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Vegetarian Nutrition and Macronutrient Intake

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

To review research to determine the dietary basis for differences in blood pressure between vegetarians and non-vegetarians.

Inclusion Criteria:

Criteria for article inclusion not specified.

Exclusion Criteria:

Criteria for article exclusion not specified.

Description of Study Protocol:
  • Search procedure not specified
  • Quality of included studies not defined.

Types of Interventions

  1. Observation differences among strict vegetarians, lacto-vegetarians and non-vegetarian controls (non-vegetarian controls were matched for age and sex to the lacto-vegetarians)
  2. Intervention studies using animal products
  3. Intervention studies altering fats and CHO intakes.

Populations Included

Vegetarians and non-vegetarians.

Design

Narrative review.

Dependent Variables

Variable One: Change in mmHg of systolic blood pressure (SBP) and diastolic blood pressure (DBP) measurements.

Independent Variables

  • Variable One: Animal products
  • Variable Two: Amount of dietary protein, fat and CHO.

Statistical Analysis

Not applicable.

Data Collection Summary:

Not applicable.

Description of Actual Data Sample:
  • This study was published in 1988 and the majority of articles included in this review were published from 1974 to 1987
  • 45 articles were referenced and about 28 were directly included in the narrative review
  • Of the selected intervention studies evaluated, sample sizes were small, ranging from 13 to 39 subjects
  • Of those with an intervention duration being reported, several were very short, lasting only three to four weeks
  • The majority of intervention studies made no attempts to describe, evaluate or control for other factors affecting blood pressure, such as sodium intake, weight status or loss, physical activity, age and race
  • The majority of intervention studies did not appear to have concurrent control groups
  • Information about the intervention dose was lacking in several of the studies.
Summary of Results:

Observation Differences Among Strict Vegetarians (N=226), Lacto-vegetarians (N=63) and Non-Vegetarian Controls (N=458; N=63)

  • Two non-vegetarian populations were age- and sex-matched with the lacto-vegetarians
  • Across age groups, blood pressure in strict vegetarians was slightly different from that of the lacto-vegetarians and blood pressure in both vegetarian groups was 10mmHg to 15mmHg lower than in non-vegetarians.

Table 1: Blood Pressure After Using Multiple-Regression Analysis to Adjust for the Effects of Age, Sex and Body Weight

  Number of Subjects SBP DBP Significant Differences*
Strict Vegetarians (SV) 226 112.5 (1.02) 65.3 (0.82)

SV<EB; SV< EB; SV<F
Lacto-Vegetarians (LV) 63 111.8 (1.63) 68.8 (1.32)

LV<EB; LV<F
Non-Vegetarians: East Boston (EB) 458 120.8 (0.66) 76.4 (0.53) EB>LV
Non-Vegetarians: Framingham (F) 63 118.9 (118.0) 79.0 (1.42) F>LV

*P<0.001

  • Dietary sodium was not quantitated across all groups, making comparison impossible
  • Examining only the strict vegetarian group (after adjusting for age, weight and sex):
    • Eating animal products occasionally (N=not reported) significantly raised blood pressure five to eight mmHg when compared to those who abstained completely from animal products 
    • Smoking cigarettes (N=not reported) significantly raised blood pressure two to four mmHg
    • Drinking coffee or tea (N=not reported) significantly raised blood pressure two to four mmHg
    • Regular meditation (N=not reported) significantly lowered SBP four mmHg
  • Examining only the lacto-vegetarian group (after adjusting for age, weight and sex):
    • Higher intake of dairy products was significantly and positively correlated with a higher blood pressure:
      • Partial correlation coefficient between intake of diary products and SBP was 0.37 (P=0.009)
      • Partial correlation coefficient between intake of diary products and DBP was 0.27 (P=0.055).

Intervention Studies Using Animal Products

Results provided on a series of dietary interventions to examine the effects of animal products on blood pressure.

Table 2: The Effects of Specific Foods on the Blood Pressure of Vegetarians

Population Studied # of Subjects Intervention Duration Change in SBP Change in DBP
Strict vegetarians 21 250 grams per day of lean beef 4 weeks +3.0 (1.1)* -0.9 (1.2)
Non-vegetarians 19 Low-fat, lacto-vegetarian diet (details not provided) 3 months -0.7 (1.1) -0.3 (1.0)
Vegetarians 17 Eggs (details not provided) 3 weeks +1.2(1.4) +0.7 (1.7)
Strict vegetarians 18 (Text and table do not match.) Variation in vegetable protein (high protein soy-wheat vs. low-protein rice patties) OR 63 grams daily vs. 119 grams daily Two 6-week periods ?? ??
Strict vegetarians 13 27 grams of casein vs. 27 grams per day of soy protein Two 3-week periods -0.7 (1.7) -1.3 (1.5)

 * P<0.5

Note: In the article there are two other ovo-lacto-vegetarian studies (N=38 and N=39) illustrated; however, no explanation is provided in the text and the population studied and intervention are unknown.

Intervention Studies Altering Fats and CHO Intakes 

Results provided on two dietary interventions to examine the effects of altering fatty acids on blood pressure:

  • In a double-blind cross-over study, 17 normotensive subjects consumed 23 grams per day linoleic acids for four weeks and 23 grams per day oleic acids for an additional four weeks. There was no significant difference in blood pressure.
  • Three dietary supplements (saturated fatty acids, linoleic acid and CHO) were given to 21 mildly hypertensive subjects for six-week periods. There was no significant differences in blood pressure when saturated fat was replaced with linoleic acid or CHO.

Review summary provided on trials of dietary protein and fat on blood pressure in non-vegetarians (details of these studies were not provided):

  • Protein has no effect on blood pressure (two studies)
  • Linoleate has no effect on blood pressure (seven studies)
  • Linoleate decreases blood pressure five to eight mmHg (one study); statistical significance not provided
  • Saturated fatty acids increase blood pressure 4 to 6mmHg (one study)
  • Saturated fatty acids have no effect on blood pressure (five studies). 

 

Author Conclusion:
  • Vegetarians have lower blood pressure than non-vegetarians
  • Blood pressure-lowering effects are not explained by animal products, saturated fatty acids, polyunsaturated fatty acids or protein
  • Combinations of unexamined nutrients in the vegetarian diet may be needed to explain blood pressure-lowering effects
  • Longer-term studies are needed.
Funding Source:
University/Hospital: Harvard Medical School and Brigham and Women's Hospital
Not-for-profit
0
Reviewer Comments:

Strengths

Provided a summary of articles on controlled dietary trials that address the impact of animal foods on blood pressure.

Limitations

  • Authors fail to identify how reserach articles were selected
  • Data abstraction, synthesis and analysis were not applied consistently across studies.
Quality Criteria Checklist: Review Articles
Relevance Questions
  1. Will the answer if true, have a direct bearing on the health of patients? Yes
  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
  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
  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
  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
  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
  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
  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
  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
  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
  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
  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
  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? No
  9. Are conclusions supported by results with biases and limitations taken into consideration? Are limitations ofthe review identified anddiscussed? No
  10. Was bias due to the review's funding or sponsorship unlikely? Yes
  10. Was bias due to the review's funding or sponsorship unlikely? Yes