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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.

Criteria for article inclusion not specified.

Criteria for article inclusion not specified.

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

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

• 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.

 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

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.

University/Hospital:

Royal Perth Hospital

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.