DLM: Vitamin E (2001)

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

To provide epidemiologic evidence of a link between dietary antioxidants and reduced risk of atherosclerosis.

Inclusion Criteria:

Male health professionals who participated in the Health Professionals Follow-Up Study.

Exclusion Criteria:
  • Men whose energy intake was outside of the range of 800 to 4,200 kcal
  • Men who left blank 70 or more questions on the dietary questionnaire
  • Men who reported in the baseline questionnaire that they had myocardial infarction, angina, stroke, coronary artery bypass graft, angioplasty, diabetes or hypercholesterolemia.
Description of Study Protocol:

Recruitment

Male health professionals from the Health Professionals Follow-Up Study.

Design

Cohort study.

Statistical Analysis

  • Relative risks were calculated by dividing the incidence rate of coronary artery disease among the men in each category of antioxidant intake by the rate for the men in the lowest category
  • Adjusted relative risks for age (in five-year categories) were derived by the Mantel-Haenszel method
  • Mantel extension test was used to test for linear trends
  • Multiple logistic regression to generate odds ratios as an estimate of relative risk
  • In multivariate logisitc models, the researchers tested for significant trends by assigning each participant the median value for the category and modeling the value as a continuous variable
  • All P-values are two-sided
  • Pearson correlation coefficients between the diet records and the dietary questionnaire were adjusted for total dietary intake and for within-person variability in reported intake.
Data Collection Summary:

Baseline Data

  • Participants completed a diet and medical history questionnaires in 1986, 1988 and 1990
  • Outcome measures were CVD, which was obtained from medical records and death records
    • MI diagnosis using WHO criteria, typical EKG changes, ­ cardiac enzymes
    • Coronary artery surgery (bypass or angioplasty)

 

Description of Actual Data Sample:

39,910 US male health professionals, 40 to 75 years of age and free of diagnosed CHD, diabetes or hypercholesterolemia.

Summary of Results:
  • The men in the highest quintile group for Vitamin E intake had an age-adjusted RR of coronary disease of 0.59 (95% CI, 0.47 to 0.75; P=0.001)
  • For a high intake of carotene, the age-adjusted relative risk of coronary disease was 0.71 (95% CI, 0.55 to 0.92; P=0.02)
  • A high intake of Vitamin C showed an initial slight inverse association (age-adjusted RR of 0.83; 95% CI, 0.64 to 1.08). Further adjustment for risk factors and the use of other antioxidants brought the RR was 1.25 (95% CI, 0.91 to 1.71; P=0.98)
  • RR for men consuming more than 60 IU vs. less than 7.5 IU vitamin E per day was 0.64 (95% CI, 0.49 to 0.83; P<0.003)
  • RR for men taking less than 100 IU vitamin E daily for more than two years compared to men not taking vitamin E was 0.63 (95% CI, 0.47 to 0.84)
  • Increased intake of antioxidants, primarily as Vitamin E, and as carotene and Vitamin C in former and current smokers, is associated with a reduced risk of coronary disease
  • The lack of significant association of coronary disease with Vitamin C in the diet or as supplements suggests a causal relationship
  • Men with a higher Vitamin E intake have somewhat healthier profiles
  • After adjustment for Vitamin E, Vitamin C was not associated with a reduction in coronary disease
  • The researchers found a moderate reduction in the risk of coronary disease with increasing intake of nonsupplemental dietary Vitamin E.
Author Conclusion:

Supplemental Vitamin E may reduce the risk of coronary disease. 

 

Funding Source:
University/Hospital: Harvard School of Public Health, Departments of Epidemiology and Nutrition; Harvard Medical School; Brigham and Women's Hospital
Reviewer Comments:
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? N/A
  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? N/A
  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) 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.) 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.) 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? N/A
  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.) 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? 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? 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? 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? No
  10.2. Was the study free from apparent conflict of interest? Yes