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HTN: Fiber (2007)

Study Design:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
To investigate the effect of a 6-month primary care intervention to increase fruit and vegetable consumption on plasma concentrations of antioxidant vitamins, daily fruit and vegetable intake, and blood pressure in a healthy general population with a wide range of eating habits.
Inclusion Criteria:
  • Healthy participants from a primary-care health center
  • Aged 25 - 64 years
  • Without serious illnesses
Exclusion Criteria:
  • Cardiovascular diseases other than HTN
  • GI disease
  • Cancer
  • Serious psychiatric disorders
  • Hypercholesterolemia
  • Patients who had undergone a recent traumatic event such as bereavement
  • Those unable to give consent
  • Using dietary supplements
  • Pregnant or trying to conceive
Description of Study Protocol:


Patients recruited from lists of 2 general practices based in a health center.  Letters were sent out to patients from August 1997 - September 1998.


Randomized Controlled Trial.  Randomized using computer-generated randomization list.

Blinding used (if applicable)

Not possible - lab tests.

Intervention (if applicable)

Negotiation method to encourage an increase in fruit and vegetable consumption to at least 5 daily portions.

Statistical Analysis

Power calculation based on previous antioxidant data.  Modified intention-to-treat analysis in which patients who were assigned but did not attend initial appointment were excluded.  Comparisons between proportions were made with chi-square and between means with t test.  Adjustment for covariates done with multiple regression analysis.  Differences in outcome between intervention group and controls are shown with 95% confidence intervals.

Data Collection Summary:

Timing of Measurements

Participants attended 2 appointments at baseline and at 6 months with trained nurse.  2 weeks after initial intervention, research nurse telephoned to reinforce message.  Letter sent at 3 months. 

Dependent Variables

  • Blood pressure taken by research nurses using electronic automatic sphygmomanometer
  • Height and weight using stadiometer and scales
  • Blood samples analyzed for antioxidant vitamins and total cholesterol concentrations
  • Fruit and vegetable intake assessed by DINE food frequency questionnaire

  • Overall intake assessed by Eating Pattern Assessment Questionnaire

Independent Variables

  • Negotiation method to encourage an increase in fruit and vegetable consumption to at least 5 daily portions.

  • Controls received intervention after 6 months

Control Variables

  • Baseline antioxidant concentrations
  • Sex
Description of Actual Data Sample:

Initial N:  1045 of 2302 responded.  729 assigned to groups. 

Attrition (final N):  690 healthy participants

Age:  mean age Intervention Group:  45.7 +/- 10.1 years, Controls:  46.0 +/- 10.1 years

Ethnicity:  Practices had few patients from ethnic minorities

Other relevant demographics:

Anthropometrics:  There were substantially more men in the intervention group than controls, but groups were similar in age, social class and BMI.

Location:  Thame, Oxfordshire, United Kingdom.


Summary of Results:


Variable Between-Group Difference in Change Adjusted Difference in Change P for Adjusted Differences
Alpha-carotene 0.007  0.009 0.027





Lutein 0.018 0.018 0.032
Lycopene -0.010 -0.013 0.29
Beta-cryptoxanthin 0.047 0.050 0.0002
Retinol 0.001 -0.008 0.73
Ascorbic Acid 1.91 2.38 0.023
Alpha-tocopherol 0.117 0.093 0.76
Alpha-tocopherol adjusted for cholesterol 0.034 0.036 0.49
Gamma-tocopherol -0.038 -0.023 0.63
Gamma-tocopherol adjusted for cholesterol -0.005 0.001 0.91
Total cholesterol 0.018 0.010 0.86
Self-reported daily intake of fruits and vegetables 1.3 1.4 <0.0001 
SBP 3.4 4.0 <0.0001
DBP 1.4 1.5 0.02





Other Findings

Plasma concentrations of alpha-carotene, beta-carotene, lutein, beta-cryptoxanthin, and ascorbic acid increased by more in the intervention group than in controls (significance ranging from p = 0.032 to 0.0002).

Groups did not differ for changes in lycopene, retinol, alpha-tocopherol, gamma-tocopherol, or total cholesterol concentrations.

Self-reported fruit and vegetable intake increased by a mean of 1.4 +/- 1.7 portions in the intervention group and by 0.1 +/- 1.3 portion in the control group (between group difference of 1.4, 95% CI:  1.2 - 1.6, p < 0.0001).

SBP fell more in the intervention group than in controls (difference of 4.0 mm Hg, 95% CI:  2.0 - 6.0, p < 0.0001) as did DBP (1.5 mm Hg, 95% CI: 0.2 - 2.7 mm Hg, p = 0.02). 

Author Conclusion:
Our results show that a primary care intervention can increase self-reported fruit and vegetable intake, raise plasma concentrations of alpha-carotene, beta-carotene, lycopene, beta-cryptoxanthin and ascorbic acid; and result in significant decreases in systolic and diastolic blood pressure.  The effects of the intervention would be expected to reduce cardiovascular disease in the general population.
Funding Source:
Foundation associated with industry:
Reviewer Comments:
Large sample size.  Self-reported dietary intake of fruits and vegetables.
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? 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.) 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? No
  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? Yes
  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? 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? Yes
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