HTN: Fiber (2007)

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

To determine if blood pressure (BP) response to fiber intake is related to type of fiber and subject characteristics such as age, sex, and hypertensive status.

Inclusion Criteria:
  • Human subjects
  • English language literature
Exclusion Criteria:
  • Lack of appropriate BP data
  • Intentional co-intervention from which the effect of fiber supplementation could not be separated
  • Lack of concurrent placebo group
Description of Study Protocol:

Recruitment

MEDLINE search with the following logic: Fiber or fibre AND blood pressure or hypertension AND trial or intervention or random or study.  Search limited to studies in human subjects and English-language literature. Original publication only.

Design:  Meta-Analysis

Blinding Used (if appllicable):  not applicable

Intervention (if applicable):

Data abstraction:

  • Data collected on:
    • Differences in systolic (SBP) and diastolic (DBP) blood pressure
      • For 2 trials this was estimated from a graph in the article
    • Trial design
    • Duration of the trial
    • Type of fiber and dose
    • Whether BP was a primary or secondary outcome
    • Sample size
    • Characteristics of the sample population
    • Mean age
    • Sex
    • Baseline BP
    • Baseline body weight
    • Baseline body mass index (BMI)
    • Change in body weight during intervention
    • Use of anti-hypertensive drugs
    • Baseline fiber intake
    • Use of anti-hypertensive drugs
    • Baseline (habitual) fiber intake
  •  BP measurements abstracted by priority:
    • Sitting position
    • Supine if sitting not available, then
    • Awake ambulatory if previous listed not available.
  • Populations were considered hypertensive if >50% of subject were on anti-hypertensive medications or baseline blood pressure > 140/90 mmHg

Statistical Analysis

  • Internal validity guaranteed by random assignment
  • Quality of studies were quantified by scoring blinding by the type of treatment
  • Variation (within and between study)- random effects model
  • For population description, averages calculated for age, proportion of men, SBP, DBP, body weight, BMI and fiber intake in control and intervention groups.
  • BP change
    • Parallel trials: net change = change in control group – change in intervention group
    • Crossover trials: net change = final control period blood pressure-final intervention period blood pressure
    • Standard errors (SE) obtained or derived from confidence intervals (CI) P values, or individual SEs for blood pressure changes
  • Q statistics: to test homogeneity of effect size
  • BP estimates – 95% CI; 2 sided P values < 0.05 were statistically significant.
  • Subgroups evaluation stratified meta-analyses for between group effects of fiber supplementation.  Repeated analysis using a multivariate model.
    • Mean age                               <= 40 years        vs.   > 40 years
    • Hypertensive status                 no                     vs.   yes
    • Sex                                         < 50% male        vs.   >= 50% male
    • BMI                                         <= 28 kg/m2       vs.   > 28 kg/m2
    • Type of fiber                           soluble                vs.   insoluble           vs.   mixed
  • Associations of fiber dose & intervention duration - linear regression analysis with intercepts & ß regression coefficients with 2-sided P values
  • Publication bias – examined by funnel graph and weighted against net changes in BP.  Nonparametric “trim and fill” method used to adjust for bias.
Data Collection Summary:

Timing of Measurements:  not applicable

Dependent Variables

  • systolic blood pressure
  • diastolic blood pressure

Independent Variables

  • fiber intake

Control Variables

  • Fiber dose
  • Duration of intervention
  • Type of intervention
  • Change in body weight 
  • Age
  • Sex
  • Hypertensive status
Description of Actual Data Sample:

Initial N:  72 trials met inclusion criteria.  48 trials were then excluded based on exclusion criteria.

Attrition (Final N):  24 trials with 25 relevant strata remained, representing 1404 subjects.  1 trial was missing data for diastolic blood pressure.  Men only trials-3; Women only trials-6; remainder included both.

Age:  Mean age:42 years (median 40 years; range, 23-63 years)

Ethnicity: not mentioned

Other relevant demographics:

Anthropometrics:

Location:  Worldwide studies

Summary of Results:

Other Findings

  • Sample size range: 12 to 201 subjects
  • Mean trial duration: 9.0 weeks (median: 8 weeks; range 2-24 weeks)
  • Soluble fiber in 11 trials; insoluble fiber in 7 trials; mixture in remaining trials.
    • Diet intervention in 4 trials
    • Fiber dose range 3.5 to 42.6 g/day.
  • Unweighted averages
    • SBP change = -1.54 mmHg (95% CI, -3.48 to 0.39)
    • DBP change = -1.34 mmHg (95% CI, -2.41 to –0.27)
  • Significant heterogeneity among trials
    • Systolic X224  = 24.7, P = 0.002
    • Diastolic X223 = 37.1, P = 0.03
  • Weighted overall effect of fiber supplementation
    • SBP: -1.13 mmHg (95% CI, -2.49 to 0.23)
    • DBP: -1.26 mmHg (95% CI, -2.04 to –0.48)

 

  Systolic BP Systolic BP Systolic BP Diastolic BP Diastolic BP Diastolic BP

Characteristic

# of strata

Univariate P value

Multivariate P value

# of strata

Univariate P value

Multivariate P value

Age

 

 

 

 

 

 

<= 40

13 (810)

.001

.04

12 (791)

.16

.53

> 40

11 (548)

 

 

11 (548)

 

 

Sex

 

 

 

 

 

 

< 50% male

10 (557)

.81

.65

10 (557)

.16

.19

>= 50 % male

14 (801)

 

 

13 (782)

 

 

Hypertension

 

 

 

 

 

 

No

17 (1083)

<.001

.41

16 (1064)

.02

.35

Yes

8 (321)

 

 

8 (321)

 

 

BMI, kg/m2

 

 

 

 

 

 

<= 28

6 (433)

.32

.25

6 (433)

.50

.43

> 28

6 (309)

 

 

6 (309)

 

 

 

 

 

 

 

 

 

 

  • Changes in BP less pronounced with increasing fiber dose
    • SBP: -1.55 + 0.032 mmHg / g fiber, P = .60
    • DBP: -1.89 + 0.047 mmHg / g fiber, P = .16
  • No relation of BP to duration of intervention
    • SBP: -0.77 – 0.046 mmHg / wk, P = .73
    • DBP: -1.15 – 0.013 mmHg / wk, P = .88
  • Type of intervention
    • Soluble fiber
      • SBP: -1.32 mmHg (95% CI, -3.19 to 0.55)
      • DBP: -0.82 mmHg (95% CI, -1.83 to 0.18)
    • Insoluble fiber
      • SBP: -0.23 mmHg (95% CI, -2.88 to 2.42)
      • DBP: -0.57 mmHg (95% CI, -1.86 to 0.72)
    • Mixture of soluble & insoluble fiber
      • SBP: -1.74 mmHg (95% CI, -4.49 to 1.02)
      • DBP: -2.22 mmHg (95% CI, -3.40 to –1.03)
  • Change in body weight appeared to not be significantly significant.
  • Significantly larger reduction of SBP in older vs. younger groups (P = .04)
  • Small trials with large SBP reductions may be overrepresented.
Author Conclusion:

The present meta-analysis shows that dietary fiber has a small BP-lowering effect.  Increasing the intake of fiber in Western populations, where intake is far below recommended levels, may contribute to the prevention of hypertension.

Funding Source:
Reviewer Comments:

Authors noted the following limitations:

  • Not able to completely rule out BP effects from magnesium and potassium in the supplemented fiber.
  • Small trials with large SBP reductions may be overrepresented.  However, these trials did have small weights in the meta-analysis.
  • Data in this study shows some support for more effect with soluble over insoluble fiber.
Quality Criteria Checklist: Review Articles
Relevance Questions
  1. Will the answer if true, have a direct bearing on the health of patients? Yes
  2. Is the outcome or topic something that patients/clients/population groups would care about? Yes
  3. Is the problem addressed in the review one that is relevant to dietetics practice? Yes
  4. Will the information, if true, require a change in practice? Yes
 
Validity Questions
  1. Was the question for the review clearly focused and appropriate? Yes
  2. Was the search strategy used to locate relevant studies comprehensive? Were the databases searched and the search termsused described? Yes
  3. Were explicit methods used to select studies to include in the review? Were inclusion/exclusion criteria specified andappropriate? Wereselectionmethods unbiased? Yes
  4. Was there an appraisal of the quality and validity of studies included in the review? Were appraisal methodsspecified,appropriate, andreproducible? Yes
  5. Were specific treatments/interventions/exposures described? Were treatments similar enough to be combined? Yes
  6. Was the outcome of interest clearly indicated? Were other potential harms and benefits considered? Yes
  7. Were processes for data abstraction, synthesis, and analysis described? Were they applied consistently acrossstudies and groups? Was thereappropriate use of qualitative and/or quantitative synthesis? Was variation in findings among studies analyzed? Were heterogeneity issued considered? If data from studies were aggregated for meta-analysis, was the procedure described? Yes
  8. Are the results clearly presented in narrative and/or quantitative terms? If summary statistics are used, are levels ofsignificance and/or confidence intervals included? Yes
  9. Are conclusions supported by results with biases and limitations taken into consideration? Are limitations ofthe review identified anddiscussed? Yes
  10. Was bias due to the review's funding or sponsorship unlikely? Yes