By how much does dietary salt reduction lower blood pressure? III--Analysis of data from trials of salt reduction

Is salt restriction necessary in hypertensive patients treated with calcium antagonists?

A moderate restriction of salt intake is recommended as a nonpharmacological approach to reducing high blood pressure, and can be used as an adjuvant to antihypertensive drug treatment. Noncompliance with a salt-restricted diet is the main limitation to this therapeutic approach. Evidence to date has shown that hypertensive patients treated with a calcium antagonist may not require a salt-restricted diet to achieve target blood pressure reductions. Further long term studies are required to confirm this.

By how much does dietary salt reduction lower blood pressure? I--Analysis of observational data among populations

OBJECTIVE

To estimate the quantitative relation between blood pressure and sodium intake.

DESIGN

Data were analysed from published reports of blood pressure and sodium intake for 24 different communities (47 000 people) throughout the world.

MAIN OUTCOME MEASURE

Difference in blood pressure for a 100 mmol/24 h difference in sodium intake. Allowance was made for differences in blood pressure between economically developed and undeveloped communities to minimise overestimation of the association through confounding with other determinants of blood pressure.

RESULTS

Blood pressure was higher on average in the developed communities, but the association with sodium intake was similar in both types of community. A difference in sodium intake of 100 mmol/24 h was associated with an average difference in systolic blood pressure that ranged from 5 mm Hg at age 15-19 years to 10 mm Hg at age 60-69. The differences in diastolic blood pressure were about half as great. The standard deviation of blood pressure increased with sodium intake implying that the association of blood pressure with sodium intake in individuals was related to the initial blood pressure--the higher the blood pressure the greater the expected reduction in blood pressure for the same reduction in sodium intake. For example, at age 60-69 the estimated systolic blood pressure reduction in response to a 100 mmol/24 h reduction in sodium intake was on average 10 mm Hg but varied from 6 mm Hg for those on the fifth blood pressure centile to 15 mm Hg for those on the 95th centile.

CONCLUSIONS

The association of blood pressure with sodium intake is substantially larger than is generally appreciated and increases with age and initial blood pressure.

By how much does dietary salt reduction lower blood pressure? II--Analysis of observational data within populations

OBJECTIVE

To determine whether the estimates of the size of the association between blood pressure and sodium intake derived from studies of individuals within populations can be quantitatively reconciled with our estimates derived from comparisons of the average blood pressure and sodium intake between different populations.

DESIGN

Examination of data from 14 published studies that correlated blood pressure recordings in individuals against measurements of their 24 hour sodium intake (within population studies).

MAIN OUTCOME MEASURE

Comparison of observed differences in blood pressure per 100 mmol/24 h difference in sodium intake in each within population study with predicted differences calculated from the between population data, after allowing for the underestimation of the true association of blood pressure with sodium intake caused by the large day to day variation in 24 hour sodium intake within individuals.

RESULTS

The underestimation bias inherent in the within populations studies reduced the regression slope of blood pressure on single measures of 24 hour sodium intake to between a half and a quarter of the true value (for example, in one study from 6.0 to 2.4 mm Hg/100 mmol/24 h). Estimates from between population comparisons of the regression slope of blood pressure on sodium intake, after adjustment to take this underestimation bias into account, were similar to the values actually observed in the within population studies.

CONCLUSION

The within population studies confirm our estimates from between population comparisons of the magnitude of the association between blood pressure and sodium intake.

Strategies of prevention revisited: effects of imprecise measurement of risk factors on the evaluation of "high-risk" and "population-based" approaches to prevention of cardiovascular disease

Imprecise measurement of risk factors causes misclassification of individuals, limits sensitivity to detect those with high true levels, and dilutes associations between risk factors and disease. The implications of these effects for two particular examples were explored using data from a large prospective study relating plasma cholesterol to coronary heart disease (CHD) mortality and diastolic blood pressure (DBP) to fatal stroke. The absolute and relative effectiveness of three "high-risk" strategies of screening and treatment and a "population-based" shift in the risk factor distribution were compared, assuming different degrees of measurement error. The absolute benefits of each strategy were greater than suggested by unadjusted estimates from survey data. For cholesterol and CHD (a linear relationship in this cohort), uncorrected estimates tended to exaggerate the effectiveness of "high-risk" strategies relative to the "population-based" approach. For DBP and stroke (an exponential relationship), the relative effectiveness of screening and treatment was underestimated if no allowance was made for measurement error. These findings are strictly applicable only to the middle-aged men from whom they were derived, but the effects of misclassification and regression dilution need to be considered in any assessment of preventive strategies.