HTN: Vitamins (2007)
Citation:
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
To examine the relation of serum antioxidant vitamins, including vitamins A, C and E, alpha-carotene and beta-carotene to blood pressure.
Inclusion Criteria:
- Civilian noninstitutionalized US population participating in NHANES III
- At least 20 years old.
Exclusion Criteria:
- Not non-Hispanic white, non-Hispanic black or Mexican
- Lack of blood pressure data
- Lack of serum antioxidant vitamin levels
- Those on antihypertensive medications were included however their data was excluded from the linear regression analysis for association of serum antioxidants with blood pressure.
Description of Study Protocol:
Recruitment
Those enrolled in the NHANES III between 1988 and 1994.
Design
- A stratified multistage probability design used to obtain a respresentative sample of general US population
- Oversampling of the very young, the elderly, non-Hispanic blacks and Mexican-Americans
- Standardized home interview followed by detailed physical examination in a mobile exam center
- Blood pressure (BP) was measured three times during the home interview and during mobile center exam; mean of all systolic and of all systolic readings was used for each individual
- Hypertension was defined as systolic BP at least 140 or diastolic BP at least 90mm Hg
- Blood samples were collected at second visit.
Blinding Used
Not discussed.
Intervention
None.
Statistical Analysis
- Statistical signicance of differences was analyzed by Student's T-test (continuous variables) and chi-squared test (categoric variables)
- Analysis of association of serum level of vitamins and supplement use or dietary intake was assessed by Pearson correlation coefficients (R)
- Linear regression analysis was used to assess the association between serum antioxidant vitamin levels and BP
- Logistic regression analysis was used to assess the relation between serum antioxidant vitamin levels and odds of hypertension
- Regression coeffeicients or odds ratios are reported as the difference in BB (mm Hg) or odds of hypertension associated with a one-SD increment in serum antioxidant vitamin levels
- Two models were used for analysis: one adjusted for age, gender and race; one adjusted for age, gender, race, education, alcohol consumption, BMI, dietary intake of sodium, potasium, saturated fat and energy.
Data Collection Summary:
Timing of Measurements
Cross-sectional between 1988 and 1994.
Dependent Variables
- Variable One: Systolic BP (mean of six measures using standard protocol)
- Variable Two: Diastolic BP (mean of six measures using standard protocol)
- Variable Three: Hypertension (defined as systolic BP at least 140 or diastolic BP at least 90mm Hg).
Independent Variables
Serum levels of vitamin A, alpha-carotene, beta-carotene, vitamn C, vitamin E (measured using isocratic HPLC).Control Variables
- One analysis model adjusted for age, gender and race
- One analysis model adjusted for age, gender, race, education, alcohol consumption, BMI, dietary intake of sodium, potassium, saturated fat and energy.
Description of Actual Data Sample:
- Initial N: 18,825 persons at least 20 years old, who participated in NHANES III.
- Attrition (final N): 15,317 (N=10,457 normotensives, 4,860 hypertensives; 48% normotensive males, 47.7% hypertensive males).
- Age: At least 20 years old; 40.1±0.38 years (mean±SE) for normotensives, 60.3±0.50 years for hypertensives (P<0.001)
- Ethnicity: Non-Hispanic blacks, non-Hispanic whites, Mexican-American
- Other relevant demographics: 79.8% high school graduates for normotensives, 65.4% high school graduates for hypertensives (P<0.001)
- Location: United States.
Summary of Results:
Descriptive Results
- Normotensives were 20 years younger than hypertensives, P<0.001
- Normotensives had a lower proportion of non-Hispanic blacks, P<0.001
- Normotensives had a higher proportion of high school graduates, P<0.001
- Diabetes was higher in hyperensives, P<0.001
- Normotensives had a lower BMI than hypertensives, P<0.001
- Mean serum levels of vitamin A, C, E and bta-carotene were lower in normotensives, P≤0.008.
Serum Vitamins, 1 SD |
Multivariate-Adjusted |
P |
Vitamin C, 0.47 mg/dL |
0.98 (0.91-1.07) |
0.7 |
Vitamin E, 496 microgrm/dL |
1.18 (1.09-1.27) |
<0.001 |
- Adjusted for age, gender, race and ethnicity, education, alcohol consumption, BMI, hx of diabetes, dietary intake of sodium, potassium, saturated fat and total energy
- A one-SD difference in the level of serum vitamin E (496micrograms per dL) is positively associated with 18% increased odds of having hypertension
- After adjusting for all confounding factors, serum vitamin C was not significantly associated with odds for hypertension, P=0.7.
Effect of Serum Vitamins C and E on SBP and DBP: Multiple Linear Regression Analysis
Serum Vitamin, 1 SD |
SBP, mm Hg Effect (95% CI) |
P |
DBP, mm Hg Effect (95% CI) |
P |
Vitamin C, 0.47 mg/dL |
-0.10 (-0.52, 0.32) |
0.6 |
-0.37 (-0.62, -0.13) |
0.003 |
Vitamin E, 496 microgrm/dL |
0.66 (0.22, 1.11) |
0.004 |
0.84 (0.59, 1.08) |
<0.001 |
- Participants on antihypertensive medications were excluded. N=12, 102 for multivariate analysis adjusted for age, gender, education, alcohol consumption, BMI, hx of diabetes, dietary intake of sodium, potassium, saturated fat and total energy
- Vitamin E was positively associated with both systolic and diastolic BP, whereas vitamin C was inversely assoicated with both systolic and diastolic BP
- Vitamin A was positively associated with both systolic and diastolic BP and the carotenoids were inversely associated with both systolic and diastolic BP (data not shown here).
Other Findings
- A one-SD difference in serum vitamin A was positively associated with a 43% increased odds of having hypertension, P<0.001
- Multivariate-adjusted differences in systolic and diastolic BP associated with a one-SD in level of serum vitamin among specified subgroups by age, gender, race and ethnicity and hypertension were analyzed
- Serum vitmain C was inversely associated with diastolic BP in those who were younger than 60 years, in men and women and in hypertensives and normotensives
- Serum vitamin C was inversely associated with both systolic and diastolic BP among blacks, P<0.05.
- Serum vitamin E was positively associated with both systolic and diastolic BP among most of the subgroups (age, gender, race, hypertension status) as analyzed by multivariate-adjusted differences, P<0.05 overall.
Author Conclusion:
- A significant and postive relation between serum levels of vitamin A and E and risk of hypertension independent of age, gender, race and other risk factors was found
- A significant inverse relation between serum vitamin C and diastolic BP only was identified
- Apha- and beta-carotene levels were also inversely associated with BP. These relationships were stronger in normotensive and younger participants.
- The cross-sectional study design does not allow inferences to be made regarding causality between antioxidant vitamins and BP.
- Authors suggest that antioxidant vitamins may play an important role in the underlying cause and prevention of hypertension.
Funding Source:
Government: | NHLBI, NIH |
Reviewer Comments:
Physical activity not included as a confounding variable.
Quality Criteria Checklist: Primary Research
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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) | Yes | |
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.) | 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? | 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? | 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? | Yes | |
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)? | N/A | |
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? | No | |
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 | |