The Yanomamo Indians ,

Transcription

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Populations, salt and blood pressure
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u atzons, sa t an
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ressure
There have been nearly 40 accounts of certain primitive populations in
which blood pressure did not rise with age - in other words they did not have
essential hypertension. Their dietary intake of salt was below 3 g per day, in
a few it was below 1 g per day, and in one it was about 0.05 g per day. At the
other end of the scale, there have been studies among several Japanese and
Portuguese communities with a high prevalence of hypertension, in one the
salt intake averaged 26 g per day. In between there are the bulk of westernised societies, which consume around 7 to 12 g of salt per day with an
average of 10 g per day. The connection between salt intake and hypertension in these intermediate populations is evident but more difficult to
discern mainly because of the narrow range of salt intake.
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Yanomamo Indians fighting'. (Chagnon NA. Yanomamo. The Fierce People.
Holt, Rinehart & Winston, New York, 1977. Reprinted by permission of Holt,
Rinehart & Winston.)
by wild fruit and insects. Their staple foods are cooked bananas and manioc.
In most villages there is little access to salt, refined sugar, alcohol, milk or
dairy products. In one group of 206 persons aged between 20 and 50 years
which comprised all the adults from three villages, the mean 24-hour
urinary excretion of salt was 0.05 g per day with a potassium excretion of 3
g per day. The mean weight of the men (50 kg) was about the same as that
of chimpanzees (mentioned in the previous chapter) their blood pressure
was much lower than that found in western populations, and there was no
rise in blood pressure with increasing age.
Admittedly there are many differences between persons living in a
western community and Yanomamo Indians, but one explanation put
forward for the difference in blood pressure was spectacularly incorrect. It
was suggested that the Yanomamos lived an arcadian existence in which the
certainty of their behaviour, which was surmised to be ruled by ritual and
taboo, was in contrast to the 'stressful uncertainties of western society in
which life is a set of individual choices and decisions'. An anthropologist,
Napoleon Chagnon, studied the Yanomamo tribe for about 15 years, living
with them part of the time. He wrote a book entitled Yanomamo. The Fierce
Primitive societies
Low salt eating primitive societies used to inhabit the tropics of Africa,
South America and the Pacific and there was one in the Arctic in a group of
Eskimos. The outstanding example is the tribe with the lowest sodium
intake - the Yanomamo Indians.
The Yanomamo Indians
They are probably the most primitive native tribe in the world. They live in
about 100,000 square miles (258,999 km 2) along the border between
Venezuela and Brazil. There are approximately 18,000 individuals scattered
through the Amazon rainforest in about 200 villages with 40 to 200 persons
in each. They have been described as semi-nomadic 'slash and burn'
agriculturists living on a diet of locally produced crop and game supported
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Salt, diet and health
People which describes how they live a life of chronic warfare with violence
except for the genes, that the absence of anyone of them or a combination
of them can cause hypertension. The Yanomamos have the lowest salt intake
of any population and consume a diet that is similar to that of early humans.
They have the lowest blood pressure of any population with no rise in pressure with age. They probably represent the ultimate human example of the
importance of salt on blood pressure.
and tension. Warfare is endemic and fluctuates in intensity from region to
region. The culture encourages aggression and though it is controlled to a
certain extent by a system of graduated violence it often breaks down.
Chagnon describes one such breakdown in the village of Patanowa-teri,
One of the young men took the wife of another because she was allegedly being mistreated by him. This resulted in a brutal club fight that
involved almost every man in the village. The fight escalated to jabbing
with the ends of the clubs when the husband of the woman in question
was speared by his rival and wounded. The headman of the village had
been attempting to keep the fighting restricted to clubs. When the
husband's rival speared his opponent, the headman went into a rage and
speared him in turn, running his own sharpened club completely
through the young man's body. He died when they tried to remove the
weapon. The wife was then given back to her legitimate husband who
punished her by cutting both ears off with his machete.
The kinsmen of the dead man were then ordered to leave the village
before there was further bloodshed. The aggrieved faction joined the
Monou-teri and Bisaatri-teri because these two groups were at war with
their native village, and they knew that they would have an opportunity
to raid their own village to get revenge. The Monou-teri and the two
Bisaatri-teri groups accepted their new arrivals - they were kinsmen
and would actively prosecute the war against Patanowa-teri. The hosts,
of course, took several women from the refugees, the price a vulnerable
group must pay for protection.
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As Denton comments, the idea that Yanomamos are emancipated from
mental stress in contrast to members of a Western community,
is hardly credible unless we are to argue that stress associated with
attack, and fear of humiliation of bodily injury and death does not
access those blood pressure raising mechanisms which are stimulated by
the anxieties of Western life, 'the rent, the corporate pecking order, etc'.
An anthropological argument has been put forward that the low blood
pressure of the many primitive societies is better explained by genetic and
cultural factors than by their intake of salt. There is an implication that the
effect of heat, cold, infections, traditional cultures and the gene pool can
explain the lack of the rise of blood pressure with age and that each prim i~ive society will have a different reason for its low blood pressure. Though it
IS true that these variables can effect the blood pressure there is no evidence,
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Solomon islanders
The effect of salt intake on the blood pressure of the Solomon islanders, some
of whom, unlike the Yanomamos, had had some modest contact with western
civilisation was studied by Lot Page in six of the islands. The physical fitness
of all the participants was excellent, in keeping with other studies in a primitive society which indicates that the low blood pressure is not due to ill
health as has sometimes been proposed. All six tribes were Melanesians
living in villages in rural areas with no electricity or other western conveniences. Their habitats varied, three were in the mountains and three on or
near the coast. Their main staple diet was potato and vegetables. Those on
the coast also ate a great deal of fish and were able to purchase tinned meat,
fish, rice and bread. Those who lived in the mountains ate less than 2 g of salt
per day, while two of the three tribes who lived near the coast ate 3 to 8 g of
salt per day, and the other cooked in copious amounts of sea water and had a
salt intake of 9 to 15 g of salt per day. The potassium intake, which was not
given, must have been substantial. An increase in blood pressure with age
occurred in the women of the three tribes with the highest salt intake, but
there was no such change in the men. In the three tribes with the lowest salt
intake only 1% had high blood pressure, in the two tribes with a moderate
intake of salt 3% had high blood pressure, and in the tribe which cooked in
sea water which had the highest salt intake 8% had high blood pressure. It is
probable that the relatively moderate effect of salt on blood pressure and lack
of rise of blood pressure with age in men was related to a high potassium
intake. Usually when individuals from primitive communities on a low-salt
high potassium diet move to a western way of life the resulant increase in salt
intake is associated with a considerable reduction in potassium, which
accentuates the rise in arterial pressure caused by raised salt intake. There
was no evidence that contact with western influences including increased
access to fats had had any consistent effect on weight, which suggests that the
change in the diet which caused the hypertension was the raised salt intake.
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The Mundurucu and the Caraja in Brazil
presumably on the surface of the numerous salt domes associated with the
vast accumulation of oil in the Persian Gulf. The most important dietary
staple was the very thin flat bread, baked daily, consumed at every meal. It
contained an average of 2% salt by weight. Salt was also added in plentiful
amounts when cooking meats and in the preparation of cheese and milk, and
it was also added at the table, even to the bread. Drinking water was usually
low in salt but sometimes natural salt water sources were used for drinking
during periods of travel. Two water samples contained 3 and 7 g of salt per
litre.
The mean urinary salt excretion in the men was 11 g per day and in the
women 9 g per day, the 24-hour potassium excretion around 2 g per day in
each. This high-salt/low-potassium intake is typical of the diet in western
culture and contrasts with the low-salt/high-potassium intakes of primitive
societies who mainly eat fruit and vegetables. The men weighed 59 kg and
the women 49 kg. The blood pressure in this nomadic primitive society consuming a diet with the same level of salt intake as that of economically developed societies, increased with age in both the men and the women, as in
developed societies, but in contrast to them there was no tendency for an
increase in body weight.
A further example, from Brazil, of the effect on blood pressure of increasing the salt intake of a primitive tribe with minimal accompanying contact
with western civilisation, while another adjoining tribe remained primitive.
Forty-five years prior to the study the Mundurucu had moved from a
savana-like forest to be near to a Fransican mission on the Cururu river
where they had come under the influence of the Catholic church and thereby
access to salt. Otherwise, there was no substantial change in their diet. The
Caraja lived on a nearby river and continued to eat their traditional low-salt
diet. There was a rise in blood pressure with age in the men of the
Mundurucu and a similar but not significant trend in the women. The blood
pressure of the Caraja did not rise with age. The Mundurucu's contact with
western ways consisted mainly of accepting some of the culture of Catholic
Brazil. There was no change in climate or their surroundings. They were
certainly not exposed to the stress of western urban life and diet. Unless it
is suggested that religious conversion induced a deeply disturbing harmful
effect rather than a joyful uplift, it is probable that the rise in arterial pressure was due to increased salt intake. Glieberman is more cautious,
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Although the salt could be the major factor, the relinquishing of traditional religious beliefs for the levels of Catholicism may be causing an
unconscious stress which would be difficult to eval uate.
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The Qash 'qai in Iran
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There is one striking study which demonstrates that, whatever subsidiary
factors may be suggested other than the dietary intake of salt, to account for
the relatively low blood pressure of primitive communities, they were not
sufficient to prevent the blood pressure rising in one primitive population
that habitually consumed a high-salt diet and lived in relative isolation from
western culture. The study, also by Lot Page, is in a population of traditional
nomadic herdsmen of the Qtsh'qai tribe of Turkish origin in southern Iran
who have been in this area for approximately 400 years. The total population was estimated to be about 400,000, a large proportion of whom continued to practice pastoral nomadism, herding goats and sheep. In response to
the needs of the animals for pasture and water, the Qash'qai migrated
summer and winter over distances varying from 50 to 600 km. The study
took place at six sites that covered an area which contained many natural
surface deposits of salt which the tribe used liberally. These salt deposits are
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Nigeria
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A recent study of two related rural tribes in Nigeria, one of whom had access
to a salt lake and used salt in its cooking, whereas the other had no access to
salt, showed that urinary salt excretion was increased in those who added salt
to their food compared to those who did not, and that there was a highly significant difference in blood pressure between the two tribes who were identical in all other characteristics that were studied.
One exception
There is one interesting primitive tribe, the Kuna Indians who live in the isolated San Bas Island chain off the Caribbean coast of Panama, who, though
the documentation of their salt intake was only an approximation, appear to
be an exception. They have no rise in blood pressure yet a dietary assessment
indicates that the consumption of salt was probably more than 8 g per day.
This assessment was mainly based on a rough intake of salt excretion and
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each subject's recollection of how many teaspoons of salt they had added to
their food. Potassium intake, however, did not appear to be raised.
subjects was weighed on scales and its nutrient content calculated using
standard tables of food composition. Once a year these subjects had a physical examination, which included a measurement of weight and blood pres-
Migratory studies
There are examples of groups of individuals from low-salt eating communities whose blood pressure rises when a change in their circumstances
causes them to eat more salt. The best of these was a carefully controlled
study from Kenya where subsistence farmers ate a low-salt/high-potassium
diet. Some of the farmers migrated to an urban community where there was
a marked increase in salt intake and a fall in potassium intake to levels similar
to those in western countries. Blood pressure in these migrants rose whereas
in a control group who did not migrate there was no change in blood pressure. It was concluded that a major part of the blood pressure rise that
occurred on migration was due to the increase in salt intake.
High salt-intake societies
There are several societies in the world that continue to have a very high salt
intake comparable to what was eaten in most western societies during the
nineteenth century before the introduction of refrigeration and canning.
These include Portugal, northern China and Japan. Of these the one that
has been studied longest is Japan.
Japan
In the late 1950s the Japanese became aware that certain parts of the country
had the highest salt consumption and deaths from stroke due to brain haemorrhage in the world, and that in these areas the prevalence of high blood
pressure was greater than in the rest of the country. The Japanese have had
a strong interest in nutrition for over 100 years dating from their demonstration that beri-beri could be prevented by altering the diet. Since the Second
World War, which caused much acute malnutrition, the Ministry of Health
has carried out a yearly nationwide survey of food intake together with an
assessment of the health of the population. Until 1964 about 1 in 2000 of the
population was picked at random each year from the census returns. Four
times a year on three consecutive days the food consumed by those selected
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The surveys showed that vegetable foods dominated the diet. Rice was
served at every meal and provided over half the intake of calories and a third
of the protein. Wheat, in the form of noodles and bread, was second in
importance. Proteins came predominantly from the rice and fish. Most of
the fat came in the form of vegetable oils. The consumption of salt was obviously very great but difficult to assess precisely with the methods in use in
the national surveys. Therefore, to obtain a more accurate estimation of its
intake the 24-hour urinary excretion of salt was measured in several groups
throughout the country. It turned out that the average salt consumption was
15 g per day, but the outstanding discovery was its regional variation with a
maximum in certain individuals up to 60 g per day. In the north east province
of Akita the mean urinary salt excretion in 300 adults was over 26 g per day.
Seasonal variations in salt consumption were equally striking with the
highest from November to February and the lowest from June to September.
The high salt consumption was due to foods that are traditionally used to
accompany the rice, which is considered to taste sweet. This was disguised
by the ample use of various salt-rich sauces. Miso, which is a product of soya
beans and yeast, has a salt content averaging 12%. It was served as a soup
(muso shiru) containing 1.5% salt. Shoy, the most popular seasoning contains
18% salt. Salted fish and vegetables, preserved in brine, were also consumed.
The seasonal variation in salt consumption was due to the ingestion of salted
vegetables which rose in the autumn, reaching a peak by the end of the year
when it fell until the next summer. In addition, it was noticeable that in Akita
not only did the miso contain more salt than in the south but the local inhabitants had a habit of eating many salted vegetables when they drank tea
between meals.
It is highly relevant that the persistence of this traditional vegetarian low
animal fat diet, which was consumed on a wide scale well into the 1950s and
1960s, was mainly due to the late introduction of the cow (milk and bee0
into Japan. The low consumption of beef, milk and milk products before thIs
time was responsible for the low incidence of coronary heart disease and
other forms of atherosclerosis. Fat consumption doubled from 1950 to 1964
and has continued to increase. It is responsible for the rapid increase in
obesity and heart disease that is now taking place.
In the early 1960s, it was noted that the death rate from what was termed
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Regional distribution of cerebral haemorrhage in Japan in 1957 - highest in the
north and lowest in the south. The intake of sodium in four areas is also indicated, 14 grams (g) per day in the south and 27 g per day in the north.
(Takahashi E. et al. Human Biology 1957; 29: 139-66.)
'apoplexy', that is, a sudden cerebral catastrophe or stroke, which in Japan
at that time was mainly due to an acute cerebral haemorrhage from the
rupture of a cerebral artery, was greater than in any other population, even
African-Americans who until this time had had the highest incidence.
However, the incidence of 'apoplexy' from a clot in an atherosclerotic cerebral artery was one of the lowest in the world. Furthermore, it emerged that
the prevalence of acute cerebral haemorrhages had a remarkable regional
distribution. It was much greater in Akita than in the southern region of
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in 1957. There was a parallel variation in dietary salt intake and blood pressure.
(Takahashi E. et al. Human Biology 1957; 29: 139-66.)
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Akimbo. This paralleled the regional difference in salt consumption.
Similarly the National Survey showed that, though the blood pressure of the
Japanese was much the same as that in the United States, there were marked
regional differences which matched those of acute cerebral haemorrhage
and salt consumption. The rise in blood pressure with age in the high-salt
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consuming Akita was much steeper than in the south. The number of hypertensive individuals aged between 50 and 60 years in one community in the
north-east was 70% whereas it was only 10% in the south. It was also noted
that in the north-east there was a seasonal fluctuation in the blood pressure
being substantially higher in winter than in summer. The highest pressures
therefore occurred when and where the salt intake was highest, which probably explained the finding that the number of strokes in the north-east was
greatest in the winter.
ated since 1960. They were underlined by the finding in the 1970s and
1980s that in some rural communities in Japan, China and Taiwan, in which
there was a wide range of salt intake between individuals but in which the
diet in each individual did not vary from day-to-day and therefore the salt
excretion was relatively constant, there was a direct relationship between
24-hour urine salt excretion and blood pressure within the individuals in a
single community.
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The relation of blood pressure to salt intake between individuals in
a population
The low average blood pressure of low-salt eating communities and the
hypertensive apoplectic levels of blood pressure in certain other societies,
including the hypertensive Japanese communities which were eating a highsalt diet, made a good foundation for the suggestion that in humans salt
intake determines the blood pressure. Indeed, several studies have confirmed, after comparing a wide range of communities, that there is a significant relationship between salt intake (measured as 24-hour salt excretion)
and blood pressure. One obstacle, however, bothered a great many researchers and continued to do so increasingly as more population studies were performed. It was that, within a single community there was no relationship
between the blood pressure and the salt intake of its individuals.
There are many reasons for this apparent lack of correlation between the
blood pressure and the intake of salt within the individuals of a single community. Blood pressure varies from day to day within an individual and also
depends on the manner in which the blood pressure is measured. More
important is the fact that while the spread of the average amount of salt
ingested by individuals of a single western community is relatively narrow,
the day-to-day fluctuations in salt ingested, and excreted by each individual, in anyone 24-hour period may vary enormously. Day-to-day variations
in 24-hour salt excretions of 3 to 18 g per day have been recorded. Such
variations depend on the type of food eaten the previous day and its salt
content. In order to obtain an accurate estimate of anyone individual's
average salt excretion it is necessary to measure the salt excretion on more
than five occasions, which is not practical when studying large numbers of
people. Such very large methodological difficulties obviously mask the
detection of differences between subjects. These facts have been appreci-
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The relationship of the blood pressure to salt intake between
populations
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Dahl was the first to relate salt intake to the incidence of hypertension across
several countries. Over a number of years he and his associates had measured
the blood pressure and the 24-hour urinary excretion of salt in Alaskan
Eskimos , Marshall Islanders in the Pacific and employees at the Brookhaven
laboratory in the United States where Dahl worked. He had, in addition, the
results of Fukuda's investigations in northern and southern Japan. When
the results were plotted, the correlation between the average daily salt intake
and the prevalence of hypertension across these five centres was excellent.
The Eskimos, whose salt intake appeared to be 4 g per day had no hypertension, the Japanese with the highest salt intake had the highest prevalence
of hypertension, and the other three were in between, lying close to the line
joining the other two. The implication was that, though the relation between
salt intake and blood pressure was not evident within the individuals in most
populations, salt intake controlled the blood pressure. These findings failed
to impress the majority of people in the West doing research into hypertension or looking after patients with hypertension. With some justification it
was pointed out that the collection of data had been patchy, the age of the
subjects and the definition of hypertension unclear, and the number of
observations between the two extremes of salt intake too small to judge
whether salt intake and hypertension in these populations might be linked.
One critic regarded Dahl's diagram 'as a work of the imagination but of
no value as evidence'.
This resistance was bolstered by the reaction of these same critics to
another of Dahl's investigations. Before embarking on full-scale epidemiological studies in humans, he had thought that it might be useful to try and
get some information about the relation of salt to blood pressure in the
employees at Brookhaven. He first studied their use of salt at the table
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where, as he put it, 'the salt-shakers were ubiquitously available'. He found
that the personnel could be classified into three groups, those who had: (1)
a low intake of salt who did not add and had never added salt to food; (2) an
average intake of salt who added salt to food only if, after first tasting it, they
found it insufficiently salty for the palate; and (3) a high intake of salt, who
were in the habit of adding salt to food without first tasting its degree of
saltiness. Later, he asked a colleague who looked after the employees to
enquire from all on whom he had made a physical examination what were
their customary salt-adding habits according to this classification. It turned
out that the incidence of hypertension was significantly different from a
random distribution. Though the mean blood pressure of the three groups
were not significantly different, among those classified as having been on a
low intake throughout their lives there were significantly fewer persons with
a high blood pressure than among those classified as having been on a high
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Sodium (g/day)
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The first international population study of the relation of the salt intake to
blood pressure. (From Meneely G.R. & Dahl L.K. Medical Clinics of North
America 1961; 45: 271-83. Reprinted by permission of WE. Saunders
Company.)
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The relation of the blood pressure of men aged 50-55 years to their salt intake
in 27 populations according to Glieberman. (From Glieberman L. Ecology of
Food and Nutrition 1973; 2: 143-56. Reprinted by permission of Gordon and
Breach Publishers.)
intake. It would perhaps have been better if Dahl had kept this little experiment to himself. It was, after all, only a preliminary reconnaissance, the
results of which encouraged him to undertake the population studies. The
'salt-shaker' experiment was easy meat for those who wished to denigrate
his work.
Ey the early 1970s many more studies had been carried out and
Glieberman, an anthropologist, reviewed all the available information using
data from men and women aged 50 to 55 years in 27 populations. The salt
intake in 11 populations had been derived from a 24-hour urinary output in
a segment of the population, and in 10 there was a dietary estimate of the
mean salt consumption. For the remaining six studies, Glieberman herself
estimated the salt intake from what she called 'non-quantitative data', i.e.
imprecise information. This she did for some primitive communities who,
though they did consume some salt, she considered that their 'consumption
of prepared foodstuffs was likely to be minimal'. Across the 27 populations
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the mean salt intakes correlated well with the mean blood pressure in both
men and women. This correlation was also positive in the 11 studies in
which urinary salt excretions were available and, though less significant,
appeared to substantiate the relationship between salt and blood pressure.
Glieberman's review added weight to the proposition that the salt intake
of populations affects their blood pressure. It certainly stimulated thought,
revived the subject and focused much constructive criticism. The rather
haphazard assessment of salt intake was the major fault, but it was also
pointed out that, as the data had been obtained from many different sites,
different criteria had probably been used to select subjects, measure blood
pressure and to collect the urine. More importantly, the effect of other
factors, such as potassium intake, body weight and alcohol intake, which
were now known to affect blood pressure, had not been taken into account.
In statistical jargon, such factors are known as confounding variables.
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The Intersalt study
In the summer of 1982, thirty clinical epidemiologists met in Tuohilampi,
Finland, for a two-week residential course under the supervision of Stamler
and Rose. They were given the task to devise an epidemiological project that
would overcome some of the objections raised against the international salt
and blood pressure hypothesis so enthusiastically launched by Dahl. To
remedy the drawbacks of previous studies it was proposed to apply standardised methods across a variety of populations, simultaneously examining
some major confounding variables, and with sufficient numbers also to
evaluate relationships in individuals. Thus was born the Intersalt study - the
largest ever international epidemiological comparative study in 52 centres
worldwide representing both the rich and poor countries. To ensure quality
control, all investigators were called to one of the five training sessions held
around the world for special retraining in blood pressure measurements.
Each centre recruited 200 men and women aged 20 to 59 years, who were
divided by age and sex into eight equal 10 year groups. Altogether 10,079
subjects were involved. The start and end of the 24-hour urine collection
was supervised and a seven-day alcohol intake was assessed by questionnaire. Urine samples were sent to a central laboratory in Belgium. An
attempt was made to assess the individual fluctuations in 24-hour urine salt
excretion by obtaining a second urine collection from 8% of the participants
who were randomly selected. The main findings were published in 1989.
Further analyses of the data has continued since.
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Urinary salt excretion over the 24-hour period varied from 0.05 g per day
in Yanomamo Indians in Brazil to 16 g per day, in Tianjin in China. The
levels recorded at the Japanese and Portuguese centres were unexpectedly
lower than in previous published reports. Thus, the overall range was narrower than had been expected - in more than half the centres (mainly in
Europe) the range was between 8 and 11 g per day. Potassium excretion
ranged from 1 to 3 g per day, being generally low in the Far Eastern centres
and highest in Finland, the Federal Republic of Germany, the Netherlands
and among the Xingu in Brazil. Overweight was rarest in Northern India,
but it was common in many populations particularly in Hawaii. Alcohol
intake varied even more. Two outstanding findings from the results
emerged. The first was that the salt intake of individuals within these diverse
populations was significantly and independently related to their blood pressure. This disposed of one of the main objections previously levelled against
population studies on the relation of the blood pressure to salt intake. The
second was that, when comparing whole populations, one with another, the
higher the salt excretion the steeper the rise in blood pressure with age.
After adjustment for body weight and alcohol intake in the 48 centres in
which there was such a rise, it was calculated that if the intake of salt was
lowered by 6 g per day over 30 years (e.g. from age 20 to 50 years) the average
blood pressure of a population would be 9 mmHg lower. On a population
basis this is a major reduction in blood pressure. This effect would be greater
the higher the potassium intake and the lower the body weight and the
alcohol consumption. It was calculated that an increase in potassium excretion of 0.6 g per day would lower the blood pressure by I mmHg. Body
weight and alcohol intake also have independent effects on blood pressure,
for instance, an intake greater than the equivalent of 300 ml of pure alcohol
per week or more than 3 to 4 alcoholic drinks per day, raises the blood pressure by 3.0 mmHg.
One is so accustomed to thinking about the blood pressure in terms of
individuals, particularly those with the highest pressure who have the worst
outlook, that the initial reaction to these findings in population studies is
how little effect salt, and the other variables studied, have on blood pressure.
A difference, therefore, in salt intake of 6 g per day over 30 years causing a
difference of 9 mmHg does not seem very much. But graphically, the distribution of blood pressure in a population has a bell-shaped curve. Of those
defined as having hypertension, only a minority at the far right of the curve
will have severe hypertension; the bulk will only have a modest to moderate
rise in pressure. And though the complications of hypertension such as
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Mean Sodium Excretion
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o Papua New Guinea Sample
" Kenya Sample
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The relation of the blood pressure to the sodium excretion in the four Intet'salt
populations with salt excretion below 3 grams per day. (From Carvalho]. et al.
Hype1"tension 1989; 14: 238-46. Reprinted by permission of the American Heart
Association. )
Urinary Sodium Excretion as grams of salt/day
The relation of salt excretion to the slope of the rise in blood pressure, adjusted
for age, sex, weight and alcohol consumption in 52 centres. (Redrawn from the
Intersalt study. British MedicalJournal1988; 297: 319-28.)
stroke and coronary heart disease affects proportionately fewer of the subjects with these lower levels of blood pressure, because of their much greater
numbers the bulk of complications occur in this group (page 139). Rose was
the first to point out that even a small fall of 2-3 mmHg in mean blood pressure in a population would reduce the number of deaths from stroke by 7%
and from heart attack by 5%. On this basis, the possibility of lowering the
mean blood pressure of a population by about 10 or more mmHg would have
a profound effect. An estimate by Law along the same lines suggests that in
the UK a reduction in dietary salt of only 3 g per day would prevent 40,000
deaths a year.
In the Intersalt study there were four isolated populations with salt excretions of 0.05 to 3 g per day who had low blood pressure, which showed little
or no rise with age. This was consistent with the many previous reports
which have failed to identify hypertension in any population with a low salt
intake. Nevertheless, it is interesting that in these four populations, all eating
I
•
a low-salt diet and with a relatively low blood pressure, the distribution of
blood pressure within the four increased in a pattern consistent with the salt
excretion - i.e. the Yanomamo had the lowest overall blood pressure while
the Kenyans, who had the highest salt intake of 3 g per day, had the highest
blood pressure.
Another interesting and relevant finding that surfaced from the Intersalt
study was the relation of the blood pressure to education. Those with less
education had on average a higher blood pressure and urinary salt excretion,
a lower potassium excretion, a greater weight and higher alcohol intake. This
confirms and explains several previous studies that showed that a lack of
education, tended to be related to hypertension .
I
Differences between developed and developing countries
A recent review of a large number of studies on the relationship between salt
intake and hypertension has revealed that salt intake is the most important
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118
119
,
,
,
predictor of blood pressure whatever the economic development of the
country in which the subjects lived, However, for a given salt intake, the
blood pressure is slightly higher in the developed countries than in the
developing countries,
,
1
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i
The effect on the blood pressure of a population of reducing the
intake of salt
I
,
,
If the habitual salt consumption of a population determines in part the
prevalence of hypertension it would seem reasonable to propose that a
reduction in salt intake should lower the blood pressure. But it is extremely
difficult to lower the salt intake of a whole population. It has been attempted
with varying measures of success in Japan and Belgium. On a smaller scale
a most impressive trial was carried out in a village community in Portugal.
,
,,
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Japan
The most comprehensive and wide-ranging endeavour took place in Japan.
As a result of much public health activity to reduce salt intake the natural
average salt consumption was reduced from 13.5 g per day to 12.1 g per day.
The change was greatest where the salt intake was highest. For instance, in
the province of Akita, between 1960 and 1989, the salt intake was reduced
from 18 g to 14 g per day. This reduction in salt consumption in Japan was
accompanied by a gradual fall in average blood pressure, the prevalence of
hypertension and a marked decline in mortality from stroke. In local studies,
the beneficial effect of reducing dietary salt was also evident in school children in whom there was a large fall in blood pressure.
During this period in which the salt consumption was coming down
many other changes were occurring inJapan. For instance there were extensive efforts to detect persons with hypertension and to treat them with blood
pressure lowering drugs. At the same time, there were also profound changes
in life style. The Japanese experience was a remarkable unplanned experiment into the effect on the blood pressure of westernisation combined with
a reduction in salt intake. A largely feudal society that was slowly becoming
westernised before the Second World War suddenly became a democratic
industrial giant after it. All the worst features of western society, including
its dietary habits, barged their way into Japan. The modest reduction in salt
intake was accompanied by an increase in fat. The 'stressful uncertainties in
Changes in mean systolic blood pressure level in school children in Grade 1
(12-13 years old) Grade 2 (13-14 years old) and Grade 3 (14-15 years old), The
blood pressure of each new intake into these grades was measured at the beginning of the year. (From Sasaki N, Prophylactic Approach to Hypertensive
Diseases, eeL Y Yamori et aL, pp, 467-74, Raven Press, New York, 1979,
Reprinted by permission of Lippincott-Raven Publishers.)
I
which life is a set of individual choices and decisions', a euphemism for the
market-led rat race of western societies became the norm. Cigarette
smoking, alcohol drinking, beef eating and the modish consumption of
sweet fizzy drinks rose, as did the suicide rate. For an employee to take a
holiday, though it might be permitted in his contract of work, became a show
of weakness and a suspicious lack of commitment. No population ever
became so westernised so quickly and with so much enthusiasm. According
to those who believe that the western influences that cause the blood pressure to rise have little to do with salt consumption, the blood pressure of the
Japanese between the 1950s and the 1980s should have increased. In spite of
all these changes and increasing obesity, there was a fall in blood pressure
with a 80% reduction. in stroke mortality. These results suggest that, by
comparison with the effect of salt intake, those other environmental factors
associated with westernisation, which have been claimed to influence blood
pressure, are of little importance.
120
Belgium
In Belgium a successful mass campaign against salt was started in 1968. A
rather limited study in six groups of elderly subjects (a total of 3,328
persons) aged between 70 and 81 years showed the same trend in blood pressure as in the Japanese. Between 1967 and 1986, the prevalence of hypertension in these subjects decreased substantially and severe hypertension
disappeared. This blood pressure fall was accompanied by a reduction in salt
excretion from 16 g to 11 g per day in men and from 12 g to 9 g per day in
women. It occurred in spite of a small rise in body weight and a fall in potassium intake in the men. Some of the overall improvement was due to treatment of those subjects who had been found to have hypertension previously,
but the fall in pressure in the untreated was the same as in the whole group.
121
Intervention
I
Trial
Two
Portuguese
Villages
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The most clear-cut example of the effect of reducing salt intake on the blood
pressure of a community occurred in Portugal, which is notorious for its high
salt consumption. The trial was carried out in two communities within the
same district, each with about 800 inhabitants, who had a very high salt intake
of about 21 g of salt per day, and 30% had high blood pressure. Each community was close-knit and rural, which was particularly favourable for health
education. One village was used as the control for the other in which the
intake of salt was reduced. Community awareness of the problem of stroke
and hypertension probably enhanced co-operation and acceptance of advice.
Active intervention lasted two years, it involved the whole community but the
responses were assessed in random samples. About half the total salt intake
was added in the kitchen, which was easy to influence, another 30% came
from salted cod fish and the rest was in bread. Those who were responsible
for the cooking were advised to add less salt and instead to use herbs and other
alternative flavourings (often previously unknown in the village), which
became popular. The community as a whole was advised to eat less cod and
fewer sausages, which also contained much salt, and the bakers were asked to
reduce the salt added to bread by about 50% during the two years of the trial.
From urine samples it was estimated that by the end of the second year
there had been a 25% fall in salt intake, but this was found to be an underestimation. A dietary survey indicated a fall of 47% from a salt intake of 21
g to 12 g per day. There was no change in potassium intake. In the control
village the salt intake was 21 g per day at the beginning and 22 g per day at
...
P<O.001
Compared to
Control Group.
Blood pressure changes in two Portuguese villages, in one of which the dietary
intake of salt was reduced. (Drawn from Forte]. G. et al. Journal of Human
Hypertension 1989; 3: 179-84.)
I
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,
I
the end of the second year. The reduction in salt intake in the trial village
was associated with a fall in blood pressure which was highly significant. At
the end of the second year the difference in blood pressure between the two
villages was 13 mmHg in systolic pressure and 6 mmHg in diastolic pressure. Over the two-year period there was an upward trend in the blood pressure of the control village.
The fall in blood pressure could not be explained by chance or a change
in body weight, lifestyle, medical care or potassium intake. The conclusion
that the reduced salt intake had been responsible for the fall in blood pressure was strengthened by finding that those individ uals with the greatest fall
in salt excretion tended to be those who showed the greatest fall in blood
pressure. The fall in blood pressure, moreover, involved the whole community, not just those with hypertension, and the response did not differ
either between the young and the old or between men and women. From the
published findings on the distribution of blood pressure in Portugal, Rose
estimated that a downward shift of 5 mmHg, as observed in this trial, would
reduce the prevalence of hypertension by about a quarter.
122
The initial salt intake, the extent of its reduction, and the rate of fall in
blood pressure were greater in Portugal than in Belgium or in most areas of
Japan. This suggests that the higher the habitual salt intake and the greater
its reduction, the greater the fall in blood pressure. This has a certain resemblance to the effect of reducing salt intake in hypertensive patients, where
the higher the patient's initial pressure the more effective is the reduction in
salt intake. One conclusion that emerges is that there does not seem to be a
threshold of salt intake that has to be reached before the blood pressure of a
population will fall. In both Portugal and northern Japan there was a fall in
blood pressure with a reduction in salt intake though it remained greater
than 12 g per day.
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Hypertension in African-Americans: an hypothesis
A particularly persuasive hypothesis has been put forward by Grim, to
explain how African-Americans come to have the highest prevalence of high
blood pressure in the world. Part of the hypothesis makes use of the now
generally held view, which is discussed in the next chapter, that it is the
kidney's ability to excrete salt which determines whether or not a high salt
intake raises the blood pressure. The prevalence of high blood pressure is
twice as high among African-Americans than in Caucasian American (38%
v. 20%) and claimed to be two to four times higher than West Africans. The
degree of hypertension appears to correlate with darkness of skin colour.
The blood pressure of African-Americans is also more sensitive to increases
in salt intake than that of American whites and they retain an intravenous
load of salt far longer than whites. Conversely it is easier to lower the blood
pressure of African-Americans with a diuretic. All of which suggest that
African-Americans have an enhanced ability to retain salt or, in other words,
they have a diminished ability to get rid of a high salt intake.
The hypothesis put forward to explain the high prevalence of hypertension in African-American proposes that the process of enslavement decimated those who were least able to conserve salt so that the survivors were
individuals who were best able to conserve salt. For 300 years, between 1500
and 1800, over 12 million black people were transported against their will
from the West Coast of Africa to the western hemisphere. Most went to
South America. Only 5.8% of the total, estimated to be about 427 ,000 slaves
ended up in North America. Many came from very low salt areas in West
Africa, such as the sub-Saharan savanna. Those who already had the best
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123
ability to conserve salt would have had a better chance of survival during the
voyage, but this would also have made them and their descendents more
likely to develop a high blood pressure, subsequently, when exposed to a
high salt intake. The slaves were unfortunates conquered by African neighbours to the north and east. After capture they were force-marched 100 to
200 miles to the coast, led by African slave handlers. There they were confined to long crowded huts known as barracoons to wait several weeks or
months for the ships to take them away. The death rate from the point of
capture to the coast was about 10% and another 12% died in the barracoons.
Conditions on board ship were terrible and on average 15% died during the
two months of the crossing. Another 5% died while waiting to be sold in the
United States and a further 10% died in the subsequent two years when they
were being 'seasoned' to their new environment. Thus, on average only
about 60% of those captured survived more than two years. The causes of
death were most often due to illness associated with loss of salt and water.
There was the heat and excessive sweating during the forced marches to the
coast, and the incarceration in the unventilated barracoons and ships' holds.
During the sea voyage, vomiting due to sea sickness was common. Diarrhoea
(the flux) was always rife and the predominant cause of death was some form
of diarrhoea. A surgeon's journal from the French slave ship Le Jeune Louis
meticulously recorded the deaths in copper plate handwriting. Presumably
the loss of valuable merchandise had to be accounted for. There is an entry
for every day during the months of May and June in 1825 and the diagnosis
was usually dissenterie. By the time the Lejeune Louis docked 30% (115) of
the captives had died.
Death left a dwindling number of survivors, but death is not random. The
hypothesis suggests that the possession of kidneys which had a well developed ability to hold onto salt would have increased the chances of surviving.
As this process of attrition by various forms of dehydration was repeated
over a period of 1 to 3 years the cumulative effect would be highly selective.
Accordingly the hypothesis proposes that it is this selective survival, among
the descendants of the surviving slaves, of genes responsible for an increased
ability to hold onto salt which is now responsible for the exceptionally high
prevalence of hypertension in African-Americans. There has been a counter
suggestion that it was not the survivor's ability to conserve salt but rather
their capacity to resist infections which was responsible for their survival.
Against this proposal records from the slave stations on the West African
coast show that mortality was highest in June, July and August, the hottest
and most dehydrating months of the year. Infectious agents, however, such
124
125
Smnmary
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. . , ' ... ' .
'
,
.
-.
...
,':::;'
.
Populations who eat less than 3 grams of salt per day do not develop high
blood pressure and do not have a rise in blood pressure with age. In other
populations, where salt intake is greater than 3 grams per day, the rate at
which blood pressure increases with age is dependent on salt intake.
Furthermore in three populations where salt intake was reduced over a prolonged period there was an associated reduction in blood pressure, and in
two of these populations a reduction in stroke mortality. In Japan this fall in
blood pressure occurred as it was becoming westernised.
'.-:
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Further reading
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Antislavery copper engraving dated 1764 of a slave trader licking a slave's face
to assess his fitness for the voyage across the Atlantic. (Reprinted with permission of the Bibliotheque Nationale, Paris.)
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,
!
as malaria and dysentery would have been present throughout the year.
Moskowitz draws attention to an engraving from 1764 which shows an
English slave trader licking the cheek of an African captive to taste his sweat
before boarding the slave ship. It is possible that it was the saltiness of the
sweat which was used as a rough guide to the likelihood of the captive surviving the Atlantic crossing. Presumably the less salty the better.
Very recent (as yet unpublished) observations in London weaken the
suggestion that slavery was responsible for the higher prevalence and severity of hypertension in the descendents of blacks who crossed the Atlantic. It
appears that the incidence and severity of hypertension in blacks living in
London who have come from the Carribean, whose ancestors, therefore,
were slaves, is not different from the immigrants who have come to the UK
directly from Africa, such as Nigerians.
'i
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!
Chagnon, N.A. Yanomamo·. The Fierce People. Holt, Rinehart & Winston,
New York, 1968.
Costa, E.C., Rose, G.A., Klein, C.H. et al. Salt and blood pressure in Rio
Grande du SuI Brazil. Bulletin of PAHO 1990; 24(2): 159-76.
Dahl, L.K. Possible role of salt intake in the development of essential hypertension. In Essential Hypertension. Block, K.D. & Cottier, P.T. (eds),
pp. 53-65. Springer, Berlin, 1960.
Denton, D. The Hunger for Salt. Springer Verlag, Heidelberg, 1982.
Dyer, A.R., Eliott, P, Marmot, M. et al. Commentary: Strength and importance of the relation of dietary salt to blood pressure. British Medical
Journal 1996; 312: 1661--4.
Gleiberman, L. Blood pressure and dietary salt in human populations.
Ecology of Food and Nutrition 1973; 2: 143-56.
Insull, W., Oiso, T. & Tsuchiya, K. Diet and nutritional states of Japanese.
American Journal of Clinical Nutrition 1968; 21(7): 753-77.
Joosens, ].v. & Kesteloot, H. Trends in systolic blood pressure, 24-hour
sodium excretion, and stroke mortality in the elderly in Belgium.
American Journal of Medicine 1991; 90 (Supp!. 3A) 3A-5S to 3A-
lIS.
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i
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!
Kesteloot, H. & Joossens, ].v. Epidemiology of Arterial Blood Pmsure.
Martinus Nijhoff, The Hague, 1980.
Law, M.R., Frost, CD. & Wald, N.]. By how much does dietary salt reduction lower the blood pressure? British Medical JOllrna11991; 302:
811-24.
!
I
126
,
,
"
Moskowitz, nw. Hypertension, thermotolerance, and the 'African gene':
an hypothesis. Clinical and Experimental Hypertension 1996; 18(1):
1-19.
National Survey on Circulatory Disorders. A summary 1980. Ministry of
Health and Welfare (KOSEISHO) Japan.
Oliver, W.]., Cohen, E.L. & Neel, ].V Blood pressure, sodium intake and
sodium related hormones in the Yanomamii Indians, a 'No-salt'
culture. Circulation 1975; 52: 146-51.
Page, L.B., Damon, A. & Moellering, R.C. Antecedents of cardiovascular
disease in six Solomon Island Societies. Circulation 1974; 49:
1132-46.
Page, L.B., Vandevert, D.E., Nader, K. et al. Blood pressure of Qtsh'qai pastoral nomads in Iran in relation to culture, diet and body form.
American Journal of Clinical Nutrition 1981; 34: 527-38.
Prior, LA.M., Grimley, Evans]., Harvey, H.P.B. et al. Sodium intake and
blood pressure in two Polynesian populations. New EnglandJournal
of Medicine 1968; 279: 515-20.
Sasaki, N. The relationship of salt intake to hypertension in the Japanese.
Geriatrics 1964; 19: 735-44.
Staessen,]., Bulpitt, C.]., Fagard, R. et at. Salt intake and blood pressure in
the general population: a controlled intervention trial in two towns.
Journal of Hypertension 1988; 6: 965-73.
Takahashi, E., Sasaki, N., Takeda,]. et at. The geographic distribution of
cerebral hemorrhage and hypertension in Japan. Human Biology
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ressure
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i,
i,
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i
The lack of an obvious mechanism whereby salt intake controls blood pressure had been one factor which has delayed acceptance of a connection
between the two. Dr. James Le Fanu, well known for his opposition to the
suggestion that dietary salt intake be reduced, wrote in The Times in London
in 1994 that,
I
the blood pressure is therefore the best defended of all physiological
functions ... with blood pressure so well protected, modest changes in
salt consumption are unlikely to be very influential, not least because
the concentration of salt in the body is rigorously controlled, so any
excess is rapidly excreted ....
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I
I,,
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1957; 29(2): 139-66.
The Intersalt Study. Journal of Human Hypertension 1989; 3(5): 179-330.
Wilson, T.W. & Grim, C.E. Biohistory of slavery and blood pressure
differences in blacks today. A hypothesis. Hypertension 1991; 17
(Supp!. 1): 122-8.
,
I,
As about half of the people over the age of 60 years have high blood pressure it is perhaps an exaggeration to claim that blood pressure is the best
defended of all physiological functions. Hyperbole aside, Dr. Le Fanu inadvertently put his finger on the nub of the matter. It is true that the amount
of salt in the body is rigorously controlled, much more rigorously than blood
pressure, and that any excess is excreted by the kidneys. But he did not
mention that some kidneys, though they may look normal and excrete waste
products normally, are less able to excrete an excess of salt than others. It is
now clear that the kidneys of humans and other mammals who develop
hypertension are less able to do so. As a result, their bodies have to make a
greater effort to excrete the large amounts of salt habitually eaten and this
induces a multiplicity of physiological changes throughout the body, particularly in the brain. Some of these changes are compensatory in that they try
to increase the excretion of salt and one way to do this is to raise the blood
pressure.
127

The Yanomamo Indians ,

Transcription

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