers LDL-C in adults with Type 2 diabetes. However, a need exists for additional randomized-controlled trials on this topic.
Acknowledgments
The authors would like to thank William Haskell, Ph.D., Stanford University, for serving as a consultant with specific responsibilities that included reviewing our reference list and providing suggestions for the coding of studies. This study was supported by a grant from the National Institutes of Health-National Heart, Lung and Blood Institute, Award #R01-HL069802 (G.A. Kelley, Principal Investigator).
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