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HTN: Fiber (2007)

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

To compare the effect on BP of a low-sodium, high potassium diet (LNAHK) and a high-calcium (HC) to a dash-type diet (OD) in community-dwelling subjects who selected and prepared their own foods after given dietary advice.

Inclusion Criteria:
  • > 25 years old
  • Office blood pressure measured by research staff at the center at their 2nd visit that was:
    • >= 120 mmHg systolic blood pressure (SBP)
    • OR >=80 mmHg diastolic blood pressure (DBP)
    • This was the mean of last 3 of 4 measurements, taken in 1-minute intervals
  • OR a home blood pressure that was:
    • >=116 mmHg SBP
    • >=78 mmHg DBP
    • This was a mean over 7days
  • Partners of participants not otherwise eligible for study were entered into study to be helpful in maintaining dietary compliance.
  • Patients on hypertensive medication as long as they agreed to continue current medication.
Exclusion Criteria:
  • Office blood pressure measured by research staff at the center at their 2nd visit that was:
    • o       >160 mmHg SBP
    • o       >90 mmHg DBP
  • Had a cardiovascular event on the past 6 months
  • Had insulin dependent diabetes
  • On Warfarin or Dilantin
  • Ate main meal outside the home >2xs/wk
  • Drank >30 standard alcohol drinks/wk (10 g alcohol)
  • Plans to change smoking habits
  • Unwilling to cease taking dietary supplements (including vitamins)
Description of Study Protocol:


Participants recruited by newspaper article advertisement. Also through BP measurement sessions at workplaces and shopping centers.

  • Written informed consent provided
  • Consent form approved by the Deakin University Human Research Ethics Committee


Randomized Crossover Trial.

  • Control diet between each test diet period to minimize interference with time-dependant reductions in blood pressure.
    • 4 randomized groups, each to test 2 of the diets
      • Group 1: Diet 1, DASH-type diet; Diet 2, LNAHK
      • Group 2: Diet 1, LNAHK; Diet 2, DASH-type diet
      • Group 3: Diet 1, DASH-type diet; Diet 2, high calcium diet
      • Group 4: Diet 1, high calcium; Diet 2, DASH-type diet
    • Trial time line

1 week

2 weeks

4 weeks

2 weeks

4 weeks


Control diet

Diet one

Control diet

Diet two

Control diet similar to DASH (Dietary Approaches to Stop Hypertension) trial control diet

    • Low calcium, potassium and magnesium
    • Presumed to be higher in this study because Australians eat more fruits and vegetables.  Macronutrient profile and fiber similar to average Australian diet.


  • Subjects randomized to 1 of 4 groups (above)
  • Performed by the chief investigator
  • Random number generator blocks of 8 (Excel, Microsoft Corporation, 2000)

Blinding Used (if applicable):  not possible - lab tests

Intervention (if applicable):

Control diet

  • Low in calcium, magnesium, potassium
    • Instructed to consume a maximum of the following daily:
    • 1 serving dairy
    • 1.5 servings fruit/fruit juice
    • 3 servings vegetables
    • limit potatoes to 3 servings
    • limit tomatoes to 2 servings per week
    • limit fish to 1 serving per week
    • usual intake of fats and salt

Intervention diets

  • DASH-type diet
    • Based on U.S. DASH study
    • High in potassium, magnesium, fiber; lower in saturated fat, sodium
    •  Advised to avoid butter, added table/cooking salt, and salty foods; and to use poly- or mono- unsaturated margarine (low sodium version provided)
    • Recommendations:
      • Daily:
        • >= 3 servings of low fat dairy
        • >= 8 servings of fruit and vegetables daily (3-4 fruit; 4-5 vegetables)
      • Weekly:
        • >= 3 servings of fish
        • >= 1 serving legumes
        • >= 4 servings unsalted nuts and seeds
        • limit red meats to <=3 servings
  • Low sodium high potassium diet (LNAHK)
    • Lower in calcium and sodium
    • No specific recommendations for dairy, fish, red meat, or use of poly-mono-unsaturated fats.
    • Salt-free bread and margarine were provided
    • Advised to avoid added table/cooking salt and salty foods
    • Recommendations:
      • Daily
        • >=4 servings fruit
        • >=5 servings vegetables
      • Weekly
        • >=2 servings legumes
        • >=4 servings unsalted nuts and seeds
  • High calcium diet
    • >= 4 servings of low-fat dairy products
    • Maximum of 40g/day of reduced fat cheese
    • No other dietary advice provided

Throughout all diet periods, subjects allowed a maximum of 4 caffeine-containing drinks and 2 standard alcoholic drinks per day.

Statistical Analysis

Descriptive statistics and regression analysis

  • Means ± SD
    • Used to describe the distribution of variables
  • Means ± SEM
    • Used when comparisons of group were made
  • Change in blood pressure
    • Subtracting mean value of test diet period from the mean value of the previous control diet period
    • The difference in changes of blood pressure between test periods were analyzed with 2-tailed paired t tests
      •        P = 0.05 considered significant
    • Multivariate regression analysis- to determine predictors of blood pressure change
Data Collection Summary:

Timing of Measurements

Weight, office blood pressure and dietary counseling every 2 weeks.  Blood pressure taken daily during control diet period and taken daily during last 2 weeks of test diet period.  24-hour urine collections every 2 weeks (compliance) and fasting (10 hour) blood samples at the end of each control diet and test diet period.

Dependent Variables

  • Home monitored blood pressure - participants trained:
    • to correctly apply the cuff
    • alone at the same time each day
    • after a 5 minute rest in a quiet room
    • 3 measurements with a 1 minute interval
  • Urine collections
  • Blood samples analyzed for serum total cholesterol, HDL-cholesterol, and triglycerides
  • LDL-calculated with Friedewald equation
  • Height and weight

Independent Variables

  • High calcium diet
  • DASH-type diet
  • Low sodium, high potassium diet
  • Diet assessment
    • 24-hour diet records every 2 weeks
    • Trained personnel verified records
    • Diet analysis program used: Food Works, Professional Edition, Version 3.02, Xyris Software

Control Variables

Description of Actual Data Sample:

Initial N:

  • Of those who responded, 348 were sent a screening questionnaire and invited to attend further screening.
    • 118 met inclusion criteria and continued with blood pressure measurements
    • 93 met blood pressure criteria and agreed to participate.
    • 4 partners that did not meet blood pressure criteria were included (to increase compliance)
  • n = 97 at start
    • 3 men dropped before completion of test diet 1 (data not included)
    • 2 women and 1 man dropped before completing test diet 2 (data was included for test diet 1 only)
    • 4 stated inability to follow diet and 2 stated illness as reasons for dropping out

Attrition (final N): 94

Age:  see below

Ethnicity:  not mentioned

Other relevant demographics:  see below


Location: Australia


All subjects (n = 94)

Antihypertensive medication (n = 42)

No antihypertensive medication (n = 52









Age (years)

55.6 ± 9.9

60.0 ± 7.3

52.1 ± 10.2

Weight  (kg)

84.0 ± 13.3

85.8 ± 13.3

82.6 ±13.3

BMI (kg/m2)

29.0 ± 3.8

30.0 ± 3.8

28.2 ± 3.6

Home SBP (mmHg)

129.4 ± 11.3

130.8 ± 10.7

128.3 ± 11.8

Home DBP (mmHg)

80.6 ± 8.6

79.9 ± 9.0

81.1 ± 8.4

Pulse (beats/min)

67.3 ± 8.5

67.7 ± 9.1

67.0 ± 8.2

Urinary sodium (mmol/d)

149.9 ± 49.9

159.4 ± 48.8

142.1 ± 49.9

Urinary potassium (mmol/d)

77.5 ± 24.5

79.4 ± 24.0

76.0 ± 25.0

Adapted from Table 2 of article

Summary of Results:

Difference between control diet 1 and control diet 2 periods

  • In the 2nd control diet, urinary potassium was 5.3 ± 2.4 mmol/d higher than the 1st  (P < 0.05)
  • In the 2nd control diet, home SBP was 1.3 ± 0.6 mmHg lower than the 1st  (P < 0.05)

Dietary Intake

  • DASH-type diet period compared to control diet had (P < 0.001)
    • Higher
      • Intake of energy (1.4 ± 0.2 MJ)
      • Percentage of energy from protein (1.4 ± 0.4)
      • Fiber (13.2 ± 1.0 g)
      • Potassium (61.9 ± 3.4 mmol)
      • Calcium (603.0 ± 42.4 mg)
      • Magnesium (189.9 ± 14.0 mg)
      • Phosphorus (622.4 ± 54.6 mg)
    • Lower
      • Fat (2.9 ± 0.8)
      • Saturated fat (1.9 ± 6.3)
      • Sodium (26.5 ± 4.9 mmol)
  • LNAHK diet compared to DASH-type diet had (test diet – control, P <0.05)
    • Increased
      • Fiber (+7 ± 3 g/d)
      • Potassium (+21 ± 8 mmol/d)
    • Decreased
      • Sodium (–38 ± 11 mmol/d)
      • Calcium (-287 ± 85 mg/d)
  • High calcium diet compared to DASH-type diet (P < 0.05)
    • Increased
      • Saturated fat (+7 ±3 g/d)
      • Sodium (+49 ± 12 mmol/d)
      • Calcium (+442 ± 77 mg/d)
    • Lesser increase in
      • Fiber (-9 ± 3 g/d)
      • Potassium (-28 ± 5 mmol/d)
      • Magnesium (-118 ± 24 mg/d)

Blood pressure

From Table 4 of article:


Change in LNAHK – Change in DASH-type

Change in high calcium – Change in DASH-type

SBP, mmHg

-3.5 ± 1.0 ***

+3.1 ± 0.9 **

DBP, mmHg

-1.9 ± 0.7 *

+0.8 ± 0.6

Pulse, beats per minute

-1.0 ± 0.7

+1.7 ± 0.8 *

*   P < 0.05                 **   P < 0.01              ***   P < 0.001

  • Multiple regression analysis- pre-intervention home SBP [b = 0.11 (0.03)], pre-intervention home DBP [b = -0.97(0.04)], and difference in urinary sodium to potassium ratio [b = 1.98 (0.50)] together explain 40% of the variance (R2 = 0.40, P < 0.001) of the difference in home SBP between DASH-type and control diet periods.

Body weight

From Table 4 of article:


Change in LNAHK – Change in DASH-type

Change in high calcium – Change in DASH-type

Weight, kg

-0.3 ± 0.2

+0.7 ± 0.2 (P < 0.01)

  • Compared with control diet, body weight in high calcium diet increased 0.9 ± 0.1 kg (P < 0.001).
  • Compared with control diet, body weight in LNAHK diet decreased 0.4 ± 0.2 kg (P < 0.05).
  • No change in body weight in DASH-type diet compared to control diet.

Serum lipids

Serum lipids did not change except HDL increased more in high calcium diet than DASH-type diet.

Author Conclusion:
  1. Dietary education facilitated a healthier self-selection of foods and in turn improved blood pressure levels.
  2. A population-wide reduction in dietary sodium (through staple food), with an increase in potassium intake would be achievable and contribute to the maintenance of optimal blood pressure in the community
  3. A lower-sodium diet, high in fruits, vegetables, and dietary fiber, which is low in saturated fatty acids and includes low-fat dairy products and more fish is a valid recommendation for good health.
Funding Source:
Dairy Research and Development Council
Reviewer Comments:

Because urinary potassium was higher and SBP was lower, the question is raised if a 2-week control diet period is enough between the 4-week intervention periods.  Good explanation of differing results from the U.S. DASH study.

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)? 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