Effects of aerobic exercise on C-reactive protein, body composition, and maximum oxygen consumption in adults: a meta-analysis of randomized controlled trials

Abstract

The aim of the study was to use the meta-analytic approach to examine the effects of aerobic exercise on C-reactive protein (CRP) in adults. Secondary outcomes included changes in body weight in kilograms, percentage of body fat, and maximum oxygen consumption (V̇o₂max) in mL kg⁻¹ min⁻¹. Studies were retrieved using computerized literature searches, cross-referencing, and hand searching. Inclusion criteria were assessment of CRP in randomized controlled trials published in the English language between January 1, 1990, and January 1, 2006. Studies were also limited to aerobic exercise interventions lasting 4 weeks or more in adults 18 years or older. Five studies representing 323 male and female subjects (171 exercise, 152 control) and 6 outcomes for CRP were available for pooling. A nonsignificant reduction of approximately 3% was observed for CRP in the exercise groups (mean ± SEM, −0.11 ± 0.14 mg/L; 95% confidence interval [CI], −0.39 to 0.17 mg/L) using a random-effects model. Statistically significant reductions of approximately 4% were found for body weight (mean ± SEM, −3.4 ± 1.0 kg; 95% CI, −5.3 to −1.5 kg) and percentage of body fat (mean ± SEM, −1.4% ± 0.4%; 95% CI, −2.3% to −0.6%), whereas a statistically significant increase of 12% was found for V̇o₂max (mean ± SEM, 3.3 ± 0.9 mL kg⁻¹ min⁻¹; 95% CI, 1.5 to 5.1 mL kg⁻¹ min⁻¹). The results of our study suggest that aerobic exercise does not reduce CRP levels in adults, but does improve measures of body composition and physical fitness.

Introduction

Cardiovascular disease (CVD) is the leading cause of mortality in the United States, affecting a total of 696 947 people (28.5% of all deaths) in 2002 [1]. Current research suggests that inflammation plays an important role in the development of atherosclerosis [2], [3]. One of the most important markers of inflammation and subsequent CVD appears to be elevated levels of high-sensitivity C-reactive protein (CRP) [4]. To support this contention, it has been shown that CRP is a better predictor of CVD when compared to lipoprotein(a), homocysteine, interleukin 6, total cholesterol, low-density lipoprotein cholesterol, serum amyloid A, apolipoprotein B, and the ratio of total cholesterol to high-density lipoprotein cholesterol [4]. Given the apparent importance of CRP in the development of CVD morbidity and mortality, it is important to determine those factors that may help to lower and maintain optimal levels of CRP.

One possible approach for improving levels of CRP may be aerobic exercise, a low-cost, nonpharmacologic intervention that is available to most of the general public. For example, a recent qualitative systematic review that examined studies dealing with physical activity and CRP concluded that habitual physical activity results in lower levels of CRP [5]. However, these conclusions were based primarily on the results from observational studies and no randomized controlled trial(s) in which aerobic exercise was the only intervention [5].

Although observational studies can provide important information, they have several potential weaknesses, most notably, selection bias and reporting bias. In contrast, randomized controlled trials are the only way to control for confounders that are not known or measured [6], [7]. However, we are not aware of any qualitative or quantitative (meta-analysis) systematic review that has examined the effects of aerobic exercise on CRP while limiting their analysis to randomized controlled trials only.

Meta-analysis is a quantitative approach in which individual studies are combined to arrive at some general conclusions regarding a body of research. The strengths of meta-analysis include the ability to increase estimates of treatment effectiveness as well as statistical power for primary outcomes and other end points [8]. Therefore, given (1) the importance of CRP as a CVD risk factor, (2) the potential for aerobic exercise to lower and maintain levels of CRP, and (3) the strength of randomized controlled trials and the meta-analytic approach, the primary purpose of this study was to use the meta-analytic approach to examine the effects of aerobic exercise on CRP in adult humans while limiting studies to randomized controlled trials. In addition, because aerobic exercise affects many other variables associated with a reduction in the risk for CVD, we also examined its effects on body weight, percentage of body fat, and maximum oxygen consumption (V̇o₂max, mL kg⁻¹ min⁻¹).