Vegetarian Nutrition

Vegetarian Nutrition and Macronutrient Intake

Citation:

Margetts BM, Beilin LJ, Armstrong BK, Vandongen R. Vegetarian diet in mild hypertension: effects of fat and fiber. Am J Clin Nutr. 1988 Sep;48(3 Suppl):801-5.

PubMed ID: 2843031
 
Study Design:
Narrative review
Class:
R - Click here for explanation of classification scheme.
Quality Rating:
Negative NEGATIVE: See Quality Criteria Checklist below.
Research Purpose:

This narrative review examines evidence for the blood pressure-lowering effects of a vegetarian diet and whether blood pressure effects can be solely attributed by the elevation of dietary polyunsaturated to saturated fatty acid (P:S) ratio or fiber.

Inclusion Criteria:

Criteria for article inclusion not specified.

Exclusion Criteria:

Criteria for article inclusion not specified.

Description of Study Protocol:

Search procedures and article identification are not described.  Study quality was not assessed. 

Three separate randomized controlled trials are discussed in detail.  These trials include:

1) an ovolactovegetarian test diet in mild hypertensive subjects

2) a high P:S ratio test diet in normotensive subjects

3) a high fiber test diet in normotensive subjects

Within each of these three controlled trials, subjects where randomly assigned to one of three groups. All test diet periods were preceded by a two week usual diet familiarization period. 

  • Group 1: consumed a usual diet for 12 weeks
  • Group 2: consumed the test diet for 6 weeks followed by usual diet for 6 weeks
  • Group 3: consumed a usual diet for 6 weeks followed by the test diet for 6 weeks
Data Collection Summary:

Protocol for selection of studies not described. Authors review the findings of three of their previously published studies.

Description of Actual Data Sample:
  • This study was published in 1988, and the majority of articles included in this reveiw were published from 1979 - 1986
  • Approximately 23 articles were breifly reviewed along with a more in-depth review of three randomized controlled trials including a ovolactovegetarian test diet, a high P:S ratio test diet, and a high fiber test diet.
  • With the exception of the three randomized controlled trials, no information was provided regarding sample sizes, study designs or analyses for the other articles reviewed.
Summary of Results:

 Blood pressure effects of vegetarian diet in hypertension

  • One referenced review article examined blood pressure effects in normotensive vegetarians
  • Although not vegetarian, one trial found blood pressure-lowering effects of a low-fat, low-salt, and high-fiber diet in hypertensive diabetic patients
  • One randomized controlled trial: ovolactovegetrain diet test diet compared to usual diet

    • 60 mild hypertensive subjects, aged 25-64 years, residing in Perth, Western Australia
    • Subjects where randomly assigned to one of three groups:  1) consumed a usual diet for 12 weeks, 2) consumed the test diet for 6 weeks followed by usual diet for 6 week, 3) consumed a usual diet for 6 weeks followed by the test diet for 6 weeks
    • Blood pressure was measured every two weeks by use of an automatic recorder; dietary records and 24-hour urine specimens were collected to assess diet compliance 
    • Baseline data were comparable in each group 
    • Systolic but not diastolic blood pressure fell significantly when subjects ate the ovolactovegetarian test diet, even after accounting for other potential confounding variables such as salt use, exercise, smoking, alcohol use, and weight.   Since the ovolactovegetarian test diet resulted in an increased in P:S ratio, Vitamin C, and fiber and a decrease in Vitamin B-12 and protein, it was not clear which dietary aspect of the test diet was responsible for the blood pressure change.

Blood pressure effects of P:S ratio

  • Although difficult to control for other confounding dietary changes, 4 intervention studies indicate a blood pressure-lowering effect of either lowering the saturated fatty acid intake or by increasing the polyunsaturated fatty acid intake
  • One randomized controlled trial: P:S ratio test diet of 1.0 achieved by supplementing the diet with formulized margarines and biscuits compared to usual diet of 0.3 P:S ratio

    • 54 healthy normotensive subjects, aged 20-59 years
    • Subjects where randomly assigned to one of three groups:  1) consumed a usual diet for 12 weeks, 2) consumed the test diet for 6 weeks followed by usual diet for 6 week, 3) consumed a usual diet for 6 weeks followed by the test diet for 6 weeks
    • Blood pressure was measured six times daily at home using an automatic recorder; dietary records, blood and 24-hour urine specimens were collected to assess diet compliance 
    • Baseline data were comparable in each group
    • Total fat and mean levels of major nutrients, except the type of fat, were kept constant
    • Blood pressure did not change significantly as a result of the high P:S ratio test diet, even after adjusting for possible confounding effects of blood and urinary electrolytes as well as lifestyle variables

Blood pressure effects of fiber

  • 3 cross-sectional and 3 intervention studies indicate no relationship between fiber intake and blood pressure
  • Although difficult to isolate the effects of fiber, 2 intervention studies indicate a blood pressure-lowering effect of a high-fiber diet
  • One randomized controlled trial: approximately 50 grams of fiber/day test diet achieved by supplementing the diet with high-fiber biscuits and dried apricots versus usual diet of approximately 15 grams of fiber/day

    • 88 normotensive subjects, aged 20-59
    • Subjects where randomly assigned to one of three groups:  1) consumed a usual diet for 12 weeks, 2) consumed the test diet for 6 weeks followed by usual diet for 6 week, 3) consumed a usual diet for 6 weeks followed by the test diet for 6 weeks
    • Blood pressure was measured every two weeks by use of an automatic recorder; dietary intake was assessed
    • Baseline data were comparable in each group
    • Blood pressures did not change significantly as a result of the high-fiber test diet, even after adjusting for possible confounding effects of blood and urinary electrolytes as well as lifestyle variable such as exercise and alcohol intake
Author Conclusion:

An ovolactovegetarian diet can lower systolic blood pressure in mild hypertensive individuals.  When intakes of P:S ratio or fiber (both in isolation from other dietary changes) were increased, blood pressure was not effected.  A combination of these factors or some other dietary aspect may be responsible for the reductions in blood pressure.

Funding Source:
University/Hospital: Royal Perth Hospital
Reviewer Comments:

The authors reasonably describe the three randomized control trials, however the sample sizes are relatively small and are not supported by a power calculation (especially important in light of the null findings on blood pressure for increases in the P:S ratio and fiber).  Futhermore, the levels of signifcance are not provided for the positive relationship found between the ovolactovegetarian test diet and systolic blood pressure. Due to lack of detail regarding the other articles reviewed, the degree of heterogenity between them and the randomized controlled trials are unclear.

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? No
  3. Were explicit methods used to select studies to include in the review? Were inclusion/exclusion criteria specified andappropriate? Wereselectionmethods unbiased? No
  4. Was there an appraisal of the quality and validity of studies included in the review? Were appraisal methodsspecified,appropriate, andreproducible? No
  5. Were specific treatments/interventions/exposures described? Were treatments similar enough to be combined? No
  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? No
  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? No
  9. Are conclusions supported by results with biases and limitations taken into consideration? Are limitations ofthe review identified anddiscussed? ???
  10. Was bias due to the review's funding or sponsorship unlikely? Yes