This review concluded that, independent of the effects of diet or weight loss, the role of physical activity in the prevention of type 2 diabetes is equivocal. The authors’ conclusions should be interpreted with caution since all studies assessed a combined intervention of diet and exercise, thus it is not possible to comment on the contribution of exercise alone.

To determine the effectiveness of physical activity at reducing the risk of diabetes or improving glucose control in people with prediabetes.

MEDLINE and EMBASE were searched from inception to February 2006; the search terms were reported. The reference lists of published primary and review articles were screened for additional studies.

Study designs of evaluations included in the review

Randomised and non-randomised controlled trials were eligible for inclusion.

Specific interventions included in the review

Studies of interventions that included an exercise programme were eligible for inclusion. Exercise programmes were defined as interventions that actively promoted and supported physical activity or structured exercise training regimens. Studies of interventions consisting of brief written or verbal physical activity advice were excluded, as were those of single or acute episodes of exercise. Most of the studies used a multi-component lifestyle intervention combining exercise and diet; one also included exercise and diet only arms.

Participants included in the review

Studies of adults (aged at least 18 years) diagnosed with prediabetes were eligible for inclusion. Prediabetes was defined as impaired glucose tolerance (IGT) and or impaired fasting glucose (IFG) using criteria recommended by the World Health Organization (WHO) or American Diabetes Association (ADA). Studies using other criteria were included if the mean value of plasma glucose was within the range of IGT or IFG defined by the WHO or ADA criteria. All studies included patients with IGT and excluded those with isolated IFG. Overall, 40% of the participants were men. Most of the trials included both men and women; one included only men. The studies were conducted in Finland, Sweden, USA, China, England and the Netherlands.

Outcomes assessed in the review

Studies that reported a measure of physical activity and a relevant clinical measure (progression to diabetes or suitable measure of plasma glucose) were eligible for inclusion.

How were decisions on the relevance of primary studies made?

One reviewer screened the results of the searches for relevance and all potentially relevant articles were obtained. Two reviewers independently assessed full-text articles for inclusion; any disagreements were resolved through consensus or by referral to a third reviewer.

The authors did not state that they assessed validity.

The data were extracted as means and standard errors or median and interquartile ranges for baseline and follow-up exercise, body mass and glucose parameters. Data were also extracted on the relative risk, with 95% confidence interval, of diabetes in the intervention group compared with the control group. Two reviewers independently extracted all data; any disagreements were resolved through discussion or, if necessary, through a third party.

How were the studies combined?

A narrative synthesis was used to combine studies by outcome.

How were differences between studies investigated?

Differences between the studies were discussed in the text.

Eight studies (3,904 participants) were included. Seven studies were randomised controlled trials; the non-randomised trials included controls that were not enrolled in the intervention programme.

Incidence of diabetes (4 studies).

All studies found a significant reduction in the incidence of diabetes in intervention groups compared with control groups; reductions ranged from 42 to 63%. Three studies that used subjective measures of exercise reported non significant to small changes in exercise levels. One study used an objective measure and reported an 8% increase in maximal oxygen uptake.

Two-hour post-challenge plasma glucose level (7 studies).

Five studies reported no significant difference in levels between intervention and control groups. Two studies reported significant smaller changes in intervention groups compared with control groups (p<0.05).

Fasting glucose (7 studies).

None of the studies reported a significant change in fasting glucose levels in intervention groups compared with control groups.

Independent of the effects of dietary or weight loss changes, the contribution of physical activity to the prevention of type 2 diabetes in people with prediabetes is equivocal.

The review addressed a focused question that was supported by clearly defined inclusion criteria. The literature search was adequate, but no attempts were made to locate unpublished studies and so the review may be subject to publication bias. Appropriate steps were taken to minimise bias in the selection of studies and extraction of data. A formal quality assessment was not undertaken and study quality was not discussed; the reliability of the findings of the primary studies therefore remains unclear.

Study details and results were tabulated clearly. A narrative synthesis was used to combine the results. A more statistical analysis (especially for the reduction in diabetes risk), which considered heterogeneity, might have produced more informative results. The authors’ conclusions comment on the effect of physical activity independent of other interventions. However, given that all studies consisted of combined interventions incorporating both dietary and exercise components, it is not possible to comment on the effect of an exercise intervention alone. The authors’ conclusions should therefore be interpreted with caution.

Practice: The authors did not state any implications for practice.

Research: The authors stated the need for rigorously designed randomised controlled trials with objective measures of physical activity. Such studies should assess strategies for increasing physical activity in patients with IGT and IFG.

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.