HTN: Fiber (2007)

Study Design:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
To investigate the effect of dietary water soluble fiber from oat bran intake on blood pressure (BP) in persons with higher than optimal BP level.
Inclusion Criteria:
  • Mean SBP from 125mm Hg to 159mm Hg, with DBP under 95mm Hg
  • Mean DBP from 75mm to 94mm Hg, with an SBP under 160mm Hg
  • Otherwise healthy adult men and women.
Exclusion Criteria:
  • Current HTN defined as average SBP at least 160mm Hg, DBP at least 95mm Hg, use of antihypertensive medication or medication that affects BP
  • Current hypercholesterolemia defined as fasting serum cholesterol at least 240mg per dL or use of cholesterol-lowering medication
  • Current diabetes defined as fasting serum blood glucose at least 140mg per dL or use of insulin or oral hypoglycemic agents
  • Severe obesity defined as BMI at least 35
  • History of CVD diseases including CHD, CHF and stroke
  • History of chronic renal failure or serum creatinine at least 2.0mg per dL
  • Peptic ulcer disease requiring treatment during the previous two years
  • Cancer (escept non-melanoma skin cancer) requiring treatment during the past year
  • Consumption of over 21 alcoholic beverages per week
  • Vegetarian diet or current use of fiber supplements
  • Prior assignment to an intervention group in a BP-lowering trial
  • Poor compliance during a run-in phase defined as intake under 85% of the assigned study foods.
Description of Study Protocol:


Subjects were recruited from the New Orleans area by worksite and community-based BP screenings, mass mailing of brochures and radio, telvision and newspaper advertisements.


  • Randomized double-blind, placebo-controlled trial
  • Subjects were screened, asked to participate in a two-week run-in period and randomized to either the intervention or control group
  • The intervention lasted for 12 weeks.   

Blinding Used

  • Data collectors were blinded to group assignment and the control group received a placebo
  • Study participants and staff were blinded to the dietary fiber sources
  • Intervention foods were provided in coded, labeled packages.


  • Intervention Group (high-fiber): Received daily servings of 60g (approximately 3/4 cup) of Quaker oat bran concentrate as a muffin and 84g (approximately 1.5 cups) of Quaker Oatmeal Squares for 12 weeks
  • Control Group (low-fiber): Received daily servings of 93g (approximately 3/4 cup) of refined wheat as a muffin and 42g (approximately 1.5 cups) of Kellogg's Corn Flakes.

Statistical Analysis

  • Baseline characteristics were compared between groups using Student's T-tests for continuous variables and chi-square tests for categorical variables
  • Mean changes in SBP and DBP from baseline were compared between groups using two-way ANOVA for repeated measures
  • Changes from baseline at six weeks and at 12 weeks were analyzed
  • Age, sex, race, baseline BP, fiber intake, weight and change in weight were adjusted for. 
Data Collection Summary:

Timing of Measurements

  • Data was collected at three screening visits, during a run-in visit, at six weeks into the intervention and at 12 weeks
  • Body weight was monitored bi-weekly during the study.

Dependent Variables

  • Change in SBP at six weeks and at 12 weeks
  • BP was measured after at least five minutes of rest in a seated position using a random-zero sphygmomanometer
  • A mean of three measures was used at each time point
  • Change in DBP at six weeks and at 12 weeks. 

Independent Variables

  • High-fiber supplement (muffin and cereal) containing 15.9g total fiber per day, of which 8.1g was water soluble
  • Low-fiber supplement (muffin and cereal) containing 2.7g total fiber per day, of which 0.9g was water soluble.

Control Variables

  • Age
  • Sex
  • Race
  • Baseline BP
  • Fiber intake
  • Weight
  • Change in weight.
Description of Actual Data Sample:

Initial N

  • 419 people were screened. Of these, 154 met eligibility criteria.
  • 44 potential subjects were excluded mainly due to poor compliance in the run-in period
  • 110 subjects (66 women) were enrolled in the trial. 

Attrition (final N)

  • 102 (final number of women, men not specified)
  • Baseline data of those lost to follow-up was similar to that of the remainder of the subjects.


Original 110 subjects at baseline, mean±SD

  • High-fiber group: 49.7±8.2 years, P=0.03
  • Low-fiber group: 46.1±8.6 years, P=0.03.


For the original 110 subjects at baseline, 51.9% and 53.6% of the high-fiber and low-fiber groups, respectively, were African-American

Other Relevant Demographics

For the high-fiber and low-fiber groups, respectively 82.1±16.1 and 83.6±15.7 had a college education.

Baseline Anthropometrics (Mean±SD)

Anthropometric Value High-Fiber Group Low-Fiber Group (Control) P-Value
Weight (kg) 82.1±16.1 83.6±15.7 0.6
BMI 28.5±5.0 29.3±4.1 0.4
Obesity (%) 35.2 48.2 0.2


New Orleans area.

Summary of Results:

 Mean Changes from Baseline in SBP and DBP

  High-Fiber Effect (95% CI) P-Value Low-Fiber Effect (95% CI) P-Value Net Change (95% CI) P-Value
Change in SBP (mmHg)            
6 weeks -3.5 (-5.8,-1.1) 0.005 -1.3 (-3.5,0.9) 0.2 -2.2 (-5.3,1.0) 0.18
12 weeks -3.4 (-5.4,-1.4) 0.001 -1.6 (-3.3, 0.0) 0.06 -1.8 (-4.3, 0.8) 0.17
Average (6,12 weeks) -3.4 (-5.2,-1.7) <0.001 -1.4 (-3.0, 0.2) 0.08 -2.0 (-4.4, 0.3) 0.09
Change in DBP (mmHg)            
6 weeks -2.0 (-3.5,-0.4) 0.02 -1.1 (-2.8, 0.6) 0.2 -0.8 (-3.1, 1.4) 0.47
12 weeks -2.2 (-3.3,-1.0) <0.001 -1.0 (-2.3, 0.4) 0.2 -1.2 (-3.0, 0.5) 0.17
Average (6,12 weeks) -2.1 (-3.1, -1.0) <0.001 -1.1 (-2.2, 0.1) 0.07 -1.0 (-2.6, 0.5) 0.19
  • The high-fiber supplement resulted in lower SBP and DBP values at six weeks and 12 weeks, however after adjusting for the change from baseline in the low-fiber group, there were no significant changes (net changes)
  • Net change is defined as the change from baseline in the high-fiber group minus the change from baseline in the low-fiber group
  • Also, when adjusted for age, race, sex, baseline BP, baseline fiber intake and change in weight, there was no significant effect on SBP (P=0.64) or DBP (P=0.24).

Other Findings

There was a significant gain in body weight (mean=0.7kg) at six and 12 weeks in the low-fiber group, however this disappeared when the data was adjusted for net change.

Author Conclusion:
A diet rich in fiber may have a moderate BP-lowering effect.
Funding Source:
University/Hospital: St. George's Hospital Medical School
Reviewer Comments:
  • Weight, weight change, activity, alcohol intake, smoking, diet and race were covariates in the analyses
  • A limitation of the study is that the sample size lacked sufficient statistical power to detect small differences in BP change between groups
  • The high-fiber group subjects were 3.6 years older than the low-fiber group and had higher SBP (P=0.08) at baseline
  • There was no significant change in SBP or DBP from baseline after adjustments were made for these changes in the low-fiber group
  • The changes occuring in the high-fiber group did reach significance at both the six-week and 12-week time points. 
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? No
  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? No
  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? N/A
  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%.) 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? Yes
  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? N/A
  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? Yes
  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? Yes
  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? Yes
  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? No
  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? No
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