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Intersalt studies A major transnational study of over 10 000 men and women described the association between urinary excretion of sodium chloride (as a measure of salt intake) and blood pressure. After adjustments for body weight, alcohol intake, sex, and age, a higher sodium intake of 100mmolday~1 was linked with a systolic blood pressure rise of 3-6mmHg in adults aged 40 years but one of 10mmHg when aged 70 years. Updated results suggest that the association between sodium excretion and blood pressure is stronger when not adjusted for body weight, but the relationship is present whether or not the adjustment is made.

Figure 1 summarizes the relationships between sodium intake and blood pressure in the INTERSALT study. Different populations may show different responses depending on the host of other environmental factors that may be involved. The figure also illustrates the fact that individuals within any population may show very different effects and that appreciable changes in salt intake may be needed before a clear change in blood pressure is evident. Part of the problem in displaying the relationship arises from the difficulty in establishing what the prevailing blood pressure of individuals is given the remarkable variation in blood pressure during the day and night; difficulty also arises because it takes many complete 24-h urinary collections to obtain a reasonable estimate of the customary sodium intakes. The age-related incline also implies longer term amplification of the pathophysiological changes in hormonal controls and in blood vessel reactivity and plasticity; thus, as the blood pressure increases the tendency to further increase is enhanced in an accelerating process. This emphasizes the potential importance of early interventions when the blood pressure is tending to rise. It also implies that interventions to alter the diet of the young may be particularly valuable. This is borne out by the observation in the Netherlands that newborn babies fed a reduced salt content in their formula milk for the first 6 months of life had very much lower blood pressures when reassessed at the age of 15 years.

Table 7 shows the estimated changes with age in blood pressures as the salt intake is increased by

Table 7 Predicted change in systolic and diastolic blood pressure (mmHg) for each 100mmolper 24 h change in sodium intake for various centiles of blood pressure distribution

Age (years) Centile

Table 7 Predicted change in systolic and diastolic blood pressure (mmHg) for each 100mmolper 24 h change in sodium intake for various centiles of blood pressure distribution

Age (years) Centile

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