CRD summary

This review concluded that there was a dose-response relationship between aerobic exercise and visceral fat reduction in obese adults without metabolic-related disorders. Energy expenditure of at least 10 total metabolic equivalent hours per week were required. The results supported a possible dose-response relationship, but a limited literature search and no consideration of study quality limit the conclusions.

Authors' objectives

To evaluate whether there was an association between aerobic exercise for weight loss and visceral fat reduction, and to determine the minimal amount of aerobic exercise needed to reduce visceral fat.

Searching

PubMed was searched between 1966 and May 2006. Additional studies were located from reference lists. Search terms were reported.

Study selection

Randomised or non-randomised controlled trials of adults aged between 18 and 65 years with a mean body mass index (BMI) less than 25kg/m² or over 25kg/m² with a small amount of visceral fat. Groups where the mean and standard deviation of the visceral fat area was less than 100 cm² were excluded as they were considered to not need visceral fat reduction.

Eligible interventions were aerobic exercises. Only studies with groups instructed to practice aerobic exercise without weight loss by additional energy intake were included. Studies of drug therapy or resistance training only were excluded. Included studies had to measure visceral fat with computed tomography (CT) or magnetic resonance imaging (MRI). Included studies assessed individual aerobic exercise with or without a weight loss programme, different types of aerobic exercise (within the one study), diet therapy and drug therapy with aerobic exercise. The duration of the aerobic exercise intervention ranged from eight weeks to 16 months. Studies were of men only, women only and both genders. Mean ages ranged from 21 to 67.5 years. Mean baseline BMI for the aerobics groups ranged from 25.3 to 35.9 kg/m². Some studies were of specific populations such as diabetics, menopausal women or post-menopausal women.

Studies were selected by two reviewers independently.

Assessment of study quality

The authors did not state that they assessed validity.

Data extraction

Visceral fat was measured in different units across studies (such as cm² and kg). For each study this was converted to a percentage change in visceral fat per week (%VF/w) for direct comparison between groups. The amount of energy expenditure per study was converted to total metabolic equivalents for hours per week (MET h/w) using either energy expenditure data as reported in the study or calculated from body weight and other exercise intensity measures (equations were reported). Details of the BMI, percentage body fat, type of exercise and session durations and intensity were extracted for the aerobic groups. Data extraction was performed by two reviewers.

Methods of synthesis

The analysis concentrated on the relationship between the amount of exercise and the reduction in visceral fat. The analysis investigated the correlation between METs h/w and change in %VF/w for the aerobic exercise groups using Pearson's correlation coefficient. Further analysis looked at the correlation between change in body weight per week (% weight/week). and change in %VFw and METs h/w. A standard meta-analysis pooling the within-study differences between the aerobic and control groups was not performed, although each study was weighted by the number of subjects.

Subgroup analyses looked at correlations in patients without metabolic-related disorders (such as type 2 diabetes or dyslipidemia), exercise duration greater or less than 16 weeks and studies of men or women only. Patients without metabolic disorders were also split into three groups by their amount of exercise and the mean %VF/w was compared using a Kruskal-Wallis test and Mann-Whitney U test.

Results of the review

Nine randomised controlled trials (n=692) provided data for 13 aerobic exercise groups and seven non-randomised controlled trials (n=263) provided data for eight aerobic exercise groups. Six groups in four studies had metabolic-related disorders.

The calculated measures of exercise MET h/w ranged from 5.9 to 47.1. The change in %VF/w ranged from -6.062 to 0.078. Overall, there was a small non-significant negative correlation between MET h/w and change in %VF/w (r=-0.28, p=0.225). Groups that excluded people with metabolic-related disorders had a statistically significant negative correlation between MET h/w and change in %VF/w (r=-0.75, p=0.01). Further analysis of patients without metabolic-related disorders divided into three groups by MET h/w showed no evidence of a difference between groups. However, each exercise group was statistically significantly higher than the control group (p<0.05). The group with the greatest amount of METs h/w showed the greatest overall reduction in change in %VF/w (median approximately -1.4%).

For subgroup analyses, significant correlations were seen for women only for all data (r=-0.89, p=0.007) and short-term intervention groups without metabolic-related disorders (r=-0.81, p=0.027). For analyses of change in percentage weight per week, this showed a significant correlation with METs h/w (r=-0.79) in all data as well as those without metabolic-related disorders (r=-0.870). There was also a strong positive correlation between change in percentage weight per week and change in %VF/w in both groups (r=0.93 and r=0.64).

Authors' conclusions

The results suggested that there was a dose-response relationship between aerobic exercise as a weight-loss intervention and visceral fat reduction in obese subjects, excluding those with metabolic-related disorders. Visceral fat reduction was related to weight reduction. At least 10 METs h/w of aerobic exercise per week was needed for significant visceral fat reduction.

CRD commentary

The review had a clear aim and study inclusion/exclusion criteria. The search was restricted to PubMed only and there were no efforts to locate unpublished studies, thus there were limitations with the literature search. Studies were selected by two authors independently, which minimised errors and bias in the selection process. It appeared that data extraction was performed in the same way. This could be problematic as it appeared that there was a lot of derivation of energy expenditure and reduction in visceral fat from the study data, so additional checking of calculations would have been needed. There was no validity assessment, so it was not possible to judge the reliablity of the studies. No distinction between the results of the randomised and non-randomised studies was made. The main aim of the analyses was to assess the dose-response relationship between aerobic exercise and fat reduction, so a standard meta-analysis was not undertaken. However, some reporting of the comparative results from those studies with a control group may have been useful, rather than just concentrating on the intervention groups only. Reporting of the individual study results was poor. The conclusion about a possible dose-response relationship seemed supported by the results presented, but the limited literature search and lack of consideration of study quality affect this review's reliability.

Implications of the review for practice and research

Practice: The authors stated that obese people should practice an initially high volume of aerobic exercise, which should be reduced to a manageable amount in the long term.

Research: Further research should examine the balance between diet and exercise for visceral fat reduction. Research was also needed to look at the influence of factors such as metabolic-related disorders, gender and exercise duration on visceral fat reduction.

Funding

None stated.

Bibliographic details

Ohkawara K, Tanaka S, Miyachi M, Ishikawa-Takata K, Tabata I. A dose-response relation between aerobic exercise and visceral fat reduction: systematic review of clinical trials. International Journal of Obesity 2007; 31(12): 1786-1797. [PubMed: 17637702]

Original Paper URL

http://www.nature.com/ijo/journal/v31/n12/abs/0803683a.html

Indexing Status

Subject indexing assigned by NLM

MeSH

Adolescent; Adult; Clinical Trials as Topic; Energy Metabolism /physiology; Exercise /physiology; Humans; Intra-Abdominal Fat /metabolism; Middle Aged; Obesity /therapy; Weight Loss /physiology

AccessionNumber

12007004104

Database entry date

29/07/2009

Record Status

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.