Meta-analysis or Systematic Review

M - Click here for explanation of classification scheme.

POSITIVE: See Quality Criteria Checklist below.

To assess the effects of exercise or exercise and diet for preventing type 2 diabetes mellitus.

• Randomized controlled trials of exercise and diet interventions of at least six months in duration and reported diabetes incidence in people at risk for type 2 diabetes
• Participants of any age, sex or ethnicity, belonging to any of the major risk groups for the development of type 2 diabetes:
  • Impaired glucose tolerance according to WHO 1999 criteria
  • Impaired fasting glucose according to American Diabetes Association 2004 criteria
  • Previous gestational diabetes
  • Hypertension equal to or greater than 140/90mm Hg
  • Family history of type 2 diabetes in first-degree relatives
  • Obesity (BMI higher than 30kg/m²)
  • Dyslipidemia (HDL cholesterol lower than 35mg per dL, triglycerides higher than 250mg per dL or both)
  • High-risk ethnic groups, such as African-Americans, Hispanic-Americans, native Americans, Asian-Americans and Pacific Islanders.

Trials where the intervention or control group comprised the administration of any pharmacological agent.

Recruitment

The following were searched: The Cochrane Library, MEDLINE, EMBASE, CINAHL, LILACS, SocioFile, databases of ongoing trials and reference lists of relevant reviews. Search terms not described.

Design

Systematic review.

Intervention

• Exercise or exercise and diet
• Interventions varied between the studies but mainly consisted of caloric restriction if the person was overweight, low fat content (especially saturated fat), high carbohydrate content and the increase of fiber intake
• Physical activity varied but on average at least 150 minutes per week of brisk walking or other activities such as cycling or jogging were recommended
• Interventions were mainly delivered by frequent individual counseling by a physiotherapist, an exercise physiologist and a dietitian.

Statistical Analysis

• Two authors independently assessed trial quality and extracted data.
• Study authors were contacted to obtain missing data
• Data on diabetes incidence and secondary outcomes were analyzed by means of random-effects meta-analysis
• Effect sizes for dichotomous data were expressed as risk ratios in all trials except Da Qing, 1997
• For continuous outcomes, weighted mean differences and 95% confidence intervals were calculated
• Heterogeneity and reporting biases were assessed and a sensitivity analysis was completed.

Timing of Measurements

• Trials of at least six months duration; study duration ranged from one to six years
• Outcomes were planned to be assessed in the middle (up to two years of follow-up) and long-term (more than two years of follow-up) according to clinical criteria.

Dependent Variables

• Development of type 2 diabetes mellitus
• Diabetes and cardiovascular related morbidity
• Development of impaired glucose tolerance
• Development of impaired fasting glucose
• Body weight, BMI and waist-to-hip ratio
• Systolic and diastolic blood pressure
• Total cholesterol, HDL and LDL cholesterol and triglycerides
• Quality of life, ideally measured with a validated instrument
• Adverse effects, such as traumatic injuries secondary to leisure physical activity and nutritional deficits
• All-cause mortality
• Costs.

Independent Variables

• Exercise or exercise and diet
• Interventions varied between the studies but mainly consisted of caloric restriction if the person was overweight, low fat content (especially saturated fat), high carbohydrate content and the increase of fiber intake
• Physical activity varied but on average at least 150 minutes per week of brisk walking or other activities such as cycling or jogging were recommended
• Interventions were mainly delivered by frequent individual counseling by a physiotherapist, an exercise physiologist and a dietitian.

Control Variables

• Compliance
• Co-morbidities
• Age.

 

• Initial N: Initial search identified 4,875 records, full papers were obtained for 46 potentially relevant studies
• Attrition (final N): 25 publications describing eight trials were included
  • Eight trials were included that had an exercise plus diet (2,241 participants) and a standard recommendation arm (2,509 participants)
  • Two studies had a diet only (167 participants) and an exercise only arm (178 participants)
• Age: Mean age 50.3 years
• Anthropometrics: Mean BMI 31.2kg/m²
• Location: 
  • United States
  • Italy
  • Finland
  • United Kingdom
  • Japan
  • China
  • India.

Key Findings

• Eight trials were included that had an exercise plus diet (2,241 participants) and a standard recommendation arm (2,509 participants).
• Two studies had a diet only (167 participants) and an exercise-only arm (178 participants).
• Study duration ranged from one to six years
• Overall, exercise plus diet interventions reduced the risk of diabetes by 37% compared with standard recommendations (relative risk = 0.63, 95% confidence interval: 0.49 to 0.79)
• This also had favorable effects on weight and BMI reduction, waist-to-hip ratio and waist circumference
• However, statistical heterogeneity was very high for these outcomes
• Exercise and diet interventions had a very modest effect on blood lipids but improved systolic and diastolic blood pressure levels (weighted mean difference -4mm Hg, 95% confidence interval: -5 to -2mm Hg, and -2mm Hg, 95% confidence interval: -3 to -1mm Hg, respectively)
• Data on two-hour plasma glucose and fasting plasma glucose were not included in the meta-analysis
• No statistical significant effects on diabetes incidence were observed when comparing exercise only interventions either with standard recommendations or with diet only interventions
• No study reported relevant data on diabetes and cardiovascular related morbidity, mortality and quality of life.

Comparison 1. Exercise Plus Diet vs. Standard Recommendations (Overall Analysis)

Outcome or Subgroup Title	Number of Studies	Number of Participants	Effect Size
Mean differences between groups in fasting plasma glucose (mmol per L)	6	3,315	-0.19 (-0.32, -0.05)
Mean differences between groups in two-hour plasma glucose (mmol per L)	3	756	-0.23 (-1.08, 0.61)
Mean differences between groups in waist-to-hip ratio (WHR)	4	2,546	-0.01 (-0.02, 0.01)
Mean differences between groups in waist circumference (cm)	4	2,983	-3.90 (-5.90, -1.91)
Mean differences between groups in HDL cholesterol (mmol per L)	5	1,154	Not estimable
Mean differences between groups in triglycerides (mmol per L)	4	1,091	-0.14 (-0.22, -0.05)
Mean differences between groups in systolic blood pressure (mm Hg)	5	2,268	-3.54 (-4.83, -2.24)
Mean differences between groups in diastolic blood pressure (mm Hg)	6	2,521	-1.79 (-2.45, -1.14)

• Interventions aimed at increasing exercise combined with diet are able to decrease the incidence of type 2 diabetes mellitus in high-risk groups (people with impaired glucose tolerance or the metabolic syndrome). There is a need for studies exploring exercise only interventions and studies exploring the effect of exercise and diet on quality of life, morbidity and mortality, with special focus on cardiovascular outcomes.
• Overall, interventions aimed at increasing exercise combined with diet are able to decrease the incidence of type 2 diabetes mellitus in participants with impaired glucose tolerance or the metabolic syndrome. There are insufficient data on exercise alone for diabetes prevention. Also, there are no data providing evidence of the effect of these interventions on morbidity and mortality. Further, no firm conclusions can be drawn from the available evidence on which strategy to follow when trying to induce behavioral change in people at risk. These results should be taken into account by health care policy makers when planning the implementation of these strategies in real-life settings. Additionally, the favorable cost-effectiveness of lifestyle measures over pharmacological intervention is to be taken into account when planning implementation of prevention programs into routine clinical practice.

Other:

Not reported

Diet and exercise interventions not very similar. Authors note that concerning the effectiveness of exercise alone, the results of the current review are insufficient to draw a final conclusion.