EAL

NA: Dietary Factors and Effect on Sodium and Blood Pressure (2010)

Citation:

Study Design:

Class:

- Click here for explanation of classification scheme.

Quality Rating:

Research Purpose:

The purpose was to study and compare effects on blood pressure of reduced sodium, high potassium intake to typical Australian sodium intake and a high-potassium diet.

Inclusion Criteria:

Adult male twins in Australia, age 30 years and older, and family members of the twins.

Exclusion Criteria:

Undergoing treatment for cancer or renal disease
Insulin-dependent diabetes.

Description of Study Protocol:

Recruitment

Letters were mailed to adult male twins in Australia with an invitation to participate in the study.

Design

Randomized crossover.

Blinding used

Subjects and researchers were blinded as to whether they were taking the slow-release sodium tablet or placebo.

Intervention

Low-sodium (50mmol per day), high-potassium (>80mmol per day) diet with and without slow-release sodium tablet supplementation (120mmol per day).

Statistical Analysis

SPSS software was used to determine descriptive statistics and linear modeling
STATA software was used to adjust blood pressure readings via regression through the origin, with robust SEM and adjusted P values since there was no independence within pairs
Results were expressed as means±SD or means±SEM
Two-tailed paired T-tests were used to compare the sodium supplementation and low-sodium diet and between baseline values and effects of low-sodium diet
Multi-variate regression analysis was used to determine potential outcomes of the order of tablet treatment, antihypertensive therapy and sex
24-hour urinary potassium and sodium were assessed using ion-selective electrodes.
RIA assessment of angiotensin 1 was used to assess plasma renin activity and concentration, using an enzyme kinetic method.

Data Collection Summary:

Timing of Measurements

Subjects followed eight weeks of a self-selected low-sodium diet, high-potassium diet.
Subjects were randomly given placebo tablets or sodium tablets for four weeks and were crossed over to the other tablet regimen for four weeks. Twin participants received the same treatment as their participating family members.
Subjects performed 24-hour home blood pressure measurements for three days during baseline and the first three weeks of each phase and daily during the last week of the study
24-hour urine measurements were obtained at baseline and at three points (weeks 2, 3, 4) during each four-week phase.
Subjects took 12 tablets daily (three tablets four times per day)
Compliance with treatment was determined by tablet count.

Dependent Variables

Blood pressure was measured using a mercury sphygmomanometer after subjects rested in a seated position for five minutes. Three measurements were taken from the left arm, with the mean of the last two readings used for analysis.
Diastolic blood pressure was determined at the point in which Korotkoff's sounds disappeared.
Subjects measured home blood pressure on the left arm with an automated blood pressure monitor, after 10 minutes of rest. Again, the mean of the last two of three measurements was used for analysis. Subjects were instructed to allow for one minute between each measurement. Additional blood pressure readings were done every 30 minutes for 17 hours and every hour from 2300 to 0600.
Body weight was obtained from a digital scale that was on a flat surface.

Independent Variables

Low-sodium, high-potassium diet.

Description of Actual Data Sample:

Initial N

12.

Attrition (final N)

108 (44 males, 64 females).

Age

Mean 47.0 years±38.5 years, range 33 years to 74 years.

Other relevant demographics

Not described.

Anthropometrics

Subject population included 43 pairs of twins, 17 live-in partners or family members and one individual twin (87 households total)
92 normotensive subjects, 16 hypertensive subjects.

Location

Melbourne, Australia area.

Summary of Results:

Key Findings

Tablet administration order did not affect blood pressure or urine electrolyte levels, but did affect body weight. Subjects who were on the low-sodium diet in the second four weeks had weight reduction of 1.5kg±0.3kg (P<0.001). The low-sodium diet group for the first four weeks experienced significantly (P=0.031) less weight loss of 0.8kg±0.2kg (P<0.001).
Urinary potassium had a significant increase of 86.6mmol per day±2.1mmol per day for the low-sodium diet and 87.1mmol per day±2.1mmol per day for sodium supplementation (P<0.001)
Urinary creatinine was unchanged from baseline during all study phases
While on the low-sodium diet, subjects' home blood pressure readings were lower than the 24-hour automated measurements at every check
For all subjects, systolic blood pressure readings measured at home were lower on the low-sodium diet compared to the sodium supplement treatment (2.5mmHg±0.8mmHg, P=0.004). Home readings of diastolic pressure for all subjects was also lower on the low-sodium diet (2.3mmHg±0.9mmHg, P=0.015). Investigator-measured systolic blood pressure showed no differences between the treatment phases.
Compared to baseline values, the low-sodium diet lowered all systolic and most diastolic measurements
Female subjects had significantly lower baseline blood pressure compared to men (P=0.049), and there was a marginally-significant interaction between diet and sex for diastolic blood pressure: decrease in diastolic blood pressure between low-sodium and sodium-supplement treatments was greater in women (2.4mmHg±1.3mmHg, P=0.063). A similar trend was seen for systolic blood pressure (2.1mmHg±1.2mmHg, P=0.077).

Author Conclusion:

A diet that is high in potassium and low in sodium is effective for reducing blood pressure. A reduction from baseline blood pressure readings may be seen in as little as four weeks.

Funding Source:

Government:

National Health and Medical Research Committee of Australia

Not-for-profit

The Rebecca Cooper Foundation

Reviewer Comments:

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?		Yes
	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?	Yes
3.	Were study groups comparable?		Yes
	3.1.	Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT)	Yes
	3.2.	Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline?	Yes
	3.3.	Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.)	Yes
	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?		Yes
	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%.)	Yes
	4.3.	Were all enrolled subjects/patients (in the original sample) accounted for?	Yes
	4.4.	Were reasons for withdrawals similar across groups?	Yes
	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?		Yes
	5.1.	In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate?	Yes
	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?	Yes
	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)?	No
	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