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HTN: Fiber (2007)

Study Design:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
To examine the magnitude of the association between blood pressure and vitamin C as well as fruit and vegetable intakes.
Inclusion Criteria:
  • Participants of the German National Health Interview and Examination Survey and the affiliated German Nutrition Survey
  • Aged 18 to 79 years.
Exclusion Criteria:
  • Known HTN at present or formerly
  • Use of either blood pressure lowering or raising medication
  • Nursing mothers.
Description of Study Protocol:


Sampling from German population registeries stratified by age, gender, community size and federal state to participate in the German Nutrition Survey, a subsample of the German National Health Interview and Examination Survey.


Cross-sectional analysis of data from the German Nutrition Survey including interviews of dietary intake, socioeconomic and lifestyle data.

Statistical Analysis

  • The relationship between BP and dietary vitamin C intake, total vitamin C intake (dietary and supplements) and fruit and vegetable intake was analyzed using multiple linear regression models separately by gender
  • Additional regression analysis was used on groupings of total vitamin C and fruit and vegetable intake as follows:
    • Group One: At least 0.15g vitamin C per day and at least 600g fruits and vegetables per day for high intakes; less than 0.15g vitamin C per day with at least 600g fruits and vegetables per day for low vitamin C with high fruits and vegetables
    • Group Two: High amounts of vitamin C with low amounts of fruits and vegetables, using the above definitions
    • Group Three: Low intakes of vitamin C and fruits and vegetables.
  • Potential confounding factors including age, BMI, socioeconomic status, smoking, physical activity, alcohol and coffee intake, vegetarian diet, health-related quality of life issues, pregnancy, use of oral contraceptives, hormone replacement therapy, region, season and energy intake were adjusted for.
Data Collection Summary:

Timing of Measurements

Cross sectional study with one-time measures and interview.

Dependent Variables

  • SBP and DBP measured after three minutes of rest in a sitting position with a sphygmanometer
  • Mean SBP and DBP of the second and third measurements were used.

Independent Variables

Dietary vitamin C intake, total vitamin C intake from diet and supplements and fruit and vegetable intake assessed through comprehensive diet interviews. 

Control Variables

  • Age
  • BMI
  • Socioeconomic status
  • Smoking
  • Physical activity
  • Alcohol and coffee intake
  • Vegetarian diet
  • Health-related quality of life issues
  • Pregnancy
  • Use of oral contraceptives
  • Hormone replacement therapy
  • Region
  • Season
  • Energy intake .
Description of Actual Data Sample:
  • Initial N: 7,124 in German National Health Interview and Examination Survey; 4,030 of the 7,124 participated in the German Nutrition Survey. Overall response rate, 61.4%.
  • Attrition (final N): 1,628 women and 1,340 men provided complete information for the German Nutrition Survey
  • Age: 18 to 79 years (40.8±14.5 years for women, mean±SD; 41.6±15.0 years for men)
  • Ethnicity: Not mentioned
  • Other relevant demographics: Socioeconomic status was analyzed as a potential confounder to the results
  • Anthropometrics: BMI for women, 25.2±4.7; for men, 26.4±3.7.
  • Location: Germany.
Summary of Results:

Blood Pressure and Vitamin C and Fruit/Vegetable Intake of the Study Population

  Women (N=1,628) Men (N=1,340)
SBP, mmHg 127.2±17.6 132.8±159
DBP, mmHg 79.1±10.3 83.3±10.5
Dietary Vitamin C, mg/day 151.7±77.8 151.2±87.1
Total Vitamin C, mg/day* 174.1±140.6 171.8±143.0
Fruit&Vegetable intake, g/day 552.9±238.7 558.0±267.0

*Total vitamin C intake includes dietary and supplement intake.

Regression Coefficients for SBP of Intakes of Vitamin C as Well as Fruit and Vegetables Among Women


  Dietary Vitamin C Total Vitamin C Fruit and Vegetable Intake
Intake -7.74±4.71 -4.28±2.62 -5.38±1.55*


  • The regression model above also analyzed for age, BMI, and smoking
  • Only the association of SBP with fruit and vegetable intake was significant for women
  • A positive association of SBP and dietary vitamin C intake occurred for men, however data is not presented
  • There were no significant associations with DBP for women or men.

Regression Coefficients for SBP of Intake Groups of Vitamin C as Well as Fruit and Vegetables

  Women Men
Group One -2.33±0.89** 1.82±0.94
Group Two -2.99±1.37* -1.30±1.44
Group Three 0.71±1.02 -0.74±1.12

Group One: High intake of vitamin C, high intake of fruit and vegetables

Group Two: Low intake of vitamin C, high intake of fruit and vegetables

Group Three: High intake of vitamin C, low intake of fruit and vegetables

**P≤0.01, *P≤0.05

  • Low vitamin C and fruit and vegetables intake was used as the reference
  • After adjustment for age, BMI and smoking, women with a high vitamin C intake and a high fruit and vegetable intake had a 2.3-mm Hg lower SBP than the reference (95% CI -4.1, -0.6 mmHg)
  • Women who consumed low vitamin C but high fruit and vegetables had a 3.0-mm Hg lower SBP (95% CI -5.7, -0.3)
  • SBP was not different among the three groups of intake for men.


Author Conclusion:
  • For women, high fruit and vegetable intake was more strongly associated with lower SBP than a high vitamin C intake
  • Women with a high fruit and vegetable intake, but a low vitamin C intake had the highest difference of SBP (-3.0 mmHg), as compared with low intakes of both
  • There were no consistent associations observed for men.
Funding Source:
Government: Robert Koch Institute (Germany), institute of epidemiology (germany)
Reviewer Comments:
Age, BMI, smoking, physical activity, alcohol and coffee intake, vegetarian diet, health-related quality of life issues, pregnancy, use of oral contraceptives, hormone replacement therapy, region, season and energy intake were evaluated for potential confounding effects.
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) 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%.) 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? 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? ???
  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.) ???
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? ???
  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? 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