The association of estimated salt intake with blood pressure in a Viet Nam national survey
The association of estimated salt intake with blood pressure in a Viet Nam national survey
Paul N. Jensen 1 2
Tran Quoc Bao 0 2
Tran Thi Thanh Huong 2
Susan R. Heckbert 1 2
Annette L. Fitzpatrick 1 2
James P. LoGerfo 2
Truong Le Van Ngoc 0 2
Ali H. Mokdad 1 2
0 Department of Preventive Medicine, Viet Nam Ministry of Health , Hanoi , Viet Nam , 3 Department of Ethics and Social Medicine, Hanoi Medical University , Hanoi , Viet Nam , 4 Department of Global Health, University of Washington , Seattle, WA , United States of America, 5 Department of Medicine, University of Washington , Seattle, WA , United States of America
1 Department of Epidemiology, University of Washington , Seattle, WA , United States of America
2 Editor: Tatsuo Shimosawa, The University of Tokyo , JAPAN
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
Funding: Funding to support data collection (TQB,
TTTH, TLVN) was provided by The Atlantic
Philanthropies Inc.; the sponsor had no role in the
study design, data analysis, writing or preparation
of the article, or decision to have it submitted for
publication. Study authors had no association with
the funding organization. http://www.
To evaluate the association of salt consumption with blood pressure in Viet Nam, a
developing country with a high level of salt consumption.
Design and setting
Analysis of a nationally representative sample of Vietnamese adults 25±65 years of age
who were surveyed using the World Health Organization STEPwise approach to
Surveillance protocol. Participants who reported acute illness, pregnancy, or current use of
antihypertensive medications were excluded. Daily salt consumption was estimated from fasting
mid-morning spot urine samples. Associations of salt consumption with systolic blood
pressure and prevalent hypertension were assessed using adjusted linear and generalized linear
models. Interaction terms were tested to assess differences by age, smoking, alcohol
consumption, and rural/urban status.
The analysis included 2,333 participants (mean age: 37 years, 46% male, 33% urban). The
average estimated salt consumption was 10g/day. No associations of salt consumption with
blood pressure or prevalent hypertension were observed at a national scale in men or
women. The associations did not differ in subgroups defined by age, smoking, or alcohol
consumption; however, associations differed between urban and rural participants (p-value
for interaction of urban/rural status with salt consumption, p = 0.02), suggesting that higher
salt consumption may be associated with higher systolic blood pressure in urban residents
but lower systolic blood pressure in rural residents.
Although there was no evidence of an association at a national level, associations of salt
consumption with blood pressure differed between urban and rural residents in Viet Nam.
Competing interests: The authors have declared
that no competing interests exist.
The reasons for this differential association are not clear, and given the large rate of rural to
urban migration experienced in Viet Nam, this topic warrants further investigation.
While numerous epidemiological studies have reported an association between dietary salt
intake and blood pressure, the majority of this evidence has come from developed countries.
[1±3] Few studies on this topic have been conducted in developing countries, and those studies
were focused on unique, geographically isolated populations with low levels of salt
consumption.[2, 4±6] The impact of salt on blood pressure in developing countries with a high level of
salt intake, such as those of South-east Asia, is unclear.[
Viet Nam has undergone a period of rapid economic growth in the past 10±20 years, during
which the country also experienced substantial rural to urban migration, increased tobacco use,
the adoption of unhealthier diets, and decreased levels of physical activity.[8±12] These changes
align with the ªepidemiological transition,º the concept that as countries become more
developed, the burden of disease shifts to chronic non-communicable diseases as the number of deaths
from communicable diseases decreases and the average life expectancy increases.[
from urban areas strongly suggests that the Vietnamese urban population is growing older and
more obese, and that the prevalence of hypertension and diabetes is on the rise.[
11, 12, 14, 15
As a modifiable risk factor, salt consumption may be an appropriate target for public health
interventions to lower population-wide blood pressure, which is hypothesized to lead to major
improvements in public health.[
1, 3, 16, 17
] Although the cost of antihypertensive medications
for an individual can be as little as pennies a day, salt reduction interventions are often cited as
the most cost-effective means by which to lower population-wide blood pressure.[16, 18±20]
Before any nation-wide salt reduction efforts are considered in Viet Nam, it is important to
understand whether the effect of salt on blood pressure among Vietnamese is similar to that
previously observed in developed countries.
Salt consumption is notoriously difficult to measure accurately, which has inhibited its
investigation in resource-limited settings and developing countries.[
] However, recent research has
shown that a single spot urine collection can be used to provide useful estimates of salt intake in
settings where multiple spot or 24-hour urine collections are not feasible.[22±27] We used spot
urine sample data from a nationally representative population in Viet Nam to evaluate the
association of salt intake with blood pressure and prevalent hypertension. We also assessed whether
this association differed by age, smoking, alcohol consumption, or rural/urban residence.
The 2009 Viet Nam STEPwise approach to Surveillance (STEPS) survey is a cross-sectional
study designed in accordance with World Health Organization (WHO) protocols to estimate
the prevalence of key risk factors for non-communicable diseases among Vietnamese adults.
] The 2009 Viet Nam STEPS design and recruitment are described in detail elsewhere.[
Briefly, probability proportional to size sampling was used to select a nationally representative
sample of 22,940 individuals aged 25±64 years from eight provinces, with each province
representing a unique ecological region within Viet Nam. Between June and October 2009, trained
interviewers conducted in-person interviews, and participants were invited to a clinic for a
physical exam and blood and urine collection. A total of 14,706 Vietnamese adults completed
an interview, physical examination, and blood collection; a spot urine sample was collected
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from a random subsample of 2,551 participants. Participants were excluded from this analysis
if they reported acute illnesses or pregnancy, or if they reported current use of antihypertensive
medications. This study was approved by the Viet Nam Ministry of Health Institutional
Review Board, and participants provided written informed consent before participating.
Each province recruited a data collection team of approximately 20 local medical personnel
who were trained by staff from the WHO, the Viet Nam Non-Communicable Disease office,
and consultants from the Menzies Research Institute. Survey clinics were set up at each
commune in a location convenient to participants, such as the People's Committee Office (the
local government administration office) or health center. The times that clinics opened were
adjusted for each area based on the activities of local participants, and varied between 6 and
7AM. Participants attended the clinic after overnight fasting.
Urine and blood samples were collected before participants ate breakfast. Samples were
collected in standard containers, and were refrigerated as they were transported to the Viet Nam
National Institute of Nutrition, where they were kept at -20 degrees Celsius until analyzed. The
concentrations of sodium and creatinine in the urine were measured using an ion selective
electrode method. Fasting blood glucose and total cholesterol were measured from capillary
whole blood using Roche Diagnostics Accutrend Plus glucometers.
At the clinic visit, participants were administered an in-person questionnaire by a study
interviewer. The questionnaire was adapted from the WHO STEPS instrument (version 2.1)
that was translated into Vietnamese.[
] Topics covered included demographic information,
tobacco and alcohol use, physical activity, and medical history (self-reported history of
hypertension, diabetes, and medication use).
Height, weight, and waist and hip circumference measurements were taken with the
participant in bare feet without headwear or heavy clothing. Blood pressure was measured using an
Omron HEM 907 digital automatic blood pressure monitor after the participants had rested
for at least 15 minutes. Two blood pressure measurements were taken; if they differed by more
than 25/15mmHg then a third measurement was taken. The average of the last two blood
pressure measurements was used in the analysis. Once per week measurement tools and
equipment were inspected by study staff and recalibrated if needed.
Daily salt consumption estimation
Daily salt consumption was estimated from a fasting, mid-morning spot urine sample using a
formula derived by Tanaka:[
eNa: Estimated 24-hour sodium excretion (mmol/day)
NaS: Sodium concentration in spot urine (mEq/L)
CrS: Creatinine concentration in spot urine (mg/L)
Pr.Cr24: estimated 24hr urinary Cr excretion (mg/day)
A validation study was conducted on a subsample of 154 participants between November
3 / 12
and December 2010. Using the same collection protocol as described above, participants
attended a study clinic after overnight fasting to provide mid-morning spot urine samples. At
this point the participants began their 24-hour urine collections, and returned to the study
clinic at the same time the following day to complete the 24-hour urine collection. After
excluding eight incomplete or biologically implausible 24-hour urine samples (24-hour
creatinine to body weight ratios that exceeded two standard deviations of the mean), spot urine
based estimates of daily salt consumption were conservative (-12%; S1 Fig), but moderately
correlated with 24-hour measured salt consumption (rho = 0.35).[30±32] In a sensitivity
analysis, use of the Kawasaki formula had similar validity (rho = 0.34), but yielded higher estimated
daily salt consumption than the Tanaka formula in this population (+15%; S2 Fig).[
Results are presented in terms of grams of salt intake (1 gram salt (sodium chloride) =
17.1mmol sodium). We excluded participants with estimated salt consumption levels that
exceeded 3 standard deviations from the mean (less than 3 or more than 17 grams of salt per day).
Associations of daily salt consumption with systolic blood pressure were assessed using
adjusted linear regression models. Relative risk regression was used to directly estimate the
relative risk of hypertension associated with daily salt consumption, using generalized linear
models with a Poisson distribution and robust standard errors.[
] Models included
adjustment terms for age, sex, height, weight, smoking, total cholesterol, diabetes, and physical
inactivity. Prevalent hypertension was defined as systolic blood pressure 140mmHg or diastolic
blood pressure 90mmHg, and diabetes was defined as fasting glucose 126mg/dL or use of
diabetes medication in the previous two weeks. Smoking was defined as current use of tobacco
products; alcohol use was defined as five or more alcoholic drinks per week. Physical inactivity
was defined as not meeting any of the following three criteria: 30 minutes of
moderate-intensity physical activity on at least 5 days every week, 20 minutes of vigorous-intensity physical
activity on at least 3 days every week, or a combination of vigorous- and moderate-intensity
physical activity that exceeds 600 metabolic equivalent (MET)-minutes per week.[
and urban classification was based on the commune's rural/urban designation in the 2009
To evaluate differences in the association of salt intake with blood pressure and prevalent
hypertension by age (<45 years vs. 45 years), smoking (non-current vs. current), alcohol use
(<5 drinks/week vs. 5 drinks/week), and rural/urban status, we tested interaction terms
between salt intake and each of these risk factors in separate models. Interaction models
included the primary set of adjustment covariates, except the age interaction model, which did
not include a continuous term for age. Interaction models for smoking and alcohol
consumption models were restricted to men due to the infrequency of reported smoking (2%) and
alcohol use (<1%) among women. As a sensitivity analysis, we repeated analyses of salt intake with
blood pressure and hypertension using Kawasaki and INTERSALT estimated salt
Analyses were conducted with STATA version 11.2 using the svy procedure. All analyses
used STEPS sample weights that adjust for non-coverage and unequal probabilities of
selection; sample weights incorporated post-stratification weights, which were calculated for each
age (25±34, 35±44, 45±54, and 55±65 years) and sex stratum within each province using data
from the 2009 Viet Nam national census.[
] Standard errors were computed using a robust
variance estimator to take into account the complex sample design.
Of the 2,551 participants who provided spot urine samples, 7 were pregnant, 133 reported
use of antihypertensive medications in the previous two weeks, 54 had missing body size or
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blood pressure measurements, and 24 had implausible salt intake levels and were excluded,
leaving data from 2,333 participants available for analysis.
Characteristics of the 1,083 male and 1,250 female participants in the study are reported in
Table 1. The average age of participants was 37 years and 11% had prevalent hypertension. On
average, women were shorter, lighter, had lower blood pressure, and were less likely to smoke
or drink alcohol than men. Participants consumed an average of 9.9 grams of salt per day, the
distribution of which was relatively normal (Fig 1); 99% consumed more than the WHO
recommended limit of 5 grams of salt per day.[
In sex-stratified models adjusted for age, height, weight, smoking, total cholesterol,
diabetes, and physical inactivity, there was no evidence of an association of salt consumption with
systolic blood pressure or prevalent hypertension (Table 2).
There was no evidence of an association of salt consumption with systolic blood pressure or
prevalent hypertension in subgroups defined by age, smoking, alcohol consumption, or urban/
rural residence (Table 3). Although we did not observe statistically significant associations of
salt consumption with blood pressure within either urban or rural subgroups, there was
evidence that these associations differed (p-value for interaction of urban/rural status with salt
consumption, p = 0.02), which suggested that higher salt consumption may be associated with
higher systolic blood pressure in urban participants but lower blood pressure in rural
participants. While the mean systolic blood pressure was similar in urban and rural participants (119
vs. 118 mmHg, p = 0.79), salt consumption levels were slightly lower in urban residents than in
rural residents (9.5 vs 10.1 g/day, p = 0.01). Restricting analyses to Kinh participants, using the
Kawasaki or INTERSALT formulas to estimate salt intake, and including salt consumption
levels that exceeded three standard deviations from the mean did not alter any of these findings
(S3 Fig; S1, S2 and S3 Tables).
Fig 1. Daily salt intake. Blue = men. Red = women.
We observed that salt consumption was not associated with systolic blood pressure or the risk
of prevalent hypertension at a national level in Viet Nam. We also observed that associations
with systolic blood pressure differed between urban and rural participants, suggesting that
higher salt consumption may be associated higher systolic blood pressure in urban residents
but lower systolic blood pressure in rural residents in Viet Nam. However, associations in each
subgroup were not statistically significant and may be due to chance.
To the best of our knowledge, this is the first study to examine the association of salt
consumption with blood pressure at a national level in South-east Asia.[
3, 7, 8
] The national average
estimated salt consumption level we observed (9.9g/day) was in line with levels recently reported
in rural areas near Hanoi (8.5g/day to 10.8g/day).[
] Our finding of no association of salt
consumption with blood pressure at a national level contrasts with a recent meta-analysis of
Includes age and body mass index as adjustment covariates
Includes age, height, weight, smoking, total cholesterol, diabetes, and physical inactivity as adjustment covariates
Systolic blood pressure 140mmHg or diastolic blood pressure 90mmHg
β = -0.20
β = 0.29
RR = 0.94
RR = 0.98
Includes adjustment terms for age, sex, height, weight, smoking, total cholesterol, diabetes, and physical inactivity
p-value for interaction
Continuous age not included as an adjustment term
Analyses restricted to men-only
thirty-four trials in developed countries, which observed a positive association of salt intake with
blood pressure (difference in systolic blood pressure (mmHg) per one-gram higher level of salt
consumption; β = 0.95, 95%CI: 0.72, 1.18).[
] A recent South Korean national survey estimated
daily sodium excretion from spot urine samples and reported an association of salt consumption
with higher blood pressure; however, this study did not collect information on antihypertensive
medication use and was not able to account for this in its analysis.[
] We did not observe the
differential association of salt consumption with systolic blood pressure or hypertension by
smoking status, alcohol consumption, or age that has been observed elsewhere.[
Reasons that salt consumption may have a different impact on the blood pressure of urban
participants than of rural participants are not clear. The varying degree to which an
individual's blood pressure responds to changes in salt intake (ªsalt-sensitivityº) is largely driven by
renal function. A number of genetic factors have been found to impair renal function and
are associated with salt-sensitivity.[42±45] Both rural and urban residents were predominantly
Kinh ethnicity (98% and 96%, respectively; p = 0.23), and restricting analyses to Kinh
participants did not alter our findings. Given that the rate of rural to urban migration from 1999 to
2010 increased by an average of 9.2% per year, and one-sixth of the urban population in 2009
had moved from rural areas in the past five years,[
] it would seem unlikely that genetic
differences between rural and urban participants fully explain the stronger association of salt
consumption with blood pressure in urban residents.
Apart from genetics, a number of other factors are associated with salt-sensitivity, including
old age and diets low in potassium or calcium. [41, 42, 47±49] While we were able to adjust for
age as a confounding variable, we were not able to adjust for dietary potassium or calcium, or
for markers of renal function, as they were not measured in this survey. If these factors were
more common among urban participants than rural participants, they could impair renal
function (thereby increasing the salt-sensitivity) of urban residents more than rural residents,
which could help explain the differential association of salt consumption with blood pressure
by urban/rural status that we observed.
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It is also possible that because rural participants were more likely to work in agriculture, they
would lose more salt in sweat than urban participants due to the physical nature of their work
and the hot climate of Viet Nam. Although there is a common perception that extra salt should
be consumed to replace electrolytes lost due to perspiration, the amount of salt needed to
maintain electrolyte balance in most conditions is actually quite low (approximately 0.6±1.2 grams
per day for an average adult), so the amount consumed in this population far exceeded
3, 50, 51
] While some sodium is indeed secreted through perspiration, the amount
is relatively small (0.6±4.1 grams per liter of sweat), with heat acclimated people on the low end
of that spectrum.[
] Because the study population would be well acclimated to the heat and
humidity, and because we adjusted for physical activity in our analyses, we do not have reason
to believe that differential perspiration loss by rural/urban status influenced our findings.
Although many studies have documented a rise in blood pressure corresponding with rural to
urban migration, few have examined the influence of salt on this association.[
] A 1991 study
of the Yi, an ethnic minority in southwestern China, collected blood pressure measurements and
24-hour urine samples from Yi farmers who lived in the rural mountains and Yi who had
migrated to nearby urban centers. Rural Yi farmers had low levels of salt consumption (5.6g/
day) and one of the lowest average blood pressures in the world (98/60mmHg); both of which
were lower than those of Yi urban migrants (9.3g/day, 107/69mmHg).[
] The study authors
concluded that, due to the ethnic similarity of the urban migrants to the rural farmers, the higher
blood pressure observed in the urban migrants was largely due lifestyle changes, including
increased salt intake. A 1984 study of Kenyan rural to urban migrants provided similar findings.
] In our population, rural participants had a slightly higher average level of salt consumption
than urban participants, but the mean systolic blood pressure was similar in the two groups.
The cross-sectional nature of the study is a limitation of this analysis. Because we cannot
assess whether salt consumption levels predated systolic blood pressure levels, we are only able
to assess the correlation of salt consumption with blood pressure and prevalent hypertension,
rather than causality. Another limitation is our use of an imperfect measure of salt
consumption. Although 24-hour urine collection is the gold standard for estimating salt consumption
in epidemiological studies, spot urine sample-derived estimates are a useful measure of salt
consumption and are gaining popularity in large-scale epidemiological studies.[
Further, the levels of estimated daily salt consumption we observed were similar to those reported
in region-specific studies of salt consumption in Viet Nam.[
] However, deriving daily
salt consumption from a single spot urine often results in overestimates among those at lower
salt consumption levels and underestimates among those at higher salt consumption levels.
25, 30, 54
] In our population where 99% consumed more than the WHO recommended limit
of salt per day, use of spot urine may have underestimated daily salt consumption and
impaired our ability to detect associations with blood pressure and hypertension, resulting in
conservative estimates of the association of salt consumption with blood pressure and
prevalent hypertension or an apparent lack of association where one may truly be present.
Finally, the relatively young upper age (64 years) of included participants inhibited our ability
to assess associations among those at greatest risk of high blood pressure.
There was no evidence of an association of salt consumption with elevated systolic blood
pressure at a national level in Viet Nam; however, the association of salt consumption with blood
pressure differed in urban versus rural residents. The reasons for this differential association
are not clear, and given the large rate of rural to urban migration experienced in Viet Nam,
this topic warrants further investigation.
8 / 12
S1 Fig. Bland-Altman plot, Tanaka estimated vs. 24-hour urine measured salt
consumption in spot urine validity subset population (N = 146).
S2 Fig. Bland-Altman plot, Kawasaki estimated vs. 24-hour urine measured salt
consumption in spot urine validity subset population (N = 146).
S3 Fig. Average daily salt consumption estimated from a spot urine sample using three
formulas. Blue = men. Red = women.
S1 Table. Sex-stratified regression models of Kawasaki and INTERSALT estimated salt
intake (g/day) with systolic blood pressure and prevalent hypertension.
S2 Table. Age-, smoking-, alcohol-, and rural/urban-stratified adjusted regression models
of Kawasaki and INTERSALT estimated salt intake (g/day) with systolic blood pressure
and prevalent hypertension.
S3 Table. Sex-stratified regression models of estimated salt intake (g/day) with systolic
blood pressure and prevalent hypertension, with and without outlier exclusion.
S1 Dataset. Spot urine validity dataset.
S2 Dataset. Analytic dataset.
The authors thank the study participants and staff, and the Viet Nam Ministry of Health for
their important contributions.
Conceptualization: Paul N. Jensen, Tran Quoc Bao, Tran Thi Thanh Huong, Susan R.
Heckbert, Annette L. Fitzpatrick, James P. LoGerfo, Truong Le Van Ngoc, Ali H. Mokdad.
Data curation: Paul N. Jensen, Tran Quoc Bao, Tran Thi Thanh Huong.
Formal analysis: Paul N. Jensen.
Funding acquisition: Tran Quoc Bao.
Methodology: Paul N. Jensen, Tran Quoc Bao, Tran Thi Thanh Huong, Susan R. Heckbert,
Annette L. Fitzpatrick, James P. LoGerfo, Ali H. Mokdad.
Project administration: Tran Quoc Bao, Tran Thi Thanh Huong, Susan R. Heckbert, Annette
L. Fitzpatrick, James P. LoGerfo, Truong Le Van Ngoc, Ali H. Mokdad.
Resources: Tran Quoc Bao, Tran Thi Thanh Huong, James P. LoGerfo, Ali H. Mokdad.
9 / 12
Supervision: Tran Quoc Bao, Tran Thi Thanh Huong, Susan R. Heckbert, Annette L.
Fitzpatrick, James P. LoGerfo, Ali H. Mokdad.
Validation: Paul N. Jensen.
Writing ± original draft: Paul N. Jensen.
Writing ± review & editing: Paul N. Jensen, Tran Quoc Bao, Tran Thi Thanh Huong, Susan
R. Heckbert, Annette L. Fitzpatrick, James P. LoGerfo, Truong Le Van Ngoc, Ali H.
10 / 12
11 / 12
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