This systematic review found that multi-component lifestyle interventions incorporating diet + aerobic exercise + RT conducted in at risk or prediabetic adult populations were efficacious for inducing modest weight loss and eliciting small improvements in glycemic control, together with improvements in aerobic fitness and dietary intake. The impact of interventions on muscular fitness and physical activity were not consistently reported, making it difficult to determine the contributions of these components towards improvements in glucose regulation.
Weight change and glucose regulation
All interventions in this review and the meta-analysis found significant weight loss compared to controls. Importantly the DPP identified weight loss as the dominant predictor of their 58% reduction in T2DM incidence . However, the effects on glucose regulation were less consistent. Meta-analysis found a small but significant reduction that would be of clinical importance in those with borderline prediabetes. The baseline FPG mean of the combined study population in the meta-analysis (5.6 mmol.L-1) was at the lower limit of the prediabetes range (5.6 - 6.9 mmol.L-1) . This suggests that scope to improve further was limited in these cohorts, a circumstance which may be common amongst prediabetic individuals who present in clinical settings. Furthermore, the small magnitude of change observed in the meta-analysis for FPG was heavily influenced by the results of the Finnish DPS, which received a 65% weighting due to its large sample size. The Finnish DPS excluded participants from the study after diagnosis of T2DM. As the majority of those developing T2DM belonged to the control group, this introduces bias, which underestimates the FPG of the control group, leading to an attenuation of the difference between the groups.
Exercise programs and measurement of related outcomes
The reporting of exercise programs was inconsistent between studies and most studies provided only general descriptions of their exercise programs. For example, “The supervised exercise group has additionally been offered supervised, progressive, individually tailored aerobic exercise programs and circuit-type resistance training sessions for 1 hour twice a week” . This makes it difficult to determine the specific modes of RT exercises that were performed (e.g., body weight, free weights, isometric exercises, isokinetic exercises, resistance band) and the volume (load, repetitions and sets) prescribed. Future studies are recommended to provide more comprehensive descriptions of the exercise programs. Most studies provided supervised individual or group exercise sessions; only one study included a home-based exercise component . This has implications for the feasibility, practicalities and dissemination costs of these programs into community and health-care settings, as few health care systems can afford to provide supervision of exercise programs by qualified personnel.
Measurement of exercise-related outcomes was also inconsistent between studies. No studies used objective measures (e.g., pedometers or accelerometers) to assess physical activity, which is a major limitation in existing studies. Physical activity levels as measured by self-report improved in intervention groups versus controls groups [42–49]. Aerobic exercise tests to measure or predict VO2max were the most widely used fitness indicator, and improvements in aerobic fitness in intervention groups were generally observed [29–31, 33, 34, 36–40]. Only one study measured improvements in muscular strength [29–31], assessing only lower body limb strength using an isokinetic dynamometer. Without evaluation of muscular performance (including upper and lower body muscle groups) it is difficult to determine whether the RT program was adhered to or whether the addition of RT in multi-component programs contributes to improvements in muscular fitness and glycemic control in prediabetes populations, as has been shown in adults with T2DM . Future studies should provide comprehensive and objective evaluation of the impact on aerobic and muscular fitness.
Type 2 diabetes incidence
A reduction in T2DM incidence is the goal for all T2DM prevention programs. Of the studies reviewed, incidence of T2DM was only reported in the Finnish DPS and SLIM studies (up to 58% reduction in T2DM incidence). This finding is of great interest, particularly since the US DPP, which did not prescribe RT as part of their physical activity recommendations, also reported a 58% reduction in diabetes incidence (after 2.8 years) . This suggests that multi-component T2DM prevention programs that include RT are effective, but whether RT provides benefits additional to dietary and aerobic components requires further investigation.
Features of effective interventions
Study design and intervention components were heterogeneous amongst the included studies, which may account for some of the variation observed in the outcomes assessed. Design characteristics of studies that achieved significant changes for weight loss and FPG [29–31, 33, 36–39, 41] included: face-to-face intervention delivery mode (individual and/or group), an average of eight contacts per month (including face to face sessions, emails and phone calls), and a minimum of six (preferably 12) months of follow up. Lifestyle intervention characteristics included: 150–210 minutes (3–5 sessions) of aerobic exercise per week; 60–120 minutes (1–3 sessions) of RT per week; recommendations for a specified macronutrient diet profile, energy restriction for weight loss and setting a weight loss goal of 5-10%.
Sex differences in lifestyle programs
Of the studies reviewed, 62% of participants were female. Since there is no reported global difference in gender distribution for diabetes , this may indicate that women are more likely to participate in diabetes prevention trials. None of the studies targeted a specific sex or reported their results by sex. Whether males and females benefit equally from these multi-component interventions is not known, but future studies should report their results by sex to reveal any differences that may exist. A recent systematic review  argued that sex-specific design features may be important influences on the effectiveness of lifestyle interventions.
Strengths and limitations
This is the first review to synthesize the evidence of multi-component interventions including diet, aerobic exercise and RT for the prevention of type 2 diabetes. It adhered to the PRISMA statement for the reporting of systematic reviews and meta-analyses; a comprehensive search strategy was performed across multiple databases with no date restrictions; high agreement levels for quality assessments were achieved; and detailed data extraction was performed to allow for comparisons between studies.
The review also has some limitations. Meta-analyses for weight and FPG were based on a small number of studies and the meta-analysis for weight was statistically heterogeneous. The sample for the meta-analyses consisted of 62% females, which introduces a sex bias. Furthermore the mean age of participants was 54.5 ± 9.7 years and only one study targeted older individuals (>65) [29–31]. This limits the generalizability of the results particularly for older individuals and highlights an evidence gap in the field. Regular resistance training may result in gains or maintenance of muscle mass; consequently weight loss as an outcome by itself would be confounded by the inability to discriminate between loss of fat mass and gains in fat free mass. Future studies need to include more comprehensive assessments of body composition. For the aforementioned reasons results from the original studies and the synthesis of results presented here must be interpreted with caution. Finally, T2DM prevention studies that employed diet + aerobic exercise, but not RT were not eligible, including the highly successful US DPP.
Direction for future research
This review has highlighted the need for high quality long-term RCTs that assess multi-component lifestyle prevention programs for T2DM. Systematic investigation of the benefits of each additional component (diet, aerobic, RT, physical activity) of multi-component lifestyle interventions is also required to provide further support for the current recommendations for T2DM prevention. Future studies should report intervention component adherence and use objective measures to detect changes in muscular fitness, aerobic capacity and physical activity. More comprehensive measures of body composition (e.g., waist circumference, dual x-ray absorptiometry or bioimpedance analysis) should be utilised to determine changes in body composition as a result of multi-component T2DM prevention programs including RT. Studies exploring interventions tailored specifically for men or women are required to determine any impact on recruitment, retention and efficacy.