• Increase Font Size
  • Decrease Font Size
  • View as PDF

DLM: Vitamin E (2001)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:

To assess the association between vitamin E intake and the incidence of major coronary disease events in the Nurses' Health Study.

Inclusion Criteria:

87,245 female nurses, 34 years to 59 years of age and free of diagnosed cardiovascular disease (CVD) and cancer, completed a diet questionnaire in 1980 and every two years thereafter for eight years to evaluate diet and supplement use.

Exclusion Criteria:
  • Women who left 10 or more items blank on the questionnaire (4%)
  • Women whose food scores were implausible (2.7%)
  • Women who had a history of cancer (except non-melanoma skin cancer), angina, myocardial infarction, stroke or CVD.
Description of Study Protocol:

Recruitment

Participants in the Nurses' Health Study using follow-up dietary questionnaires.

Design

Cohort study.

Statistical Analysis

  • Person-months were allocated according to exposure variables on the most recent questionnaire until death or an end-point was reached
  • Stratified analyses adjusted for age and one other variable
  • To adjust simultaneously for multiple risk factors, proportional-hazard models were used, with vitamin E supplements used as a time-dependent variable
  • P-values were two-tailed.

 

Data Collection Summary:

Outcome Measures

CVD-medical records and death records:

  1. Myocardial infarction diagnosis: World Health Organization symptoms, typical electrocardiogram changes, ­ cardiac enzymes
  2. Coronary artery surgery (bypass or angioplasty)
  3. Strokes: Ischemic or hemorrhagic.
Description of Actual Data Sample:

87,245 female nurses, 34 years to 59 years of age and free of diagnosed CVD and cancer, completed a diet questionnaire in 1980 and every two years thereafter for eight years to evaluate diet and supplement use.

Summary of Results:
  • Women in the highest quintile compared with the lowest quintile for vitamin E intake had relative risk of major coronary disease of 0.66 (95% confidence interval, 0.50-0.87) after adjustment for age and smoking
  • Length of supplement use: Users of vitamin E supplement for fewer than two years had no significant reductions in risk for coronary disease
  • Length of supplement use: Women taking vitamin E for more than two years had relative risk of major coronary disease of 0.59 (95% confidence interval, 0.38-0.91) after adjustment for age, smoking status, risk factors for coronary disease and use of other antioxidant nutrients. These same women had a decrease in risk of 41%.
  • Dosing: Vitamin E 100 IU to 250 IU per day had a relative risk of 0.56 (95% confidence interval, 0.21-1.51)
  • Dosing: Vitamin E 300 IU to 500 IU per day had a relative risk of 0.56 (95% confidence interval, 0.33-0.96).
Author Conclusion:

The use of vitamin E supplements in middle-aged women is associated with reduced risk for coronary heart disease.

Funding Source:
University/Hospital: Northwestern Medical School, Loyola Medical Center, University of Melbourne (Australia), Saint Mary's Hospital (London), Akademisch Ziekenhuis St. Rafael Leuven (Belgium), University of Pennsyvania Medical School, University College London,Tulane University
Reviewer Comments:

No differentiation for the type of vitamin E used.

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) 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? 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? 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? 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.) 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%.) N/A
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? ???
  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.) N/A
  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? 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? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? No
  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? 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? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? N/A
  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)? No
  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? ???
  10.2. Was the study free from apparent conflict of interest? Yes