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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:

The research purpose was to determine if a higher intake of dietary fats would be associated with a lower insulin sensitivity.

 

Inclusion Criteria:

1. 40 to 69 yr of age

2. African-American, Hispanic or non-Hispanic white.

3. Normal glucose tolerance status or impaired glucose tolerance

Exclusion Criteria:
1. Previously diagnosed type 2 diabetes mellitus
Description of Study Protocol:

Recruitment: The Insulin Resistance Atherosclerosis Study (IRAS) was conducted in 4 locations in 3 states.  Subjects were recruited between October 1992 and April 1994.  Subjects were recruited with a goal of including an equal distribution of ethnic groups, sex and age among the 4 clinical centers.

Design:

Cross-sectional analysis of OGTT results, insulin sensitivty measured by FSIGT, anthropometrics (height, weight, waist and hip circumference), physical activity, and dietary intake.

Blinding used (if applicable):  Not applicable

Intervention:  Not applicable

Statistical analysis:

Regression assumptions were evaluated in their data by using the rank transformation and log transformation. A regression model was constructed with total dietary fat as the independent variable with adjustment for potential confounders. Each type of dietary fat (g/d) was included in separate multiple-linear-regression models as the independent variable of interest, with adjustment for energy intake using total energy/d as a covariate.  To evaluate whether associations between dietary fat and insulin sensitivity were independent of obesity, BMI and WHR were then added to the models.

Data Collection Summary:

Timing of Measurements

  • 2-hr, 75 g glucose OGTT to classify glucose status; those taking oral hypoglycemic agents at study entry were classified as diabetic.
  • Insulin sensitivity: 50% glucose solution (0.3 g/kg body weight) andregular human insulin (0.03 U/kg) were injected at 0 and 20 minutes; blood samples were taken at –5, 2, 4, 8, 19, 22, 30, 40, 50, 70, 100 and 180 minutes; insulin sensitivity was calculated comparing the time course of glucose and the plasma insulin values.
  • anthropometrics: height, weight, waist and hip circumference.
  • medical history and health habit interviews

Dependent Variables:

  • Insulin sensitvity as measured by an intravenous glucose tolerance test

Independent Variables:

  • Dietary Fats
  • physical activity: Questionnaire: usual frequency of vigorous activities and interviews to obtain detailed physical activities which were converted to METs.
  • dietary intake: NCI-Health Habits and History Questionnaire was modified to include regional and ethnic foods
Control Variables:
  • Obesity
  • Energy/day
  • Sex
  • Ethnicity
  • Smoking status
  • Self-report of a special diet
  • Alcohol intake

Description of Actual Data Sample:

Initial N:  1625

Attrition (Final N):  1173 subjects, 55% women

Age:  40-69 years

Ethnicity:  39% non-Hispanic white, 33% Hispanic, 28% African American (n=1173)

Other relevant demographics:

55% women; 57% had normal glucose tolerance, 28% had IGT, 15% had previously diagnosed undiagnosed NIDDM; 30% were sedentary and 52% were classified as obese using the National Center for Health Statistics cutpoints

Anthropometrics:  52% were obese; obese had a mean BMI of 33.0

Location:

Recruited from clinical centers in Los Angeles and Oakland, CA; San Luis Valley, CO; and San Antonio, TX

Summary of Results:

 Insulin sensitivity measured by the FSIGT according to glucose tolerance status, 1992-1994

Other Findings:

Mean dietary intake:

1888 + 822 kcal

34.7% fat

12.2% saturated fat

6.7% PUFA

12.8% oleic acid

Percentage of dietary intake from fat was associated with rank of insulin sensitivity (r = - 0.06, P = 0.03) with log fasting insulin (r = 0.10, P <0.001) and with BMI (r = 0.10, P<0.001).

Higher fat intake was associated with lower insulin sensitivity in the obese (ß = -1.40, P = 0.03) but not among the nonobese (ß = 0.16, P = 0.80).

Insulin sensitivity (Si) by quintile of percentage of energy from total dietary fat, 1992-1994  

Quintile Fat Si Si (rank transformed)
  % of energy (min-1 x pmol-1 x L) 10-5 (min-1 x pmol-1 x L) 10-5
1 (n=230) 10.0-28.7 3.6±22.72 621.2 ± 333.2
2 (n=235) 28.8-33.5 3.5 ± 18.5 617.2 ± 332.8
3 (n=239) 33.6 -36.9 3.3 ± 23.7 578.1 ± 351.9
4 (n=236) 37.0-40.7 2.8 ± 22.3 550.2 ± 331.5
5 (n-233) 40.7-58.0 3.2 ± 23.5

569.2 ± 340.0

 

 

Glucose Tolerance Status Insulin Sensitivity
  (min-1 x pmol-1 x L) 10-5
Normal glucose tolerance (n=667) 4.4 (0-23.6)
Impaired glucose tolerance (n=330) 2.15 (0-13.7)
Previously undiagnosed NIDDM2 (n=176) 1.13 (0-16.2)
All (n=1173) 3.27 (0-23.6)

Author Conclusion:

Among individuals already at increased risk for insulin sensitivity due to obesity and perhaps a sedentary lifestyle, intakes of dietary fat, may worsen insulin sensitivity. This effect may be largely due to a mediating role of obesity in response to habitual consumption of a high fat diet.

Funding Source:
Government: NIH,NHLBI
Reviewer Comments:
These findings from this study are interesting but a cause and effect cannot be shown by this study. A clinical intervention trial is needed to determine if dietary fat intake or obesity is the primary cause of insulin resistance.
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? 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? 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) N/A
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  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? Yes
  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.) Yes
  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? No
  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.) No
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? No
  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? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? Yes
  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? Yes
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  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? N/A
  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? Yes
  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