EAL

NA: Effect on Blood Pressure (2009)

Citation:

Study Design:

Class:

- Click here for explanation of classification scheme.

Quality Rating:

Research Purpose:

To assess the blood pressure response to short-term alteration of sodium intake and to determine the mechanism behind differences between African Americans and whites.

Inclusion Criteria:

Patients with essential hypertension who had not received treatment for this condition or had stopped treatment three months before the study.

Exclusion Criteria:

Not reported.

Description of Study Protocol:

Recruitment

Local patients with essential hypertension.

Design

Randomized trial.

Intervention

Normal diet
High-sodium diet
Low-sodium diet.

Statistical Analysis

Results are reported as mean±SEM
Paired T-tests were used to analyze changes in continuous variables within each group
Comparisons between ethnic groups at baseline were made with unpaired T-tests for continuous variables and X² test for categorical data
Multiple linear regression was used for comparison between the two ethnic groups with adjustment for potential confounders
All statistical analyses were performed with the Statistical Package for Social Science (SPSS).

Data Collection Summary:

Timing of Measurements

Patients were studied on their normal diet, on the fifth day of high-sodium intake and on the fifth day of low-sodium intake. The potassium intake was not altered for either diet.

Blood pressure was measured in the same arm by nurses using semiautomatic ultrasound sphygmomanometers with attached recorders. The measurements were free from observer bias. Supine and standing blood pressures were the mean of five readings taken at one- to two-minute intervals in the corresponding positions. Two 24-hour urine samples were collected during the last two days of each dietary period for measurement of sodium, potassium and creatinine. Blood was also taken at the end of each diet period.

Dependent Variables

Variable 1: Supine blood pressure
Variable 2: Standing blood pressure
Variable 3: Blood and urine measurements.

Independent Variables

High-sodium intake was achieved by supplementing the normal diet with 20 slow sodium tablets (200mmol per day). A low sodium diet was provided by the Metabolic Unit kitchen.

Description of Actual Data Sample:

Initial N

104:

33 white males, 15 African American males (P=1.000)
38 white females, 18 African American females.

Attrition (final N)

104.

Age (Values are Mean±SEM)

	Whites	African Americans	P
Mean age, years	49±1	44±1	0.022
Age range, years	19 to 70	29 to 62

Ethnicity

71 whites, 33 African Americans.

Anthropometrics (Values are Mean±SEM)

Weight : 72.1±1.7kg in whites, 76.0±1.9kg in African Americans (P=0.167).

Blood Pressure	Whites	African Americans	P
Systolic blood pressure, supine	174.9±2.7	174.9±3.8	0.999
Dystolic blood pressure, supine	108.5±1.3	113.97±2.4	0.050
Systolic blood pressure, standing	173.3±2.6	171.5±3.6	0.699
Dystolic blood pressure, standing	116.0±1.5	124.3±2.3	0.003

Urine	Whites	African American	P
Sodium, mmol per 24 hours	141.3±5.6	159.5±8.0	0.069
Potassium, mmol per 24 hours	59.9±2.3	63.2±4.3	0.467
Creatinine, mmol per 24 hours	9.6±0.6	12.9±1.2	0.009
Volume, ml per 24 hours	1,598±71	1,466±73	0.196

Plasma	Whites	African Americans	P
PRA, ng • mL^-1 • h^-1	1.08±0.09	0.48±0.08	0.000
Ang II, pmol per L (N=43)	39.8±9.6	16.6±1.4	0.116
Aldosterone, pmol per L	441.7±31.4	403.2±39.6	0.472
Sodium, mmol per L	139.8±0.2	138.4±0.4	0.002
Potassium, mmol per L	3.98±0.05	3.88±0.07	0.240
Creatinine, μmol per L	81.1±1.7	91.4±3.3	0.003

	Whites	African Americans	P
Plasma volume, L	2.71±0.07	2.78±0.09	0.519

	Whites	African Americans	P
Total blood volume, L	4.45±0.12	4.38±0.16	0.724

Location

Blood Pressure Unit, St George’s Hospital Medical School, London, UK.

Summary of Results:

Variables

Whites

High Low Diff. Salt Salt

Blacks

High Low Diff. Salt Salt

Statistical Significance

Supine blood pressure

Systolic

Diastolic

172.1±2.5	155.4±2.3	-16.6
107.7±1.4	101.6±1.4	-6.1

175.7±4.4	153.7±3.8	-22.0
111.6±2.6	101.5±2.4	-10.1

P<0.001 high salt vs. low salt.

Standing blood pressure

Systolic

Diastolic

171.2±2.4	152.6±2.5	-18.6
117.5±2.0	107.5±1.7	-9.99

173.2±3.97	151.2±3.7	-22.0
121.5±2.6	108.6±2.6	-12.9

P<0.001 high salt vs. low salt.

Other Findings

Taking all patients together, the fall in systolic blood pressure with the low-sodium diet was significantly correlated with age (R=0.20, P<0.05) and with systolic blood pressure on normal diet (R=0.23, P<0.05)
Subgroup analyses showed that most significant inverse correlations between the changes in blood pressure and PRA, Ang II or aldosterone were present in whites alone but no significant correlations were found in the African Americans alone
There were no significant differences between African Americans and whites in gender, supine and standing systolic blood pressures, 24-hour urinary sodium and potassium excretion, plasma potassium, plasma volume and total blood volume while subjects were on their usual diet
After five days on a low-sodium diet, blood pressure fell significantly to 155/102±2/1mm Hg in white patients with hypertension. In African Americans with hypertension, blood pressure fell significantly to 154/102±4/2mm Hg after five days on the same diet.
Compared with whites, African American patients had a 7/4mm Hg greater fall in supine blood pressure (systolic, P<0.05; diastolic, P=0.068; adjusted for age and supine blood pressure on the normal diet), with a similar reduction in urinary sodium excretion.

Author Conclusion:

Short-term sodium restriction produces a greater fall in blood pressure for hypertensive African Americans compared with whites. This greater fall in blood pressure may be partly due to their decreased renin-angiotensin-aldosterone system. The findings support the evidence that short-term sodium restriction yields changes in blood pressure that are largely modulated by the reactivity of the reninangiotensin-aldosterone system.

Funding Source:

University/Hospital:

Blood Pressure Unit, St George’s Hospital Medical School, London, UK.

Reviewer Comments:

A fall in blood pressure may be related in part to the renin-angiotensin system.

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?		No
	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?		Yes
	5.1.	In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate?	N/A
	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?	N/A
	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)?	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