HTN: Omega-3 Fatty Acids (2007)

Study Design:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
To investigate whether or not there may be additive or synergistic effects of weight loss and type of dietary fat on blood pressure (BP) during and following a 10-week hypocaloric diet intervention.
Inclusion Criteria:
  • Mild to moderate hypertension (supine diastolic BP 90mm to 100mm Hg and systolic BP 140mm to 180mm Hg) off medication
  • Female subjects were pre-menopausal
  • 130% to 170% of ideal body weight and adult-onset obesity
  • Non-smoking.
Exclusion Criteria:
  • History of syndrome X, including diabetes, cardiac, renal or cerebrovascular disease
  • Secondary cause for HTN
  • Severe HTN difficult to control pharmacologically.
Description of Study Protocol:


Not discussed.


Randomized control trial with repeated measures in three phases:

  • Phase One, baseline: One week in duration involving inpatient stay in the clinical research center to obtain baseline data and meals provided by the metabolic kitchen and instruction on diet recording
  • Phase Two: Intervention of 10 weeks with subjects randomized to one of three groups. The first nine weeks were outpatient and the final week had subjects as inpatients in the clinical research center for data collection, while remaining on intervention diets. All diets were calorie-restricted with 1,200kcal per day for women and 1,800kcal per day for men. Each diet differed in terms of fat composition. Group "saturated fatty acid (FA)" had a P:S of 0.25; group "omega-3 FA" had P:S of 1.0 with omega-3 FAs as 25% of total dietary FA; group "omega-6 FA" had a P:S of 1.0 with omega-6 FAs as 25% of total dietary FA. Sodium, potassium, magnesium and calcium intake were kept constant across the groups. Meals were provided by the clinical research center.
  • Phase Three: Maintenance of four weeks as outpatient for subjects to maintain their weight loss. However, subjects were inpatients for the last two days for data collection. FA composition of the diet remained the same as in Phase Two, however total calorie intake was increased for weight maintenance rather than weight loss. Meals were provided by the clinical research center.

Blinding Used

  • Blinding is not described for the subjects

  • Data collection staff was blinded to the subject's intervention group.


Described in Phases Two and Three above.

Statistical Analysis

  • Change in each measure was analyzed by one-sample T-test

  • Differences among groups in the change in each measure was analyzed by analysis of covariance, adjusting for initial values

  • Statistical significance of differences was determined by multiple comparisons adjustment.

Data Collection Summary:

Timing of Measurements

  • At baseline, daily measurement of BP and heart rate (HR), along with single determination of resting metabolic rate and body composition

  • Additionally, baseline blood samples were collected and an IV glucose tolerance test was conducted

  • During the last week of Phase Two (Week 10 of Phase Two), the above measures were repeated

  • During the last two days of Phase Three (Week Four of Phase Three), body composition, BP and HR measurements were conducted.

Dependent Variables

  • Variable One: Systolic and diastolic BP (BP was measured with a sphygmomanometer in the supine and upright positions after 10 minutes of rest)
  • Variable Two: Weight
  • Variable Three: Percentage body fat (underwater weighting and calculation)
  • Variable Four: Fat mass
  • Variable Five: Waist circumference
  • Variable Six: Insulin sensitivity
  • Variable Seven: Resting metabolic rate
  • Variable Eight: Heart rate
  • Variable Nine: Plasma concentrations of glucose, insulin, cholesterol, LDL-C, HDL-C, triglyceride, norepinephrine, renin activity and aldosterone.

Independent Variables

All diets contained 18% protein, 32% fat and 50% carbohydrate, with 1,200kcal per day for women and 1,800kcal per day for men, during the kcalorie-restricted Phase Two

Diets contained 150mEq sodium, 60mEq potassium, 800mg calcium and 120mg magnesium

Additionally by group:

  • Saturated FA Group: P:S=0.25
  • Omega-3 FA Group: P:S=1.0 with omega-3 PUFAs used
  • Omega-6 FA Group: P:S=1.0 with omega-6 PUFAs used. 

Control Variables


Description of Actual Data Sample:


Initial N: 34 females, 18 males

Attrition (final N): None discussed

Age: Mean, 47.65 years for women; 45.67 years for men

Ethnicity: Not discussed

Other relevant demographics: None mentioned

Anthropometrics: Baseline measurements (significant differences between sexes for weight, percentage body fat and fat-free mass (P<0.0001) and for height (P<0.0005).

Baseline Measurements

Females (N=34)

Males (N=18)

Height (cm)

162.4±0.9 SEM


Weight (kg)






Percentage body fat



Fat mass (kg)



Fat-free mass (kg)



Waist circumference (cm)



There were no significant differences between gender groups for body weight, percentage body fat, fat-free mass or waist circumference at baseline after subjects were randomized into groups.



Saturated FA Group

Omega-3 Group

Omega-6 Group

Weight (kg)















Percentage body fat 1 40.24±1.23 41.52±1.46 41.57±1.56












Fat mass (kg)
















Fat-free mass (kg)
















Waist circumference (cm)















  • Mean±SEM for Phase One values, percentage change for Phase Two and Three values
  • Weight loss was similar across groups. There were no significant differences between the groups for change in weight, body composition or waist circumference.


Vanderbilt University, Nashville, Tennessee.

Summary of Results:

Blood Pressure (mm Hg) at baseline and percent change



Saturated FA Group

Omega-3 Group

Omega-6 Group

DBP supine


90.46 + 1.89

93.41 + 2.05

91.38 + 2.41












DBP upright


101.77 + 2.08

102.08 + 2.44

102.29 + 2.30












SBP supine


139.25 + 3.54

142.66 + 3.41

134.92 + 3.10












SBP upright


143.22 + 3.68

143.94 + 4.37

140.92 + 3.46









-8.7% -8.3%

DBP=diastolic blood pressure
SBP=sistolic blood pressure
Mean±SEM for phase one values
*Significantly lower, compared with omega-3 FA group, P<0.05
^Significantly lower than omega-3 FA group, P<0.06.

Other Findings

Findings also reported for change in HR, serum glucose, insulin, insulin sensitivity, total cholesterol, LDL-C, HDL-C, triglycerides and HDL-C.

Author Conclusion:
  • Weight loss of 10% of body weight, irrespective of dietary fat composition, was effective in reducing BP in moderately obese, moderately hypertensive subjects to levels that don't require pharmacological intervention
  • There was a general tendency for omega-3 FAs to modestly reduce BP during the weight loss phase and a strong tendency for BP to rise in the omega-3 group during weight maintenance
  • Overall, there was no additional effect on the reduction of BP by the type of FA consumed.
Funding Source:
Government: NIH
Reviewer Comments:
  • Well-explained study, with weekly dietitian reinforcement to encourage compliance both in- and outpatient
  • Physical activity level did not vary significantly
  • Neutral rating for ethnicity not discussed, thus question if representative sample
  • Process of randomization not discussed
  • Withdrawals not discussed and blinding only mentioned for clinic staff and not for subjects.
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? ???
  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? ???
3. Were study groups comparable? ???
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) No
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? ???
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) No
  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? ???
  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%.) No
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? N/A
  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? ???
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? ???
  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? N/A
  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