Clinical Science and Molecular Medicine (1978) 55,373s-375s
Relationship of blood pressure to sodium excretion in a
population survey
F . 0. S I M P S O N , H . J . W A A L - M A N N I N G , P . B O L L I , E . L. P H E L A N A N D
G. F . S . S P E A R S
Wellcome Medical Research Institute, Department of Medicine, and Department of Preventive and Social Medicine, University of
Otago Medical School, Dunedin. New Zealand
Spears, 1978b). Investigations included keighf
(without shoes; to nearest cm) and weight (without
shoes, coat or jacket to nearest 0.1kg), from which
Quetelet’s index, weight/height*,was derived; blood
pressure and heart rate (seated; electronic version
of the London School of Hygiene and Tropical
Medicine blind manometer; phase IV for diastolic;
3 readings of which the 3rd has been used in the
present analysis of results); 24 h collection of urine
on the previous Sunday; measurement of sodium
(Na+), potassium (K+) and creatinine in the urine
samples. The subjects were unaware that there was
a particular interest in salt.
The data were subjected to multiple-regression
analysis. In addition, the highest and lowest
quintiles for various parameters have been compared. Data from persons on antihypertensive
therapy, or on diuretics or b-adrenoreceptorblockers from any cause, were excluded.
Summary
1. Blood pressure, height, weight and 24 h
urinary output of sodium, potassium and creatinine
were measured in over 500 adults of each sex
during a health survey of the population of a small
town.
2. Both systolic and diastolic pressure were
significantly
related
to
Quetelet’s
index
(weight/height*)and heart rate.
3. There was no significant relationship between
blood pressure and output of sodium or potassium,
sodium/potassium ratio or sodium/creatinine ratio.
Key words: blood pressure, population, Quetelet’s
index, sodium excretion.
Introduction
The relationship between blood pressure and 24 h
urinary output of sodium was examined during a
survey of the population of Milton, a small New
Zealand country town.
Results
Method
The survey was carried out from 7 a.m. to 12 noon
daily during one week of May 1975, in the country
town of Milton, 54 km south of Dunedin. Just over
1200 people, aged 16 and over, took part, i.e. about
83% of the available population. The procedures
and methods have already been described
(Simpson, Nye, Bolli, Waal-Manning, Goulding,
Phelan, de Hamel, Stewart, Spears, Leek &
Stewart, 1978a; Simpson, Waal-Manning, Bolli &
Correspondence: Professor F. 0. Simpson, Wellcome
Medical Research Institute, University of Otago Medical
School, P.O. Box 913, Dunedin, N.Z.
Blood pressure, especially systolic, rose with age; it
was lower in women than in men in the youngest
age groups but was higher in women than in men in
the older age groups, in spite of antihypertensive
treatment being more common in women (see
S i p s o n el al., 1978b for details).
Mean 24 h Na+ excretion (all ages) was 173 k
75 (SD)mmol for men and 140 k 53 mmol for
women. There was no age-related trend after the
age of 20 years; below 20 years, excretion of Na+
was lower in both sexes (details given in Simpson et
al., 1978a).
Stepwise regression analysis showed that both
systolic and diastolic blood pressure were significantly related to age, Quetelet’s index and heart
313s
F. 0. Simpson et al.
374s
TABLE1. Mean valuesfor several variables in the highest and lowest quintiles of sysiolic blood pressure
*** P < 0.005,Student’s I-test for significance of difference between values for highest and lowest quintiles of systolic
pressure.
Men
Systolic blood pressure (mmHg)
Diastolic blood pressure (mmHg)
Heart rate (beatdmin)
Body weight (kg)
Quetelet’s index
Nat output (mmo1/24 h)
Age (years)
Women
Highest
quintile
Lowest
quintile
Whole
group
Highest
quintile
Lowest
quintile
Whole
group
( n = 113)
( n = 113)
(n = 565)
( n = 107)
(n = 107)
( n = 537)
105
69
68
12
2.41
174
39
121
81
12
75
2.52
114
39
158
94
84
69
2.61
137
40
105
68
16
60
2.33
138
40
129
79
80
64
2.50
138
40
155
96
71
81
2.72
175
40
***
***
***
***
rate but not to Na+ or K + output or to Na+/K+ or
Na+/creatinine ratio. Quetelet’s index was significantly related to sodium output and age in both
sexes.
The relationships between the variables was
examined also by dividing each 10-year age group
(men and women separately) into quintiles for a
given variable and then comparing the other
characteristics of the highest and lowest quintiles.
For the whole group (i.e. all ages) each quintile was
made up of a quintile from each 10-year age group
so that all ages would be fully represented.
The quintile with the highest systolic pressure
(Table 1) had, for both men and women, significantly higher mean diastolic pressure, heart rate
and Quetelet’s index than the quintile with the
lowest systolic pressure. However, Na+ output was
identical in these two quintiles.
The quintile with the highest Na+ output (not
shown) had, in both sexes, significantly higher
mean Quetelet’s index than the quintile with the
lowest systolic pressure. However, both systolic
pressure and diastolic pressure were practically
identical in these two quintiles.
Discussion
These results indicate that the amount of sodium
intake (as measured by sodium output) does not, at
least in the range found in this population, influence
blood pressure. This finding runs counter to wellestablished beliefs and traditions based on animal
studies, on epidemiological studies comparing
various populations, and on studies of dietary
manipulation or salt-testing threshold (see reviews
by Dahl & Love, 1957; Dahl, 1961; Joossens,
Willems, Claessens, Claes & Lissens, 1970;
Meneely & Battarbee, 1976).
***
***
‘
‘
I
***
Previous studies of the relationship between
blood pressure and sodium output within a
population have given conflicting results. Whereas
Joossens et al. (1970) in Belgium, and Morgan,
Carney & Wilson (1975) and Doyle, Chua &
Duffy (1976) in Australia, have shown a correlation, Miall (1959) and Dawber, Kannel, Kagan,
Donabedian, McNamara & Pearson ( 1 967) found
none. The Belgian population was, however, not
homogeneous, the elderly being drawn from a
separate group and having a particularly high
sodium/creatinine ratio. Also the correlation between sodium output and blood pressure in women
was not significant when age was taken into
account (as it must be). The Australian papers
compare sodium excretion of 428 mild hypertensive subjects and 53 normotensive subjects from
a screening survey and found 24 h sodium
excretions of 141 and 118 mmol respectively.
However, there were proportionately more female
subjects in the normotensive group and women
have, on average, a lower excretion of sodium than
men. Morgan et al. (1975) compare these values
with a mean sodium excretion of 185 mmol in 150
hypertensive men referred to their hospital clinic. In
the Dunedin Hypertension Clinic, mean 24 h
sodium excretion in newly referred patients is about
175 mmol for men and 137 mmol for women, i.e.
no higher than the mean for the population of
Milton.
Regardless of whether the Belgian and
Australian studies have provided the right answers,
possible errors in our own results have to be
considered. Blood pressure recordings during a
busy survey are not ideal and single 24 h
collections of urine in a population almost inevitably contain some irregularities in the collections.
However, we believe that the vast majority of
Population survey of blood pressure
subjects did their best to make accurate collections.
The use of sodium/potassium ratio and
sodium/creatinine ratio should, to some extent,
overcome inaccuracies in the collection of urine but
these variables fared no better than sodium in the
regression analysis.
It is evident that the relationships between blood
pressure, age, Quetelet’s index and sodium output
are complex. However, exclusion of Quetelet’s
index from the regression analysis does not bring
out any correlation between sodium output and
blood pressure.
We are forced to conclude that if there is a
relationship between sodium intake and blood
pressure, it is not a simple one. No doubt the
average intake of salt is quite unnecessarily high
but the epidemiological evidence on which to base a
major public health campaign to reduce salt intake
is shaky. Freis (1976) has suggested that the reason
why a link between sodium intake and blood
pressure is so hard to find is that once the intake is
over 60 mmol/day, the excess is immaterial. More
studies are clearly needed.
Acknowledgments
This work was supported by the Medical Research
Council of New Zealand and the National Heart
Foundation of New Zealand. The assistance of the
Milton Rotary Club is gratefully acknowledged.
375s
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