The review concluded that there was evidence to suggest that risk of developing coronary heart disease tended to decrease as walking dose increased. Given the potential for language bias, a lack of validity assessment and the exploratory nature of the analyses, the authors' conclusions should be regarded as tentative.

To quantify the dose-response relationship between walking and reduction in risk of coronary heart disease for men and women in the general population.

MEDLINE, SPORTDiscus and Cochrane Database of Systematic Reviews were searched from 1954 to September 2007 for studies in English. Search terms were reported. Reference lists of retrieved articles and systematic reviews were searched for additional studies. Conference proceedings (details not specified) were scanned for unpublished studies.

Studies that evaluated walking as a primary prevention of coronary heart disease in men and women in the general population were eligible for inclusion. Included studies had to report relative risks for the effect of walking on coronary heart disease or report enough data for these to be calculated. Studies that combined walking with other types of physical activity were excluded. The main outcome was coronary heart disease defined as fatal or non-fatal myocardial infarction as reported by the primary study authors. Included studies had at a minimum to adjust for age as a confounder.

In the included studies walking was described as leisure time walking, regular walking or overall walking. Walking was assessed by distance travelled, walking time, energy expenditure, pace and frequency. Most studies included only men, some included only women and two studies included mixed populations. Participant age ranged from 35 to 93 years. Most of the studies were conducted in USA and the others in UK.

Two reviewers independently selected papers for inclusion. Discrepancies were resolved through discussion.

The authors did not state that they assessed validity.

Data were extracted on the relative risks (RR) and 95% confidence intervals (CI) of the effect of walking on coronary heart disease. Where these data were missing, relative risks and 95% CI were calculated from raw data or graphical images. Where a study reported several relative risks due to different levels of adjustment for confounders, only those with adjustment levels comparable to that of other studies were used. Walking measures were converted to a uniform metric MET-hours per week. Where only walking time or pace were reported, normal walking for half an hour a day on average was assumed (defined as average 2.5miles per hour on a firm surface).

Two reviewers independently extracted data. Discrepancies were resolved through discussion.

Data for dose-response were combined using random-effects meta-regression models using methods by Greenland and Orsini, and Hamling et al. Where it was not possible to perform a dose-response meta-analysis due to lack of data, a random-effects model was used to calculate overall relative risk for all levels of working exposure. Heterogeneity was assessed using Cochran Q and the I² statistics.

Sensitivity and subgroup analyses were used to explore potential sources of heterogeneity (gender, mean age, duration of follow-up). Studies that evaluated walking exposure in at least three levels were included in the meta-analysis. Three studies were excluded from meta-analysis as walking variables were only classed in two levels (high versus low). Publication bias was assessed with Egger's regression asymmetry test.

Twelve studies (n=295,177) met the inclusion criteria: one randomised controlled trial (RCT) and 11 prospective cohort studies. Sample sizes ranged from 196 to 73,743. Follow-up ranged from two to 16 years. There were 7,094 cases of coronary heart disease at follow-up.

Risk of developing coronary heart disease was reduced when walking intensity was increased by 8 MET per hour per week, equivalent to an increase of approximately 30 minutes of normal walking a day for five days a week (19%, 95% CI 14% to 23%, I²=37%; eight studies). Risk of developing coronary heart disease was also reduced when walking pace was increased by 2km per hour (21%, 95% CI 15% to 27%, I²=0%; eight studies) and when normal walking time was increased by 3.5 hours per week (32%, 95% CI 11% to 48%, I²=16%; eight studies).

There was no evidence of heterogeneity or publication bias (p=0.39) for any of these analyses. Results were similar for sensitivity and subgroup analyses.

The analysis suggested that the risk of developing coronary heart disease tended to decrease as walking dose increased.

The review question was clear with clearly defined with appropriate inclusion criteria. Several relevant sources were searched. It appeared that some efforts were made to locate unpublished data. Limitation to studies in English meant there was potential for language bias. Validity was not assessed, so the results from these studies and any synthesis may not be reliable. Appropriate methods to reduce reviewer error and bias were undertaken for study selection and data extraction. Data were combined in a regression model, which included a number of exercise exposure variables; control for confounding factors appeared to have been assumed, based on control for confounding factors in the primary studies (not reported in detail). Studies were conducted either in USA or UK and few participant characteristics were reported; therefore, it was difficult to assess the generalisability of the findings. The authors stated that due to incomplete reporting in the primary studies of walking intensity, duration and frequency, adherence to basal walking levels and participation in other physical activity, it was difficult to accurately quantify the walking dose.

Given the potential for language bias, a lack of validity assessment and the exploratory nature of the analyses of data, the authors' conclusions should be regarded as tentative.

Practice: The authors stated that walking should be prescribed as an evidence-based effective exercise modality for coronary heart disease prevention in the general population.

Research: The authors stated that further research was required to evaluate the effect of walking in reducing coronary heart disease risk among populations with different basal levels of physical activity and fitness. Research was needed to determine the relative accuracy and validity of different assessment methods and measures in order to identify the most reproducible, valid and accurate measures.

Not stated.

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.