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DM: Prevention of Type 2 Diabetes (2007)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
To examine the influence of obesity and prevention of weight gain on the incidence of type 2 diabetes.
Inclusion Criteria:
  • Aged 25 - 64 years at enrollment
  • Randomly selected from low-income, middle-income and high-income neighborhoods in San Antonio, Texas
Exclusion Criteria:
None specifically mentioned.
Description of Study Protocol:

Recruitment

Participants from the San Antonio Heart Study, a prospective population-based study of Mexican Americans and non-Hispanic whites residing in San Antonio, Texas.  A 7-8 year follow-up began in 1987 and was completed in the fall of 1996.

Design:  Prospective Cohort Study

Blinding used (if applicable):  not applicable 

Intervention (if applicable):  not applicable

Statistical Analysis

Logistic regression was used to estimate the decrease in risk of developing type 2 diabetes associated with prevention of increases in various BMI units.  BMI values were inverse-transformed to more closely approximate a normal distribution and to improve the fit of the model to the data.  Analyses were conducted separately for Mexican Americans and non-Hispanic whites.  Chi-square tests were used to determine statistical differences in proportions and t tests were used to determine statistical differences in means.

Data Collection Summary:

Timing of Measurements

Measurements made at enrollment and after 7-8 years of follow-up.

Dependent Variables

  • Number and proportion of incident cases prevented by targeting BMI category
  • Decrease in risk of developing type 2 diabetes associated with weight gain prevention across BMI and age

Independent Variables

  • BMI category: 

    BMI was stratified into 4 categories:

    • normal (<25)
    • overweight (>25 and <30)
    • obese (>30 and <35)
    • very obese (>35)
     
  • Waist to hip ratio
  • Subscapular to triceps skinfold ratio
  • Blood chemistry measurements:  fasting plasma glucose, fasting serum insulin and total, LDL and HDL cholesterol, and triglyceride concentrations

Control Variables

  • Age
  • Family history of diabetes
Description of Actual Data Sample:

Initial N: Study initially enrolled 3301 Mexican Americans and 1857 non-Hispanic white men and nonpregnant women

Attrition (final N):  3682 (73.7% of survivors) from the 2 phases completed the follow-up.

Age:  aged 25 - 64 years at enrollment

Ethnicity:  1996 Mexican Americans and 1232 non-Hispanic whites

Other relevant demographics:

Anthropometrics:

Location:  San Antonio, Texas

 

Summary of Results:

 

BMI Category and Number of Subjects 7.5-year incidence (%) Relative Risk Proportion of Population

Proportion of Incident Cases
Mexican Americans:

 

      

<25 (n=633)

3.8

 1.0

633/1996 (31.7)

24/226 (10.6)
>25 to <30 (n=809) 10.1 2.7 809/1996 (40.5) 82/226 (36.3)
>30 to <35 (n=352) 19.3 5.1 352/1996 (17.6) 68/226 (30.1)
>35 (n=202) 25.7 6.8 202/1996 (10.1) 52/226 (23.0)
Non-Hispanic Whites:        
<25 (n=621) 1.6 1.0 621/1232 (50.4) 10/69 (14.5)
>25 to <30 (n=420) 6.4 4.0 420/1232 (34.1) 27/69 (39.1)
>30 to <35 (n=141) 16.3 10.1 141/1232 (11.4) 23/69 (33.3)

>35 (n=50)

18.0

 11.2

50/1232 (4.1)

9/69 (13.0)

Other Findings

226/1996 Mexican Americans and 69/1232 non-Hispanic whites developed type 2 diabetes at follow-up (mean 7.5 years).

Among Mexican Americans, individuals who developed type 2 diabetes were older, had a lower SES, and were more likely to live in a barrio neighborhood than those who did not develop type 2 diabetes.  They also had higher BMI at baseline, weighed more, had higher subscapular to tricep skinfold ratio, had higher fasting glucose, 2-hour glucose, fasting insulin, SBP and DBP, and triglycerides, and had lower HDL cholesterol.

Among non-Hispanic whites, individuals who developed type 2 diabetes were older and more likely to live in a transitional neighborhood compared to those who did not develop type 2 diabetes.  They also had higher BMI, were heavier, had a higher subscapular to tricep skinfold ratio, had higher fasting glucose, 2-hour glucose, fasting insulin, SBP and DBP, total cholesterol and triglycerides, and had lower HDL cholesterol.

Preventing normal individuals from becoming overweight would result in the greatest reduction in type 2 diabetes incidence.

This would result in a 62% reduction in Mexican Americans and 74% reduction in non-Hispanic whites.

Preventing the entire population from gaining 1 BMI unit on average would result in a reduction in incidence of type 2 diabetes of 12.4% in Mexican Americans and 13.0% in non-Hispanic whites. 

Author Conclusion:
We conclude that the majority of cases of type 2 diabetes were from individuals who were overweight or mildly obese and who were not from the upper end of the obesity spectrum.  Public health resources should be directed at preventing normal and overweight individuals from becoming overweight and obese in order to prevent the maximum number of type 2 diabetes cases at the population level.
Funding Source:
Government: NHLBI
Reviewer Comments:
Authors note that they did not adjust for confounders related to weight gain and diabetes, such as exercise and diet.  Measures of obesity at only 2 time points, enrollment and 7.5 years later.
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) N/A
  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) N/A
 
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? Yes
  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? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  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? ???
3. Were study groups comparable? ???
  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? ???
  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? ???
  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? N/A
  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? Yes
  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? N/A
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  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? N/A
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? N/A
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? N/A
  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? Yes
  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? ???
  7.7. Were the measurements conducted consistently across groups? N/A
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? ???
  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)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? ???
  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