Vegan Diets and the Prevention and Treatment of CVD


Turner-McGrievy GM, Jenkins DJ, Barnard ND, Cohen J, Gloede L, Green AA. Decreases in dietary glycemic index are related to weight loss among individuals following therapeutic diets for type 2 diabetes. J Nutr 2011;141(8):1469-74.

PubMed ID: 21653575
Study Design:
Randomized Controlled Trial
A - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

The purpose of this study is to assess the effects of changes in GI and GL on weight loss and changes in HbA1c among individuals with type 2 diabetes in the context of a vegan diet and the 2003 ADA dietary recommendations while controlling for dietary and demographic variables.

Inclusion Criteria:
  • Provided written informed consent
  • Type 2 diabetes (fasting plasma glucose concentration over 6.94mmol per L on two occasions or a prior diagnosis of type 2 diabetes with the use of hypoglycemic medications for at least six months).
Exclusion Criteria:
Not described.
Description of Study Protocol:
  • Recruitment: Participants with type 2 diabetes were recruited in two cohorts between 2003 and 2004
  • Design: Randomized clinical trial
  • Blinding used: Implied with measurements.


  • Participants were randomly assigned to follow a low-fat, low-GI vegan diet or individualized diets based on the 2003 American Diabetes Association (ADA) dietary recommendations for 22 weeks
  • The vegan diet (10% of energy from fat, 15% protein, 75% carbohydrate) consisted of vegetables, fruits, grains and legumes and participants were not given an energy intake restriction
  • The ADA diet (15% to 20% protein, less than 7% saturated fat, 60% to 70% carbohydrate and monounsaturated fats) was individualized based on body weight and plasma lipid concentrations.

Statistical Analysis

  • Two sample T-tests were conducted to determine whether outcomes were significantly different between groups and paired sample T-tests were used to examine within-group differences
  • ANOVA was used to examine differences in changes in variables among the three tertiles of change in GI
  • The Tukey's test was used for post hoc analyses among the three groups
  • Intention-to-treat analysis was conducted by bringing baseline values forward in cases where participants did not complete 22-week assessments
  • A linear regression model was analyzed with weight loss as the dependent variable and GI as an independent variable, adjusting for age, race, sex, group assignment and changes in energy intake, fiber, carbohydrate, fat and steps per day
  • SPSS for Windows, Version 17.0 was used for the analyses with a P-value of 0.05 used to indicate significant differences.
Data Collection Summary:

Timing of Measurements

Measurements were made at baseline, 11 weeks and 22 weeks.

Dependent Variables

  • Body weight: Methods described elsewhere
  • HbA1c: Methods described elsewhere.

Independent Variables

  • Participants were randomly assigned to follow a low-fat, low-GI vegan diet or individualized diets based on the 2003 American Diabetes Association (ADA) dietary recommendations for 22 weeks
  • The vegan diet (10% of energy from fat, 15% protein, 75% carbohydrate) consisted of vegetables, fruits, grains and legumes and participants were not given an energy intake restriction
  • The ADA diet (15% to 20% protein, less than 7% saturated fat, 60% to 70% carbohydrate and monounsaturated fats) was individualized based on body weight and plasma lipid concentrations
  • Glycemic index and glycemic load: Collected using three-day weighted food records that were analyzed using Nutrition Data System for Research (NDS-R) software version 5.0
  • All foods and beverages, including alcohol, were included in the GI and GL calculations
  • Results were totaled for each day and and averaged for the three days of dietary recording. The GI and GL values were obtained from NDS-R calculations. 

Control Variables

Physical activity was assessed over a three-day period by using a pedometer and with the Bouchard three-day physical activity record.
Description of Actual Data Sample:
  • Initial N: 1,049 individuals initially screened; 99 met the participation criteria and were randomly assigned to either the Vegan (N=49) or ADA (N=50) Group
  • Attrition (final N): A total of 99 included in an intent-to-treat analysis (49 in the Vegan Group, 50 in the ADA Group)
  • Age: Mean age in Tertile One of change in GI, 53.7 years: in Tertile Two of change in GI, 59 years; in Tertile Three of change in GI, 53.9 years; all based on Tertile One of GI change (GI at least zero), Tertile Two (-0.001 to -6.46) and Tertile Three (at least 6.47).


  • Tertile One of Change in GI
    • Black non-Hispanics: 47%
    • White: 38%
    • White Hispanic: 10%
    • Asian non-Hispanic: 5%.
  • Tertile Two of Change in GI
    • Black non-Hispanics: 30%
    • White: 55%
    • White Hispanic: 4%
    • Asian non-Hispanic: 11%.
  • Tertile Three of Change in GI
    • Black non-Hispanics: 53%
    • White: 41%
    • White Hispanic: 3%
    • Asian non-Hispanic: 3%.

Other Relevant Demographics
63% female in Tertile One of change in GI, 59% female in Tertile Two and 56% female in Tertile Three.


  • Mean BMI (kg/m2)
    • Tertile One of change in GI: 34.8
    • Tertile Two of change in GI: 34.6
    • Tertile Three of change in GI: 35.1.
  • Baseline characteristics, measurements, or nutrient intakes also did not differ between diet groups
  • There were no differences in baseline demographic characteristics among those in the tertiles of GI change.

Washington, DC.

Summary of Results:

Key Findings

  • Body weight was reduced by 3.7±3.8kg in Tertile One, 4.8±4.4kg in Tertile Two and 7.0±4.7kg in Tertile Three (P<0.01)
  • The Vegan Diet Group reduced their GI (-5.4±8.2) to a greater extent than did the ADA Group (-1.7±8.6) (P=0.03), whereas the reduction in GL was greater in the ADA Group (-37.4±52.9) than the Vegan Group (9.5±56.2) (P<0.001)
  • GI predicted changes in weight (P=0.001), adjusting for changes in fiber, carbohydrates, fat, alcohol, energy intake, steps per day, group and demographics, such that for every point decrease in GI participants lost 0.2kg (0.44 lbs)
  • GI was not a predictor for changes in HbA1C after controlling for weight loss (P=0.33)
  • Weight loss was a predictor of changes in HbA1C (P=0.047)
  • GL was not related to weight loss or changes in HbA1C.
Author Conclusion:
  • Consumption of low-GI foods, but not a low-GL diet, appears to be one of the determinants of success of vegan or ADA diets in reducing HbA1c and body weight
  • Additional studies to lower the dietary GI further than was accomplished in the present study by specifically incorporating low-GI foods such as peas, beans and lentils would be of interest to examine the extent to which they can demonstrate further improvements in the metabolic advantages of therapeutic diets.
Funding Source:
Government: Grant #R01 DK059362-01A2 from the National Institute of Diabetes and Digestive and Kidney Diseases and by the Diabetes Action Research and Education Foundation
Reviewer Comments:
  • Some study details reported elsewhere
  • This study has several strengths including controlling for fiber intake, an intention-to-treat design and applicability outside the research setting
  • A study limitation is that the effect of GI or GL on glycemic control was not the primary objective of the study
  • Other limitations include the reliance on self-reported dietary intake.
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? ???
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? No
  2.2. Were criteria applied equally to all study groups? ???
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? Yes
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? N/A
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? Yes
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? Yes
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? No
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) Yes
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? N/A
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? Yes
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? Yes
  6.6. Were extra or unplanned treatments described? Yes
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? Yes
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? ???
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? ???
  7.5. Was the measurement of effect at an appropriate level of precision? ???
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? Yes
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes