Salinity in Drinking Water and the Risk of (Pre)Eclampsia and Gestational Hypertension in Coastal Bangladesh: A Case-Control Study
et al. (2014) Salinity in Drinking Water and the Risk of (Pre)Eclampsia and Gestational
Hypertension in Coastal Bangladesh: A Case-Control Study. PLoS ONE 9(9): e108715. doi:10.1371/journal.pone.0108715
Salinity in Drinking Water and the Risk of (Pre)Eclampsia and Gestational Hypertension in Coastal Bangladesh: A Case-Control Study
Aneire Ehmar Khan 0
Pauline Franka Denise Scheelbeek 0
Asma Begum Shilpi 0
Queenie Chan 0
Sontosh Kumar Mojumder 0
Atiq Rahman 0
Andy Haines 0
Paolo Vineis 0
Pal Bela Szecsi, Gentofte University Hospital, Denmark
0 1 MRC-HPA Centre for Environment and Health and Dept. of Epidemiology and Biostatistics, School of Public Health, Imperial College London , London , United Kingdom , 2 Grantham Institute for Climate Change, Imperial College London , London , United Kingdom , 3 Dhaka Children's Hospital , Dhaka, Bangladesh, 4 Upazilla Health Complex Dacope, Khulna , Bangladesh , 5 Bangladesh Center for Advanced Studies , Dhaka , Bangladesh , 6 London School of Hygiene and Tropical Medicine , London , United Kingdom
Background: Hypertensive disorders in pregnancy are among the leading causes of maternal and perinatal death in lowincome countries, but the aetiology remains unclear. We investigated the relationship between salinity in drinking water and the risk of (pre)eclampsia and gestational hypertension in a coastal community. Methods: A population-based case-control study was conducted in Dacope, Bangladesh among 202 pregnant women with (pre)eclampsia or gestational hypertension, enrolled from the community served by the Upazilla Health Complex, Dacope and 1,006 matched controls from the same area. Epidemiological and clinical data were obtained from all participants. Urinary sodium and sodium levels in drinking water were measured. Logistic regression was used to calculate odds ratios, and 95% confidence intervals. Findings: Drinking water sources had exceptionally high sodium levels (mean 516.6 mg/L, S.D 524.2). Women consuming tube-well (groundwater) were at a higher disease risk than rainwater users (p,0.001). Adjusted risks for (pre)eclampsia and gestational hypertension considered together increased in a dose-response manner for increasing sodium concentrations (300.01-600 mg/L, 600.1-900 mg/L, .900.01 mg/L, compared to ,300 mg/L) in drinking water (ORs 3.30 [95% CI 2.005.51], 4.40 [2.70-7.25] and 5.48 [3.30-9.11] (p-trend,0.001). Significant associations were seen for both (pre)eclampsia and gestational hypertension separately. Interpretation: Salinity in drinking water is associated with increased risk of (pre)eclampsia and gestational hypertension in this population. Given that coastal populations in countries such as Bangladesh are confronted with high salinity exposure, which is predicted to further increase as a result of sea level rise and other environmental influences, it is imperative to develop and evaluate affordable approaches to providing water with low salt content.
Funding: The research study was funded by the Grantham Institute for Climate Change, Imperial College London (https://www3.imperial.ac.uk/climatechange).
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
Hypertensive disorders in pregnancy are among the leading
causes of maternal and perinatal death in low-income countries
. Of these, (pre)eclampsia is one of the top five causes of direct
maternal deaths [1,2], and is associated with perinatal death
through increased risk of preterm birth and intrauterine growth
restriction, together with higher childhood blood pressure (BP) in
offspring, and future cardiovascular disease in mothers [3,4].
There is a wealth of epidemiological evidence associating high
salt intake with the risk of hypertension in children and adults .
However, the role of salt in the aetiology of hypertensive disorders
in pregnancy remains largely unclear, according to a Cochrane
In a survey conducted in 2008, rates of (pre)eclampsia and
gestational hypertension were found to be higher in Bangladeshs
coast compared to non-coastal areas . The rates were also
considerably high in the dry season, when salinity levels in surface
and groundwater are higher than in the monsoon season [8,9].
Bangladeshs coastal population, comprising approximately 40
million people, relies heavily on natural water sources like ponds,
rivers and tube-wells for obtaining drinking water. These sources
have become severely saline from seawater intrusion caused by
environmental changes, and man-made factors including poor
water management and shrimp farming . Salinity has already
encroached .100 km inland from the Bay of Bengal, and the
impacts are projected to be exacerbated by sea level rise due to
climate change and excessive groundwater withdrawals from
In light of this, this epidemiological study was conducted to
investigate the association between consumption of highly saline
drinking water and the risk of (pre)eclampsia and gestational
hypertension among pregnant women in coastal Bangladesh.
Study design and participants
A population-based case-control study was conducted between
October 2009 and April 2011 in Dacope Upazilla, a rural coastal
sub-district in the Khulna district of south-west Bangladesh,
among a population of approximately 143,000 people.
Cases of (pre)eclampsia and gestational hypertension were
obtained from the Upazilla Health Complex, Dacope (UHCD).
Cases were also identified and recruited from the local community
within the study area. Health assistants measured the BP of all
identified pregnant women (aged 1345) within Dacope, at
gestation week 20 and onwards. In the presence of high BP (.
140/90 mmHg) and/or oedema, they were referred to the
UHCD, where diagnoses were made by study physicians. BP
was measured in millimeters of mercury (mmHg) directly by
manual sphygmomanometers, using a standard protocol .
Controls included pregnant women at gestation week 20 and
onwards, without hypertension, randomly selected from the same
study population, and within the same time frame as the selection
of the cases. Four controls per case were selected.
The study protocol was approved by the ethics committee of the
Bangladesh Medical Research Council (BMRC). All participants
gave written informed consent before initiation of study activities.
Drinking water and confounders. Information on drinking
water sources, previous pregnancies, socioeconomic conditions,
occupation, lifestyle, diet, and medical histories was collected
through interviews. The questionnaire was a modified version of
the one used in the Health Effects of Arsenic Longitudinal Study
(HEALS) cohort . Study participants visited the UHCD,
where study physicians completed the questionnaires clinical
sections, measured BP again and proteinuria, and made disease
diagnoses, if relevant. The study physicians were blind to the water
sources. Definitions of the health outcomes are summarised in
Sodium measurements. Drinking water samples (250 ml)
were collected during the time of the interview, and sent to Dhaka
University for analysis. Sodium was measured by the Atomic
Absorption method, using A Analyst 800, Perkin Elmer, USA, and
expressed in milligrams per litre (mg/L).
To further check sodium intake and excretion, we collected
urine samples amongst the controls who were given appropriate
containers and instructions for collecting 24-hr urine samples. A
laboratory technician homogenised the specimens, recorded the
total volume, and stored 10 ml of each sample at 4 C.
Spot urine, instead of 24-hr urine, was collected from cases to
reduce distress caused to the ill pregnant women. This was
considered to be less intrusive for women who were unwell.
However, because of lack of comparability with controls, these
samples were not used in the case-control comparisons. Only
controls samples were used to validate water measurements.
Urine samples were sent to the International Centre for
Diarrhoeal Disease Research in Bangladesh (ICDDR, B) for
analysis. Urinary sodium was measured by Ion Selective Electrode
method (ISE), using Automated Chemistry Analyzer, Olympus,
Model AU640, Beckman Culter International, Japan. Spot
urinary excretion values were measured in millimoles per litre
(mmol/L). Individual 24-hr sodium excretion values were
calculated as the product of concentrations in urine and the total
urine volumes, measured in millimoles per day (mmol/d).
All statistical analyses were performed using Stata 11.0 for
Windows. Water sources were grouped as: rainwater (salt-free),
filtered pond water, pond water (unfiltered) and tube-well water.
The reference category contained women who reported rainwater
combined with those who reported rainwater with another source
due to low numbers in the rainwater only group. Water sodium
levels were divided into groups: ,300.01 mg/L, 300.01600
mg/L, 600.01900 mg/L and .900.01 mg/L, based on the
distribution among cases.
Analyses on (pre)eclampsia and gestational hypertension were
carried out separately, as well as a single (combined) health
outcome as they appear to have similar relationships with salt
intake from drinking water.
Summary descriptive statistics for all continuous and categorical
variables were calculated separately for cases and controls. Logistic
regression was used to calculate the odds ratios (ORs), and
corresponding 95% confidence intervals (CI) for the health
outcomes in relation to water sources and sodium levels in
drinking water. Multivariate regression analyses were adjusted by
age, parity, and mid-upper arm circumference (as a proxy measure
of adiposity; BMI was not used because of oedema in
preeclamptic women) based on a priori knowledge about potential
confounders. In the matched analysis, cases and controls were
matched by age, parity and the area of residence within Dacope.
Summary descriptive statistics
We recruited 206 cases and 1,020 randomly selected controls.
Four cases were excluded because they were known to have
preexisting hypertension. Among controls, women with missing data
for urinary sodium (n = 2) and BP (n = 12) were excluded. In the
Raised BP of .140/90 mmHg and significant proteinuria (0.3 grams on at least two random clean catch urine samples), with or without pathological oedema, occurring
after the 20th week of pregnancy, in a previously normotensive and non-proteinuric patient.
Pre-eclampsia complicated with convulsions that could not be attributed to other causes and/or coma.
Raised BP (.140/90 mm Hg) detected for the first time after mid-pregnancy and without proteinuria or pathological oedema.
final analysis, 202 cases and 1,006 controls were included. For the
first nine months, drinking water sodium was analysed on all
controls. For the subsequent nine months of the study, because of
resource constraints, water sodium was analysed on a random
subsample of the enrolled controls. Therefore, water sodium levels
were measured for 202 cases and 553 controls (Figure 1).
There were 162 cases of pre-eclampsia, 33 cases of gestational
hypertension, 7 cases of eclampsia. Since the sample population
was measured for BP at the 20th week of gestation, it was possible
to diagnose and treat pre-eclampsia early, and disease progression
into eclampsia reduced drastically, compared to previous years
(personal communication from the UHCD).
The socio-demographic characteristics of the 202 cases and
1,006 controls, and the crude risk of the disease with different
exposures are described in Table 2. Mean water sodium levels of
drinking water sources were significantly higher in the cases
(728 mg/L) than the controls (440 mg/L) (p,0.001) (Table 3).
This difference was not significantly affected when the comparison
was made separately for the first and second nine-month periods of
Age in years (mean, s.d.)
Age in categories, n, %
, = 19
Years in education, n, %
No education (0 y)
Up to primary school (6 y)
Incomplete secondary (10 y)
Secondary School (12 y)
Higher Secondary or above (.14 y)
Socioeconomic status Index
Service, business, daily labourer
Previous children, n, %
Multiparous, n, %
2 children, n, %
3 or more children, n, %
Table 2. Summary descriptive statistics, mean (sd) or percentage (%) of cases and controls and crude risks with (pre)eclampsia and
Cases (n = 202)
Controls (n = 1,006)
Odds ratio and 95% CI
Urinary sodium and blood pressure in controls
In controls, drinking tube-well water was associated with the
highest mean 24-hr urinary sodium excretion, compared to the
other sources, while rainwater combined with another source had
the lowest level (199 mmol/d for tube-well versus 119 mmol/d for
rainwater; p,0.001) (Figure 2). A crude linear regression showed
that women who drank filtered pond water, pond water or
tubewell water had higher urinary sodium levels of 30.1 mmol/d (95%
CI: 14.745.5), 37.5 mmol/d (95% CI: 25.050.0), and
60.0 mmol/d (95% CI: 48.171.7) respectively, than those who
drank rainwater plus one other source (p,0.001).
Both systolic and diastolic BP in controls were associated with
24-hr urinary sodium excretion, showing a regression coefficient of
0.02 mmHg/mmol of sodium for systolic BP (p,0.001) and
0.01 mmHg/mmol of sodium for diastolic BP (p,0.001).
Sodium levels by drinking water source
Tube-well water had the highest water sodium levels (714
mg/L), followed by filtered pond and pond water (Table 4).
Blood pressure in all study participants by drinking water
Both systolic BP and diastolic BP levels were associated with the
type of drinking water source, with higher levels in sources that
had higher salinity levels (p,0.001) (Table 5).
(Pre)eclampsia and gestational hypertension risk and
drinking water source
Among the 1,208 pregnant women, none of the cases reported
rainwater as their only drinking water source, compared to 5
women in the control group (,1%) (Table S1). River water was
excluded from the regression model (n = 14) as there were no cases
in this group, and multiple sources (except those with rainwater
Cases (n = 202)
Controls (n = 1,006)
n = 202
n = 553
Table 3. Comparison of blood pressure, anthropometric variables and mean urine and drinking water sodium levels in cases and
Systolic BP (mean, s.d.)
Diastolic BP (mean, s.d.)
Weight in kg (mean, s.d.)
Height in m (mean, s.d.)
Mid upper arm circum in cm (mean, s.d.)
Urinary sodium mmol/L (mean, s.d.)*
Water sodium mg/L (mean, s.d.)**
*Not directly comparable, but correlated. See text for further explanation.
**See text for explanation.
and another source) were excluded as they cannot be classified and
interpreted meaningfully (n = 92). The final model included age,
parity, mid-arm circumference, and socioeconomic status as
The adjusted ORs for (pre) eclampsia and gestational
hypertension among those who drank filtered pond, pond and tube-well
water were 5.32 (95% CI 2.4111.7), 5.31 (2.6010.9) and 8.30
(4.2016.4), compared to those who drank rainwater combined
with another source (p,0.001) (Table 6).
A sensitivity analysis by matching the area of residence (to
control for unknown confounders), which included 147 cases and
147 controls, showed a similar adjusted risk trend with OR: 3.72
(95% CI 1.3010.6) for women using filtered pond water, OR:
4.14 (95% CI 1.6110.7) for pond water and OR: 4.60 (95% CI
1.6110.7) for tube-well water (p,0.001).
Compared to the rainy season, the ORs for the different sources
of water were consistently higher for the dry season, but the
differences were not statistically significant (data not shown).
(Pre)eclampsia and gestational hypertension risk and
water sodium levels
Mean water sodium levels were universally high for all nine
unions within Dacope (Figure 3). The adjusted ORs were 3.30
(95% CI 2.005.51), 4.40 (2.707.25) and 5.48 (3.309.11) for
increasing sodium concentrations (OR = 1.0 for lowest; p,0.001)
(Table 7). In the analysis including 147 cases and 147 controls
with complete measurement of water sodium and matched by area
of residence, the adjusted ORs were 2.40 (1.174.90), 6.23 (2.85
13.6) and 6.30 (2.9013.6), p,0.001.
The adjusted ORs for pre-eclampsia cases only in relation to
increasing sodium (300.01600 mg/L, 600.1900 mg/L, .
900.01 mg/L, compared to ,300 mg/L) in drinking water were
3.03 (95% CI 1.805.25), 4.12 (95% CI 2.407.00) and 4.60 (95%
CI 2.667.90) (p,0.001). For gestational hypertension only, the
adjusted ORs were 5.95 (95% CI 1.6621.4), 7.00 (95% CI 2.00
24.3), and 13.4 (95% CI 4.1743.0) (p,0.007).
Figure 2. Mean 24-hr urinary sodium (mmol/d) in controls (n = 912) by water source.
n = 755
Mean water sodium (SD)
g/day with 2 L intake
Mean Systolic BP
Mean Diastolic BP
1. Rain has been combined with any other water source because of small numbers in the rainwater only group.
2. For brevity we refer to filtered pond water as filter. 3. Those with multiple sources (except those who reported rainwater) and river have been grouped as other. doi:10.1371/journal.pone.0108715.t005
1. For brevity we refer to filtered pond water as filter. doi:10.1371/journal.pone.0108715.t004
Our study provides a novel insight into understanding the
aetiology of hypertensive disorders in pregnancy among pregnant
women living in coastal Bangladesh. This population is exposed to
exceptionally high levels of sodium in drinking water, as shown by
measurements in water and urine.
A positive association was found between salinity in drinking
water and the risk of both (pre)eclampsia and gestational
hypertension. A significant dose-response relationship was found
with drinking water sodium levels, which persisted after adjusting
for multiple potential confounders, and in the analysis matched by
the area of residence. We observed a stronger association in the
dry season but not statistically significantly so. These findings are
consistent with a previous observation of an excess of
(pre)eclampsia and gestational hypertension in the dry season, when salinity
levels in surface and groundwater are higher than in the monsoon
Preeclampsia and gestational hypertension risks were also
independently associated with water sodium levels. Similarities in
the patterns of risk factors indicate that at least some of the
biological mechanisms underlying the two conditions may be
Despite having the highest water sodium level, tube-wells were
the commonest source of drinking water, implying that there is an
urgent need for promoting alternative sources. The mean water
sodium level found in all the water sources combined was 516 mg/
L. Assuming an intake of 2 L, it is equivalent to 1.1 g/day of
sodium intake from drinking water alone, which is .27 times
higher than the intake from the recommended sodium limit of
20 mg/L in drinking water (equivalent to 0.04 g/day intake) set by
the US Environmental Protection Agency . The mean water
sodium levels from drinking water alone, detected in this
population, contribute to almost 52% of the dietary goal of 2 g/
day set by the WHO . Moreover, 38% of pregnant women
consumed .1.2 g/day of sodium from drinking water, showing
that a large number of people in this population are exposed to
unacceptable levels of sodium.
The INTERSALT study reported that a median sodium
excretion of .100 mmol/d over 30-years was associated with an
1. Rain has been combined with any other water source because of small numbers in the rainwater only group.
2. For brevity we refer to filtered pond water as filter. doi:10.1371/journal.pone.0108715.t006 OR Adjusted by age, parity, SES, mid-upper arm circumference
Figure 3. Map of Dacope sub-district showing the mean sodium levels of drinking water measured in various water sources.
OR Adjusted by age, parity, SES,
mid-upper arm circumference (95% CI)
increase of systolic BP by 3.16.0 mm Hg and diastolic BP by 0.1
2.5 mmHg, after adjusting for BMI . In the present study, the
mean 24-hr urinary sodium among the healthy pregnant women
was 164 mmol/d and the median was 155 mmol/d, both of which
are well above the recommended daily sodium intake of ,
85 mmol/d (or 2 g/day of sodium) . Using the conservative
BMI-adjusted estimates of the INTERSALT study, the present
studys mean intake of 164 mmol/d would increase systolic BP by
6.012 mmHg and diastolic BP by 0.25 mmHg, potentially
leading to a large num-ber of pregnant women becoming
Direct comparison of urinary sodium between cases and
controls is complicated by the use of spot urine for the cases and
24-hr urine for the controls, although these are known to be closely
correlated . The urinary sodium-BP relationship in the
controls in the present study (coefficient = 0.02 mmHg/mmol)
relates well with that found in the INTERSALT study where the
systolic BP-sodium coefficient was 0.035 mmHg/mmol sodium
and 0.015 for diastolic BP in non-pregnant adults , which
strengthens the validity of the findings.
Limitations of the study
We cannot exclude the possibility that some of the cases in the
study had pre-existing hypertension. However, we excluded
pregnant women (n = 4) who were known to have essential
hypertension, which is uncommon among young women in rural
Bangladesh. We also repeated the analysis excluding pregnant
women .30 years of age in whom pre-existing hypertension may
be more common; however, the results remained the same.
Urinary sodium between cases and controls could not be
directly compared because of the different approaches used in
collecting the samples (24-hr urine versus spot urine).
Furthermore, sodium handling in cases and controls was likely to be
different because of disease. The literature suggests that the
capacity to excrete sodium is compromised in women with
preeclampsia and gestational hypertension compared with normal
pregnancy, and therefore excretion values do not always indicate
correctly the amount of sodium intake . Urine samples were
collected mainly to demonstrate the association with sodium
measurements in water, and thus the controls were used to fulfill
Water sodium samples could not be collected for all controls in
the last nine months of the study, but rather from a random
subsample of the enrolled controls (n = 553). This was mainly due to
resource constraints. However, we conducted a matched analysis
to ensure that this did not affect the results, and also found that the
differences in cases and controls did not vary over the period of the
study; therefore, the results are considered to be robust.
Previous evidence and pathophysiology
It is well established that dietary sodium is a major risk factor for
hypertension in children and adults and contributes significantly to
the risk of death and disability from cardiovascular diseases,
accounting for approximately 62% of strokes and 49% of coronary
heart disease in high-income countries . The sodium-BP
relationship in pregnancy, however, remains unclear, with studies
showing conflicting results . The role of excessive salt intake in
the aetiology of (pre)eclampsia and gestational hypertension is
even less clear.
Two prospective studies conducted in Massachusetts  and
Chicago  indicated that both systolic and diastolic BP were
higher in groups with higher sodium exposure compared to low
sodium in drinking water. A cross-sectional study in Arizona
showed no difference in the prevalence of BP between those who
consumed high- versus low-sodium drinking water . However,
the highest water sodium levels studied in the latter was much
lower than the mean level reported in the present study.
High sodium intake was associated with the onset of
preeclampsia by Lauro et al. . A multicentre case-control study in
Colombia showed that a dietary sodium intake of .2200 mg/day
increased pre-eclampsia risk (OR: 3.18, 95% 1.198.48) . A
case-control study in Cairo, Egypt showed that a salty diet was
associated with a higher preeclampsia risk (OR: 1.99, 95% CI:
1.023.91) . A Cochrane systematic review that included only
two trials with 603 pregnant women concluded that the evidence
was inadequate to provide unequivocal information about the
effects of a low sodium diet . It is important to note that the
present study addresses a population exposed to environmental
conditions causing them to consume sodium from drinking water
at much higher levels (.25 times) than most Western populations
where the trials have been conducted .
Oxidative stress is proposed to play a major role in inducing
endothelial dysfunction and the consequent clinical manifestations
of pre-eclampsia and hypertension, with studies showing low
antioxidant capacity and increased oxidative stress in hypertensive
individuals and pre-eclamptic mothers . Meanwhile, salt has
been shown to induce oxidative stress in salt-sensitive individuals
. Several mechanisms control the sodium balance in the body,
such as the renin-angiotensin-aldosterone system, and a disruption
of their physiological functions may lead to the development of salt
sensitivity . The nutritional status of women in
Bangladeshi women in rural areas is sub-optimal, and it is possible that
excessive sodium exposure triggers the onset of preeclampsia in
women with low antioxidant status .
The present case-control study strongly suggests that high levels
of sodium may be intimately involved in the causal pathway of
(pre)eclampsia and gestational hypertension. Given the
considerable burden of these diseases in the coastal areas of Bangladesh,
and the associated adverse maternal and fetal outcomes, it is
imperative to develop and evaluate affordable approaches to
providing water with low salt content such as rainwater harvesting.
A 2uC rise in sea-surface temperature and 0.3 metre sea-level rise
is predicted to increase flood risk area in Bangladesh by 15% more
than the present risk area, and depth of flooding by 23% within
20 km from the coastline . It is likely that climate change
effects will considerably exacerbate the current situation, which is
likely to affect many coastal populations in low-income settings,
and this adds further impetus to the need for intervention.
We thank the advisors that were involved in the study, namely, Dr. Amirul
Khusru from Shahid Sheikh Abu Naser Specialized Hospital, Khulna; Dr.
Afroza Begum from Khulna Medical College; and, Dr. Habib Ahsan from
University of Chicago. The advisors helped in the design of the study and
the questionnaire, and in setting up the management structure, as well as
provided input on clinical issues such as definition of health outcomes. We
thank Dr Saleemul Huq for his advice during the course of the study.
Conceived and designed the experiments: AEK PV SKM ABS AR.
Performed the experiments: AEK ABS SKM PV AR. Analyzed the data:
AEK PV AH QC. Contributed reagents/materials/analysis tools: ABS QC
SKM. Wrote the paper: AEK PFDS ABS QC AH AR SKM PV.
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