Introduction

Diabetes is a significant health problem and is one of four priority non-communicable diseases (NCDs) targeted for action worldwide.1,2 Its prevalence in adults aged 18-99 years was estimated at 8.8% (451 million) of the global population in 2017, and is expected to reach 9.9% (693 million) in 2045.3 The estimated global direct cost of diabetes was USD825 billion annually,4,5 and the International Diabetes Federation estimated that diabetes corresponded to one of every eight dollars spent on healthcare.3 The rapid growth of the diabetes population is concerning given the serious consequences of diabetes, including impact on quality of life and high risk of developing devastating long term health complications such as cardiovascular disease, kidney failure, vision loss, amputations and premature mortality.1,3 In addition to impacts on an individual's health, diabetes and its complications impose direct and indirect economic costs on individuals, families, the healthcare system and nation due to treatment costs, diabetes care management expenditure, and loss of employment. There are three main types of diabetes: type 1 diabetes (i.e. T1D), type 2 diabetes (i.e. T2D) and gestational diabetes (GDM), with type 2 diabetes being the most common, accounting for 90% of all diabetes worldwide.3,6 The proportion of children and adolescents living with type 2 diabetes has increased in some countries, along with an increase in likelihood of developing diabetes complications at an earlier age.3 The evidence demonstrates that this change has been associated with obesity, an unhealthy diet and sedentary lifestyle.1,3

Although it is a vast majority group, type 2 diabetes is a largely preventable condition through the modification of potential risk factors.3 There are two categories of risk factors associated with type 2 diabetes, classified as non-modifiable and modifiable factors. General demographic characteristics such as age, sex, ethnicity, family history and a history of gestational diabetes are non-modifiable factors. The modifiable risk factors include factors that could be changed to decrease the chance of developing type 2 diabetes such as physical activity, diet, overweight and smoking. There is strong evidence supporting the notion that the onset of type 2 diabetes can be prevented, or delayed, through modifying changeable risk factors, particularly overweight, poor diet and physical inactivity.1,7-10 The burden of diabetes and its effects on global health and health economics suggest that it is worthwhile investing in early detection and management, as well as focusing on reducing the incidence, or delaying the onset of diabetes in high risk groups through healthy behaviors and a healthy eating program.4,5,11,12 Obesity and overweight are believed to be significant attributes in the rising trend of type 2 diabetes and projected healthcare costs related to diabetes.13,14 Huge efforts have gone into preventing obesity, but disappointingly evidence demonstrates little success.15 A number of large-scale studies have been conducted in many nations to examine the effectiveness of lifestyle interventions in reducing the incidence of type 2 diabetes. The most common objectives of these trials were increasing physical activity, improving healthy eating, maintaining a healthy weight and modifying health behaviors through lifestyle change programs, offered as group or individual sessions, and via interactive or digital services.16

The three pivotal trials on prevention of type 2 diabetes include: i) the China Da Qing Diabetes Prevention Study (CDQDPS), a large randomized controlled trial conducted in China in 1986;17 ii) the Finnish Diabetes Prevention Study (DPS), a national, multicenter RCT commenced in 1993 in Finland;18 and iii) the Diabetes Prevention Program (DPP), a large-scale RCT conducted in the USA in 1996.19 Positive conclusions for the effectiveness of lifestyle change programs on the reduction of diabetes risk were reported by these pivotal trials, including 58% reduction in type 2 diabetes incidence;20,21 and a 51% lower incidence of type 2 diabetes over a 6-year period in people with impaired glucose tolerance (IGT) compared to the control group.17 Despite the consistent outcomes, the differences in various salient factors of each trial should be considered, such as the elements of interventions (e.g. physical activity, diet, face-to-face or phone services); demographic characteristics of participants (e.g. age, genetics, culture and life pattern); the healthcare system (e.g. the universal health coverage and health insurance) and the methodologies of the studies.22 For instance, the DPS offered only one lifestyle change intervention involving physical activity and diet; the DPP offered two options (lifestyle changes and medication); whist the CDQDPS provided three options of interventions, including exercise only, diet only and exercise plus diet. Furthermore, the CDQDPS included participants with IGT and either lean or overweight participants. Whereas the DPS and the DPP included people with IGT and overweight participants.

There were some systematic reviews conducted to examine the effectiveness of type 2 diabetes prevention studies and programs with different methodologies and various aspects. In 2008, Orozco et al.23 performed a systematic review of RCTs to assess the effect of exercise or exercise and diet for preventing type 2 diabetes, which was updated and reviewed by Hemmingsen and colleagues24 in 2017. The systematic review performed by Dunkley et al.25 in 2014 was mainly summarizing the pragmatic lifestyle intervention of both experimental and observational studies, they also examined the influence of guidelines adherence on the effectiveness of the prevention program. Ashra et al.16 conducted another systematic review in 2015 which aimed to update and extend the previous systematic review25 and performed a meta-analysis to assess the effectiveness of "real-world" interventions for type 2 diabetes prevention. Similar to the former systematic review, they included both experimental and observational studies; however, they also identified elements of the intervention that contribute to a successful program. These systematic reviews provided valuable pieces of information to improve interventions for type 2 diabetes. Nevertheless, there are research gaps for new systematic reviews focusing on specific criteria such as adults aged 18 years and over with a high risk of developing diabetes, evidence-based lifestyle interventions based on either DPP or DPS only, and measurement of the diabetes incidence.

This systematic review aims to evaluate studies including only adults aged 18 years and over with high risk of developing type 2 diabetes, to provide a clear picture of the outcomes in this population. Addressing bias in the heterogeneity of data derived from mixed experimental and observational studies, plus the diversity of study settings (e.g. hospital, primary care center and community center) underpins the rationale for performing a systematic review focusing on experimental studies conducted in community-based settings. Establishing a specific evidence-base of the effectiveness of diabetes prevention programs in a "real world" setting would be highly beneficial when considering evidence for program implementation.

A preliminary search of the Joanna Briggs Institute Database of Systematic Reviews and Implementation Reports, the Cochrane Database of Systematic Reviews, PROSPERO, and MEDLINE was conducted and revealed that there are no current nor underway systematic reviews on the effectiveness of evidence-based lifestyle interventions conducted in a community-based setting to prevent type 2 diabetes among adults with high risk of developing type 2 diabetes. The aim of this systematic review is to establish the effectiveness of lifestyle interventions for adults with high risk of diabetes in preventing or delaying the onset of type 2 diabetes.

Inclusion criteria

Participants

This review will consider studies that include adults (female and male) aged 18 years and over, explicitly identified as being at high risk of developing type 2 diabetes by health professionals based on their biomedical results, or by using standard type 2 diabetes risk assessment tool with assistance from health professionals.

Intervention

This review will consider studies that evaluate lifestyle interventions comprising either physical activity or diet elements based on the DPP study and/or the DPS. The DPS intervention involved seven face-to-face sessions, comprising physical activity and dietary components.18 The DPP intervention was a face-to-face goal-based behavioral program, providing 16 sessions of a combination of physical activity and diet components within 24 weeks.19 The interventions must be identified as either based on DPP and/or DPS by the authors of the primary studies. The length of these interventions must be designed for at least six months, with at least 12 months follow-up. Interventions comprising of the administration of any pharmacological agent will be excluded.

Comparator

This review will consider studies that compare the intervention to usual care, alternative interventions or no intervention.

Outcomes

This review will consider studies that include the following outcomes:

Primary outcome: Incidence of type 2 diabetes by any validated scale or standard measurement or index.

Secondary outcomes:

i. Any outcome of interest relating to diabetes such as HbA1c level, IGT, impaired fasting glucose (IFG), lipid profile, blood pressure, weight, and Body Mass Index (BMI), comparing between the baseline and the endpoint of the intervention.

ii. Quality of life during and after participating in the intervention measured by any validated tool or standard measurement or index.

Types of studies

This review will consider all RCTs and quasi-randomized controlled trials conducted in community-based settings, globally. In the absence of RCTs, any experimental study design such as non-RCTs and quasi-experimental, before and after studies will be considered for inclusion.

Methods

The proposed systematic review will be conducted in accordance with the Joanna Briggs Institute (JBI) methodology for systematic reviews of effectiveness evidence.26

Search strategy

The search strategy will aim to locate both published and unpublished studies. A three-step search strategy will be utilized in this review. An initial limited search of MEDLINE (Ovid) and Cumulative Index of Nursing and Allied Health Literature (CINAHL) (EBSCO) will be undertaken followed by an analysis of text words contained in the titles and abstracts of relevant articles, and the index terms used to describe the articles. The second search strategy, including all identified keywords and index terms will be adapted for each included information source. Thirdly, the reference list of all studies selected for critical appraisal will be screened for further relevant studies. Studies published in English language will be included. Studies published from 1996 to the present will be included. The search strategy is presented in Appendix I.

Information sources

The databases/sources to be searched include: MEDLINE (Ovid), CINAHL (EBSCO), Scopus, JBI Database of Systematic Reviews and Implementation Reports, Embase (Ovid), Cochrane Central Register of Controlled trials, Web of Science, and Google Scholar (first 200 records).

Sources of unpublished studies and gray literature to be searched include: ProQuest Theses and Dissertations, OpenGrey, Trove, Advanced Google (first 200 record), and Relevant websites: http://www.nhmrc.gov.au/, http://www.ncbi.nlm.nih.gov/.

Study selection

Following the search, all identified citations will be loaded into EndNote X9 (Clarivate Analytics, PA, USA) and duplicates removed. Titles and abstracts will then be screened by two independent reviewers for assessment against the inclusion criteria for the review. Potentially relevant studies will be retrieved in full and their citation details imported into the Joanna Briggs Institute System for the Unified Management, Assessment and Review of Information (JBI SUMARI) (Joanna Briggs Institute, Adelaide, Australia). The full text of selected citations will be assessed in detail against the inclusion criteria by two independent reviewers. Reasons for exclusion of full text studies that do not meet the inclusion criteria will be recorded and reported in the systematic review. Any disagreements that arise between the reviewers at each stage of the study selection process will be resolved through discussion or with a third reviewer. The results of the search will be reported in full in the final report and presented in a Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) flow diagram.27

Assessment of methodological quality

Eligible studies will be critically appraised by two independent reviewers for methodological quality using standardized critical appraisal instruments from JBI for experimental and quasi-experimental studies26 prior to inclusion. Authors of papers will be contacted to request missing or additional data for clarification, where required. Any disagreements that arise between the reviewers will be resolved through discussion or with a third reviewer. All studies, regardless of the results of their methodological quality, will undergo data extraction and synthesis (where possible). The results of the critical appraisal will be reported in a tabular and narrative form.

Data extraction

Quantitative data will be extracted from studies included in the review by two independent reviewers using the standardized JBI data extraction tool in JBI SUMARI.26 The data extracted will include specific details about the populations, study methods, interventions, elements of the interventions and influence factors. Any disagreements that arise between the reviewers will be resolved through discussion or with a third reviewer. Authors of papers will be contacted to request missing or additional data, where required.

Data synthesis

Studies will, where possible, be pooled with statistical meta-analysis using JBI SUMARI. Effect sizes will be expressed as either odds ratios (for dichotomous data) or weighted (or standardized) final post-intervention mean differences (for continuous data) and their 95% confidence intervals will be calculated for analysis. Heterogeneity will be assessed statistically using the standard chi squared and I² tests. Statistical analyses will be performed using the random effects model.28 Subgroup analyses will be conducted, where sufficient data of interest factors and potential confounders can be identified, such as age group, gender, ethnicity, elements of intervention and effect of evidence based interventions (DPP and DPS). Where statistical pooling is not possible the findings will be presented in narrative form including tables and figures to aid in data presentation, where appropriate. A funnel plot will be generated to assess publication bias if there are 10 or more studies included in a meta-analysis. Statistical tests for funnel plot asymmetry (Egger test, Begg test, Harbord test) will be performed where appropriate.

Assessing certainty in the findings

The Grading of Recommendations, Assessment, Development and Evaluation (GRADE) approach for grading the certainty of evidence will be followed and a Summary of Findings (SoF) will be created using GRADEPro GDT software (McMaster University, ON, Canada). The SoF will present the following information where appropriate: absolute risks for the treatment and control, estimates of relative risk, and a ranking of the quality of the evidence based on the risk of bias, directness, heterogeneity, precision and risk of publication bias of the review results. The outcomes reported in the SoF will be presented in a tabular form.

Appendix I: Search strategy for MEDLINE

References