EAL

NA: Effect on Blood Pressure (2009)

Citation:

Study Design:

Class:

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Quality Rating:

Research Purpose:

To study the relationship between salt intake and blood pressure in children and adolescents.

Inclusion Criteria:

Four years to 18 years of age
Live in a private household
Only one child per household could participate in the study
Participated in the United Kingdom National Diet and Nutrition Survey and had salt intake and blood pressure recorded.

Exclusion Criteria:

Participated in the United Kingdom National Diet and Nutrition Survey, but salt intake and blood pressure not recorded.

Description of Study Protocol:

Recruitment

National Diet and Health Survey participants. Survey carried out in Great Britain in 1997 and was a nationally representative sample of children between four years to 18 years of age.

Design

Cross-sectional.

Statistical Analysis

One-way ANOVA for comparisons of blood pressure and other continuous variables according to tertiles of salt intake
General Linear Model with sex and other potentially confounding factors as covariates for comparisons of blood pressure
Sex-standardized Z-scores of blood pressure to compare tertiles of salt intake for each age group
Independent samples T-test used to compare two groups for continuous variables
X²test to analyze categorical data
Multiple linear regression analysis performed to examine whether there was a significant relationship between salt intake and blood pressure with adjustment for potential confounding factors.

Data Collection Summary:

Dependent Variables

Blood pressure measured using Dinamap 8100 oscillometric monitor
Pulse pressure.

Independent Variables

Salt intake assessed by seven-day dietary record.

Control Variables

Body weight (ages four years to 18 years)
Standing height (ages four years to 18 years)
Mid-arm circumference (ages four years to 18 years)
Waist and hip-circumference (ages 11 years to 18 years).

Description of Actual Data Sample:

Initial N: 2,127
Attrition (final N): 1,658 (842 boys and 816 girls)
Age: Mean 11±0.1 years, ranging from four years to 18 years
Ethnicity: British
Other relevant demographics: Participants grouped into five-year age groups (four years to eight years, nine years to 13 years and 14 years to 18 years)
Anthropometrics:
- Significant difference in age among tertiles of salt intake for each age category
- Significant difference in sex among tertiles of salt intake for each age category.
Location: Great Britain.

Summary of Results:

Average Salt Intake and Mean Blood Pressure

4.7±0.2 grams per day and 101±0.9/55±0.8mm Hg at age of four years
6.8±0.2 grams per day and 118±1.3/59±0.8mm Hg at age of 18 years.

Increasing Salt Intake and Blood Pressure

Significant increase in mean systolic blood pressure (mm Hg) from 101±0.6, 103±0.7, 103±0.7 (P<0.01) according to tertiles of salt intake for youth aged four years to eight years
Significant increase in mean systolic blood pressure (mm Hg) from 112±0.8, 116±0.9, 119±0.9 (P<0.001) according to tertiles of salt intake for youth aged 14 years to 18 years
No significant increase in mean systolic blood pressure (mm Hg) from 107±0.7, 107±0.6, 109±0.6 (P=0.054) according to tertiles of salt intake for youth aged nine years to 13 years
No significant increase in diastolic blood pressure for any age group
Sex-standardized Z-scores of systolic pressure compared with tertile one were increased from -0.17±0.07, 0.04±0.07, 0.12±0.07 (P<0.01) for youth aged four years to eight years; -0.05±0.07, -0.08±0.07, 0.13±0.07 (P=0.62) for youth aged nine years to 13 years; and -0.19±0.07, 0.05±0.08, 0.13±0.08 (P<0.01) for youth aged 14 years to 18 years.

Increasing Salt Intake and Pulse Pressure

Significant increase in mean pulse pressure (mm Hg) from 47±0.6, 47±0.7, 49±0.6 (P<0.01) according to tertiles of salt intake for youth aged four years to eight years
Significant increase in mean pulse pressure (mm Hg) from 51±0.6, 52±0.6, 54±0.7 (P<0.01) according to tertiles of salt intake for youth aged nine years to 13 years
Significant increase in mean pulse pressure (mm Hg) from 55±0.7, 58±0.8, 63±0.9 (P<0.01) according to tertiles of salt intake for youth aged 14 years to 18 years
Sex-standardized Z-scores of systolic pressure compared with tertile one were increased from -0.17±0.07, 0.04±0.07, 0.12±0.07 (P<0.01) for youth aged four years to eight years; -0.05±0.07, -0.08±0.07, 0.13±0.07 (P=0.62) for youth aged nine years to13 years; and -0.19±0.07, 0.05±0.08, 0.13±0.08 (P<0.01) for youth aged 14 years to 18 years.

Increasing Salt Intake and Blood Pressure and Pulse Pressure After Adjusting for Age, Sex and BMI

After adjusting for age, sex and BMI, no significant increase in mean systolic pressure. Mean systolic pressure (mm Hg) was 101±0.7, 103±0.6, 103±0.7 (P=0.350) according to tertiles of salt intake for youth aged four years to eight years; 107±0.6, 107±0.6, 108±0.6 (P=0.365) for youth aged nine years to 13 years; and 114±0.8, 117±0.8, 117±0.8 (P=0.058) for youth aged 14 years to 18 years.
After adjusting for age, sex and BMI, the mean pulse pressure (mm Hg) was 45±0.7, 47±0.7, 48±0.7 (P=0.522) according to tertiles of salt intake for youth aged four years to eight years; 51±0.6, 52±0.6, 53±0.6 (P=0.172) for youth aged nine years to 13 years; and 57±0.8, 58±0.7, 60±0.8 (P=0.036) for youth aged 14 years to 18 years
Taking all participants together, there was a significant association between salt intake and systolic blood pressure (R= 0.30, B=1.80±0.14mm Hg grams of salt per day, P<0.001), as well as pulse pressure (R=0.32, B=1.78±0.13, P<0.001).

Multiple Linear Regression Analysis

Significant association between salt intake and systolic pressure after adjusting for age (B=0.86±0.07, P<0.001), sex (B=2.06±0.49, P<0.001) and BMI (B=0.80±0.08, P<0.001)
Significant association between salt intake and pulse pressure after adjusting for age (B=0.63±0.07, P<0.001), sex (B=2.20±0.48, P<0.001) and BMI (B=0.61±0.07, P<0.001)
Increase of one gram per day in salt intake related to an increase of 0.4mm Hg in systolic pressure and 0.6mm Hg in pulse pressure.

Author Conclusion:

In conclusion, our analysis of the data from the National Diet and Nutrition Survey for young people shows that in a free-living population of British children and adolescents, differences in salt intake are associated with differences in blood pressure of public health relevance. The magnitude of the association between salt intake and systolic blood pressure in our analysis is very similar to that observed in a recent meta-analysis of controlled trials where salt intake was changed. These consistent findings provide further evidence for a reduction in salt intake in childhood. The high salt intake may pre-dispose them to develop high blood pressure later in life. A modest reduction in salt intake is likely to cause a fall in blood pressure in children and adolescents, and a reduction in salt intake combined with other dietary and lifestyle changes, for example, reducing obesity, may prevent the development of hypertension and therefore cardiovascular disease in the future.

Funding Source:

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)	N/A
	3.2.	Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline?	N/A
	3.3.	Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.)	N/A
	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?	Yes
	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?		N/A
	4.1.	Were follow-up methods described and the same for all groups?	N/A
	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%.)	N/A
	4.3.	Were all enrolled subjects/patients (in the original sample) accounted for?	N/A
	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?		N/A
	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?	???
	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?		N/A
	6.1.	In RCT or other intervention trial, were protocols described for all regimens studied?	N/A
	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?	N/A
	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?	N/A
	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?		N/A
	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?	N/A
	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?	No
	10.2.	Was the study free from apparent conflict of interest?	Yes