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Journal of Human Hypertension (2009) 23, 451–455
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ORIGINAL ARTICLE
Dietary intake, blood pressure and
osteoporosis
J Woo1, T Kwok1, J Leung2 and N Tang3
1
Department of Medicine and Therapeutics, The Chinese University of Hong Kong, Shatin, NT, Hong Kong;
Jockey Club Centre for Osteoporosis Care and Control, The Chinese University of Hong Kong, Shatin,
NT, Hong Kong and 3Department of Chemical Pathology, The Chinese University of Hong Kong, Shatin,
NT, Hong Kong
2
Both hypertension and osteoporosis have common
underlying nutritional aetiology, with regards to dietary
cations intake. We tested the hypothesis that sodium
intake reflected in urinary Na/Cr and blood pressure
would be negatively associated with bone mineral
density (BMD), whereas other cations may have opposite associations. Subjects were part of a study of bone
health in 4000 men and women aged 65 years and over.
A total of 1098 subjects who were not on antihypertensive drugs or calcium supplements and who provided
urine samples were available for analysis. Logistic
regression was used to examine associations between
total hip and lumbar spine BMD, age, gender, body
mass index (BMI), urinary Na/Cr, K/Cr, calcium and
magnesium intake, systolic blood pressure and diastolic
blood pressure. Total hip BMD was inversely
associated with age, being female and urinary Na/Cr,
and positively associated with BMI, urine K/Cr and
dietary calcium intake. Lumbar spine BMD was inversely
associated with being female and urinary Na/Cr,
and positively associated with BMI, dietary calcium
intake and SBP. We conclude that sodium intake,
reflected by urinary Na/Cr, is the major factor linking
blood pressure and osteoporosis as shown by the
inverse relationship with BMD. The findings lend further
emphasis to the health benefits of salt reduction in our
population both in terms of hypertension and osteoporosis.
Journal of Human Hypertension (2009) 23, 451–455;
doi:10.1038/jhh.2008.156; published online 18 December 2008
Keywords: blood pressure; osteoporosis; sodium; calcium; magnesium; potassium
Introduction
The contribution of nutritional factors to the development of hypertension is well recognized, in
particular with reference to the four cations sodium,
potassium, calcium and magnesium,1–6 both in
terms of nutrient intake quantities as well as
frequency of consumption of food group.7,8 The link
between hypertension and osteoporosis have not
received as much attention, in terms of common
underlying pathogenetic mechanism, perhaps as
both conditions are common with ageing. One
pathway would be the increased obligatory urinary
calcium excretion with increased urinary sodium
excretion, which occurs with increased dietary
sodium intake. This has been confirmed in the
Hong Kong Chinese population.9 Increased urinary
calcium loss predisposes to increased bone loss and
osteoporosis. A direct connection may also occur
Correspondence: Professor J Woo, Department of Medicine and
Therapeutics, The Chinese University of Hong Kong, 9/F,
Clinical Sciences Building, Prince of Wales Hospital, Shatin, NT,
Hong Kong.
E-mail: [email protected]
Received 2 September 2008; revised 5 November 2008; accepted 5
November 2008; published online 18 December 2008
through the calcium-dependent vasoactive properties of calcium-regulating hormones, which are
linked to the activity of the rennin–angiotensin
system.10 Salt and calcium intake exert reciprocal
effects on these two hormone systems and on blood
pressure. For example, a recent population study
showed an inverse relationship between systolic
blood pressure (SBP) and serum 25 hydroxyvitamin
D concentrations.11 Both SBP and diastolic blood
pressure (DBP) were higher in patients with low
calcium intake and high serum parathyroid
hormone concentration12–14 and age-related rise in
SBP was inversely associated with calcium intake.15
However, a randomized controlled trial of calcium
supplementation in older New Zealand women
showed only a small and transient hypotensive
effect in most women, the effect being more marked
in those with a calcium intake of o600 mg day1,16
and no effect in normotensive or hypertensive
people in another study.17 A recent Cochrane
Database analysis concluded that the causal relationship between calcium supplementation on
blood pressure is weak.18
Earlier studies in Chinese populations showed a
positive association between sodium intake,19 but an
inverse association with calcium intake was only
Diet, blood pressure and osteoporosis
J Woo et al
452
observed in older vegetarian women, among whom
the prevalence of osteoporosis is high.20 In a study of
bone health, among 4000 men and women aged 65
years and over living in the community, we took the
opportunity to examine the relationship between
blood pressure, bone mineral density (BMD) and
intake of calcium, sodium, potassium and magnesium. We hypothesize that sodium intake as reflected by urinary Na/Cr and blood pressure would
be negatively associated with BMD, while other
cations may have opposite associations.
Materials and methods
A total of 2000 men and 2000 women aged 65 years
and over living in the community were invited to
attend a health check carried out in the School of
Public Health of the Chinese University of Hong
Kong, by placing recruitment notices in community
centres for the elderly and housing estates. Several
talks were also given at these centres explaining the
purpose, procedures and investigations to be carried
out. Subjects were volunteers, and the aim was to
recruit a stratified sample so that approximately
33% were in each of these age groups: 65–69, 70–74,
75 þ years. The study was approved by the Clinical
Research Ethics Committee of the Chinese University of Hong Kong, which requires informed consent
to be obtained.
A questionnaire containing information regarding
drug history and dietary intake was administered by
an interviewer. Dietary intake was assessed using a
7-day food frequency questionnaire, and man nutrient quantitation per day was calculated using
food tables derived from McCance and Widdowson21 and the Chinese Medical Sciences Institute.22
The food frequency questionnaire consisted of items
in the following seven categories: bread/pasta/rice
(16 items); vegetables (63 items); fruits (26 items);
meat (39 items)/fish (31 items)/eggs (5 items);
beverages (37 items); dinsum/snacks (39 items);
soups (10 items); and oil/salt/sauces. Items chosen
were those most frequently consumed, based on
previous local surveys. Each subject was asked to
complete the questionnaire—the food item, the size
of each portion, the number of times of consumption
each day and each week. Portion size was explained
to subjects using a catalogue of pictures of individual food portions. The amount of cooking oil was
estimated according to the method of preparing
different foods: 0.2 tablespoon for steaming fish or
stir frying half a portion of vegetables, and 1
tablespoon for stir frying one portion vegetables or
one portion of meat.23
Blood pressure was measured after 5 min rest in
the sitting position using a standard mercury
sphygmomanometer (WA Baum Co. Inc., Copiague,
NY, USA). The first and fifth Korotkoff phases were
recorded as SBP and DBP. One reading was taken.
Body weight was measured, with subjects wearing a
Journal of Human Hypertension
light gown, by the Physician Balance Beam Scale
(Healthometer, Chicago, IL, USA). Height was
measured by the Holtain Harpenden stadiometer
(Holtain Ltd, Crosswell, UK). Body mass index
(BMI) (weight in kg divided by the square of height
in m) was calculated. BMD at the hip and lumbar
spine region was measured using dual energy X-ray
absorptiometry (Hologic 4500 W, software version
11.2).
A fasting urine sample was collected for analysis
of sodium (Na), potassium (K) and creatinine (Cr),
and the ratios Na/Cr and K/Cr were used as
indicators of dietary sodium and potassium intake.
All urinary analytes were analysed in the routine
hospital laboratory on a Roche service analyzer.
Urine Na and K were measured by the principle of
indirect Ion Selective Electrode and the analysis
coefficient of variation (CVs) were 2.2 and 1.6%
respectively. Urine creatinine was measured by
kinetic colourimetric assay based on modified Jaffé
Reaction, and CV of the assay was about 2.0%.
Statistical analysis
Subjects who were on hypertensive medication or
calcium supplements or who did not provide a urine
sample were excluded from statistical analysis.
Multiple logistic regression was carried out with
total hip BMD as dependent variable. Independent
variables were age, female gender, BMI, urine Na/Cr
and K/Cr ratio, magnesium and calcium intake, and
SBP and DBP. The analysis was repeated with
lumbar spine BMD.
Results
Of the 4000 subjects, after excluding those using
antihypertensive medication and calcium supplements (n ¼ 2061) and those without urine samples
(n ¼ 963), data from 1098 subjects were available for
analysis (809 women, 289 men). Total hip BMD was
inversely associated with age, being female and
urinary Na/Cr, and positively associated with BMI,
urine K/Cr and dietary calcium intake. Together the
variables account for 37% of the variance (Table 1).
Urine Na/Cr, K/Cr and calcium intake have similar
magnitude of association with BMD, being less than
age, gender or BMI. No association was observed
between SBP or DBP and BMD. Lumbar spine BMD
was inversely associated with being female and
urinary Na/Cr, and positively associated with BMI,
dietary calcium intake and SBP (Table 2). These
variables account for 27% of the variance. Dietary
magnesium intake was not associated with BMD at
either site.
Discussion
In our Chinese population, an earlier population
survey showed a high sodium and low calcium
Diet, blood pressure and osteoporosis
J Woo et al
453
Table 1 Regression of total hip BMD (g cm2) (n ¼ 1098)
Variable
Age
Female
BMI (kg m2)
Urine Na/Cr (mmol mmol1)
Urine K/Cr (mmol mmol1)
Dietary magnesium per day (mg)
Dietary calcium per day (mg)
SBP
DBP
Mean (s.d.)/prevalence
Unit
72.3 (5.3)
73.7%
23.2 (3.6)
17.3 (10.4)
7.8 (3.3)
379.9 (195.9)
579.6 (282.2)
141.7 (18.9)
77.7 (8.9)
5.3
No/yes
3.6
10.4
3.3
195.9
282.2
18.9
8.9
Percent difference per unit (95% CI)
3.5
16.1
8.6
1.2
1.2
0.1
1.7
0.2
0.1
(4.4, 2.6)
(18.2, 14)
(7.8, 9.5)
(2.1, 0.3)
(0.2, 2.1)
(0.9, 1)
(0.7, 2.7)
(1.2, 0.9)
(1, 1.1)
R2 ¼ 0.3741
Abbreviations: BMD, bone mineral density; BMI, body mass index; CI, confidence interval; DBP, diastolic blood pressure; SBP, systolic blood
pressure.
Bold values (Po0.05).
Table 2 Regression of lumbar spine BMD (g cm2) (n ¼ 1098)
Variable
Age
Female
BMI (kg m2)
Urine Na/Cr (mmol mmol1)
Urine K/Cr (mmol mmol1)
Dietary magnesium per day (mg)
Dietary calcium per day (mg)
SBP
DBP
Mean (s.d.)/prevalence
Unit
72.3 (5.3)
73.7%
23.2 (3.6)
17.3 (10.4)
7.8 (3.3)
379.9 (195.9)
579.6 (282.2)
141.7 (18.9)
77.7 (8.9)
5.3
No/yes
3.6
10.4
3.3
195.9
282.2
18.9
8.9
Percent difference per unit (95% CI)
0.8
16.9
9.1
1.4
0.8
0.5
2.9
1.5
1.2
(0.4, 1.9)
(19.6, 14.2)
(8, 10.3)
(2.6, -0.2)
(0.5, 2.1)
(0.8, 1.7)
(1.6, 4.1)
(0.1, 2.9)
(2.6, 0.2)
R2 ¼ 0.2728
Abbreviations: BMD, bone mineral density; BMI, body mass index; CI, confidence interval; DBP, diastolic blood pressure; SBP, systolic blood
pressure.
Bold values (Po0.05).
intake.24 Only 22% of the population had an intake
of sodium p2300 mg day1, while the average
calcium intake was 570 mg day1. The finding of
this study would be indicative of associations for a
population with high sodium and low calcium
intake. Both low hip and spine BMD were associated
with higher sodium intake, as indicated by urinary
Na/Cr ratio. However, this adverse effect may be
ameliorated by higher calcium intake and also
higher intake for potassium (as indicated by higher
urinary K/Cr) for hip BMD. This finding supports
the pathway of increased obligatory calcium excretion with increased sodium excretion as a result of
increased dietary sodium intake, as the common
underlying pathogenetic mechanism for both hypertension and osteoporosis.
Furthermore, BMD appeared to be more correlated
with Na/Cr and K/Cr compared with BP, as one
would expect the same associations to be present
between BP and BMD, but no significant association
was observed between BP and hip BMD. Although
an association between SBP and lumbar spine BMD
was observed, the direction of the association was
positive instead of negative. Although the explanation for this observation is unclear, salt intake
reflected by urinary Na/Cr would not be a factor as
it is negatively associated with BMD. Therefore it is
possible that in this population, the adverse effect of
high salt intake on BMD could be greater, if not at
least as great as that on BP. In this setting, increasing
calcium intake would be all the more important.
This study also highlights the beneficial role of
potassium and magnesium in relation to hypertension. Increasing potassium intake would also be
beneficial for bone health. The major food source of
magnesium and potassium are vegetables, fruits and
dairy products, suggesting that the finding from the
DASH study using a dietary approach to hypertension with fruits and vegetables among the major
components may also be relevant for bone health.
There are several limitations in this study. There
are inherent inaccuracies in nutrient quantitation
from food frequency questionnaires, and casual
urine collection (rather than 24 h urine collection)
as indicators of sodium and potassium intake.
Estimation of sodium intake is particularly difficult
in this population owing to the liberal use of
added salt, soya sauces and monosodium glutamate
in preparing food. The 24 h urine collection quantifying sodium excretion is the accepted method in
Journal of Human Hypertension
Diet, blood pressure and osteoporosis
J Woo et al
454
many epidemiological studies examining the relationship between salt intake and blood pressure.25,26
However, collection of 24 h urine in elderly people
is more prone to inaccuracies as the procedure is
dependent on adequate manual dexterity and
cognitive function. The volume collected tends to
be less than the true 24 h urine output. Therefore, a
casual urine sample would be more feasible in a
large-scale epidemiological study of the elderly.
Furthermore, studies using casual urine specimens
have yielded similar results to studies using 24 h
urine collections in examining the relationship
between dietary cation intake and BP.27–29 We did
not carry out urine Ca/Cr measurements to show
that Na/Cr is associated with Ca/Cr. Blood pressure
was only measured once. However, we were able to
study a large number of subjects with DEXA
measurements, dietary intake and urinary measurements. In spite of the limitations, we conclude that
sodium intake, reflected by urinary Na/Cr, is the
major factor linking blood pressure and osteoporosis
as shown by the inverse relationship with BMD. The
findings lend further emphasis to the health benefits
of salt reduction in our population both in terms of
hypertension and osteoporosis.
What is known about topic
K The prevalence of hypertension and osteoporosis increases
with age.
K Sodium intake is a well-established risk factor for both
conditions.
K Relative contributions of other cations such as calcium,
magnesium and potassium to both conditions are not as
clear.
What this study adds
K Sodium is the major factor linking blood pressure and
osteoporosis.
K Calcium and potassium may ameliorate the adverse effect of
sodium on bone health.
Acknowledgements
This study received support from the Jockey Club
Charities Trust, the Research Grants Council of Hong
Kong CUHK 4101/02M.
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