CLIN. AND EWER. HYPER.-THEORY AND PRACTICE, A6(4) , 897-903 (1984)

HYPOTHESIS DISCREPANCY BETWEEN INTRA- AND INTERPOPULATION STUDIES OF THE RELATIONSHIP BETWEEN DIETARY SALT AND BLOOD PRESSURE:

FACT OR FICTION?

Per Omvik Medical Department

University of Bergen 5016 Haukeland Sykehus

Bergen Norway

Kev-words:

Hypertension, hypothesis, population study, salt intake.

ABSTRACT

show significant correlation between sodium excretion and blood pressure while a clear relationship exists in cross- cultural studies. Since daily variation of sodium excre- tion is high, the discrepancy between the two observations could be due to non-comparable data on sodium excretion. This is a discussion of the hypothesis that the finding o f a significant correlation or not between sodium excretion a n d blood pressure depends on the statistical analysis of the data.

It is a paradox that intra-population studies fail to

Cross-cultural studies have shown a highly significant

positive correlation between mean daily sodium excretion

and both blood pressure level ( 1 , 2 ) and occurrence of

hypertension ( 3 , 4 ) . This and other findings ( 5 ) has led to

the hypothesis of a cause- and effect relationship between

sodium intake and development of hypertension ( 6 ) .

However, it is a paradox that most studies fail to show

897

Copyright 0 1984 by Marcel Dekker, Inc. 07304077/84/06044897$3.50/0

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a positive correlation between sodium excretion and blood

pressure within each separate society ( 7 , 8 , 9 ) . One hypo-

thesis claims that higher levels of dietary sodium in

high-salt societies might trigger genetic or other factors

initiating the development of hypertension in "salt-

sensitive" individuals while "salt-resistants" would

remain normotensive, enduring the strain of the high-salt

diet (10,ll). In a low-salt society a smaller fraction of

the population would become hypertensive because o f less

stimulation o f the salt-sensitive mechanism. According to

this hypothesis, no significant intra-population corre-

lation between sodium excretion and blood pressure would

be expected since both normotensives and hypertensives

would be exposed to the same amount of salt as part of the

dietary habits o f their society.

An alternative hypothesis is that the discrepancy

between the intra- and the interpopulation studies of the

relationship between sodium excretion and blood pressure

is due to non-comparable data on sodium excretion: In most

intrapopulation studies individual sodium excretion rates

are determined by one or a few 24-hour urine samples

( 7 , 8 , 9 , 1 2 ) whereas in cross cultural comparisons the mean

sodium excretion rates are calculated from large groups o f

subjects, thereby minimizing the effect of the intra-

individual variability o f sodium excretion.

In each subject sodium excretion varies widely from one day to another depending on a number o f factors including

neurRhumora1 activity, blood pressure and sodium intake.

The m o s t powerful determinant is sodium intake: following

a change in dietary salt the sodium excretion rapidly

readjusts to match the new level of sodium intake ( 1 3 , 1 4 1 .

At the extreme ranges of salt intake the sodium excretion

can be altered by a hundred - or even a thousand fold

( 1 5 ) . Thus, it is not astonishing that the individual day-to-day variation o f sodium excretion is high with a

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RELATIONSHIP BETWEEN SALT AND BLOOD PRESSURE? 899

coefficient of variation of 4 0 % ( 9 , 1 2 , 1 6 , 1 7 ) and with

correlation coefficients between two 24-hour observation

periods ranging between 0 . 4 0 and 0.60 ( 9 , 1 7 1 . I n compari- son the daily variation of blood pressure is small ( 1 8 ) .

As shown by Stamler et al. (12) a large number of urine

samples from each subject is needed to minimize the large

intra-individual variation of sodium excretion. After 14

24-hour collections this variation was less than the

inter-individual variability and the "true" relationship

between sodium excretion and blood pressure, became

apparent.

Alternatively, as discussed by Watt et al, the

number o f subjects needed to detect a significant corre-

lation between the two parameters would be high (in the order of six thousand) if single urine collections were

used for each individual ( 1 9 ) . However, the group mean

level of sodium excretion is essentially the same whether

single or multiple urine samples from each rubject are

used in the calculation; by both methods t h e total number

of urine collections is high. Similarly the reproduci-

bility o f the mean rate of sodium excretion within a

population is remarkably good ( 9 , 1 7 , 2 0 ) , possibly because of the large number of samples masking the intra-

individual variability. Thus, although studies based on

single urine collections fail to demonstrato significant

correlations between individual sodium excretion rater and blood pressure, the mean rates of sodium excretion are

close to the true level of sodium intake within the society and might be used for cross-cultural comparison.

B y analogy the same analysis would apply if subgroups

within a single population were compared: despite lack of

'overall correlation between blood pressure and sodium

excretion within the whole population mean values of sodium excretion for subgroups might still be correlated

t o the mean blood pressure of each subgroup. This was

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900 OPNIK

shown in a recent study of blood pressure and daily sodium

excretion among middle-aged men living in three districts

of different geographical location in the Sogn county of

Western Norway (17).

The above analysis suggests that the discrepancy

between intra- and inter-population studies of the

relationship between dietary salt and blood pressure is a

fiction due to non-comparable data on sodium excretion.

Thus, it is probable that blood pressure is interrelated

with sodium excretion within communities as well as

between different communities. However. it is important to

note that the demonstration of a positive correlation

between the two variables in population studies does not

prove a cause-and-effect relationship between salt and

hypertension. From acute studies both in man and in

experimental animals it is well known that changes i n

blood pressure might induce large changes in sodium

excretion by the pressure natriuresis mechanism (21,22,23)

while even monstrous increments in sodium intake might

cause only minor alterations in blood pressure ( 1 5 ) .

Furthermore, most studies have shown that essential

hypertensives are in either perfect sodium and body fluid

volume balance (24,251 or slightly hypovolemic (26,27,26)

rather than hypervolemic.The higher levels o f sodium

intake in Western societies might therefore reflect more

craving for salt in order to maintain sodium balance in

the face of higher levels of sodium losses due to pressure

natriuresis (14).

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RELATIONSHIP BETWEEN SALT AND BLOOD PRESSURE? 901

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