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salt intake and cardiovascular health in adults: analysis of INTERMAP study and NDNS
Project Code: N02032
Department of Epidemiology & Public Health, Imperial College London
Robertson, C ; Hurley, J; Elliot, P
Over a decade ago, the Committee on Medical Aspects of Food Policy (COMA) recommended a reduction in the average salt intake of the adult population to 6g salt per day (Department of Health, 1994). This was based on evidence of an association between high salt intakes and on the risk of developing high blood pressure. The recent “Salt and health” review by the Scientific Advisory Committee on Nutrition (SACN) concluded that the evidence to support this association has increased since 1994 (SACN, 2003). This project was initiated to provide further information on the association of salt with blood pressure and other cardiovascular risk factors, to help inform policy in this area.
Data collected for the National Diet and Nutrition Survey (NDNS) of British adults (n=1,724 men and women aged 19-64 years) and the INTERnational collaborative study of MAcro- and micro-nutrients and blood Pressure (INTERMAP) study (n=4,680 men and women aged 40-59 years from 17 population samples in the UK, USA, Japan and Peoples Republic of China) were analysed to: i) examine interrelationships between dietary sodium and other dietary variables which potentially influence blood pressure; ii) clarify the association of sodium with body mass index, smoking, alcohol, social class; iii) quantify the independent association of sodium with blood pressure (including by population subgroups); and iv) summarise and account for methodological issues concerning sodium measurement (diet, urine) reliability when quantifying the sodium-blood pressure association.
A major effort was undertaken to assemble and prepare the NDNS dataset for use, applying several exclusion criteria and quality control checks to improve the validity and accuracy of the data. A detailed comparison was done of the nutrient-coding systems used in the two studies and the reliability of the data, specifically relevant to sodium intake estimation, was investigated. Descriptive and correlation-regression analyses were conducted to describe the samples and data, examine interrelationships between nutrient variables and to quantify the association between sodium intakes and blood pressure while adjusting for potential confounding factors.
Estimates of mean sodium intakes from 24 hour urine collections were 161 mmol (3703 mg)/day in men and 127 mmol (2921 mg)/day in women (9.26g and 7.30g salt respectively) in the INTERMAP UK sample (n=501) and 188 mmol (4324 mg)/day and 134 mmol (2082 mg)/day (10.81g and 5.21g salt) respectively in the NDNS. In both studies, sodium intakes were modestly higher for overweight people compared with non-overweight, and in people with low education compared with high (using education as a proxy for social class). In addition, smokers had a higher sodium/potassium intake ratio than non-smokers (reflecting mainly lower potassium intakes among smokers).
In the complete INTERMAP sample (n=4,680), for men and women combined, the estimates of association of sodium with systolic BP ranged from 0.8 to 3.0 mmHg/100 mmol depending on whether or not BMI was included in the regression models; for diastolic BP corresponding estimates were 0.2 to 1.4 mmHg/100 mmol sodium. These results were based on the average sodium excretion from two 24 hr urine collections without further statistical adjustment for unreliability of sodium (i.e., because of large day-to-day variability in sodium intakes, the regression dilution problem), and are therefore around half of comparable estimates from INTERSALT (which were corrected for reliability). Stronger relationships were found in women than in men.
In the NDNS adult survey, based on complete and consistently timed 24-hr urine data, estimates for the association of sodium with systolic BP (men and women combined, n=549) ranged from 0.7 to 2.1 mmHg/100 mmol sodium depending on choice of covariates in the regression models and whether or not BMI was included. There was no association with diastolic BP. Again, associations were stronger in women than in men. In women (n=283), regression estimates (all significant, p<0.05 to p<0.001) for systolic BP ranged from 3.3 to 5.3 mmHg/100 mmol sodium; none of the associations in men were significant.
The INTERMAP study confirmed the results of INTERSALT and other epidemiological studies (and the randomised controlled trials) in finding higher sodium intakes associated with higher blood pressures. In the NDNS adult survey, there were strongly positive associations in women but not in men. Reasons for the stronger associations in women than men are unclear, but might reflect different body size and hence kidney size between the two sexes. These two studies from the late 1990s (INTERMAP) and 2000-2001 (NDNS adult survey) confirm that intakes of sodium in the UK are well above recommended intakes of 100 mmol/day (6 g salt/day). These findings add to the wealth of knowledge implicating high salt intakes with high population blood pressures, and underscore the need for preventive efforts to reduce salt intakes at the population level.
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