Intervention Studies
A well controlled double-blind study in just under 500 new born babies showed that when salt intake was reduced by about 30% in the reduced salt group, as judged by spot urinary sodium concentrations, there was a progressive difference in systolic blood pressure (1). At the end of six months the babies on the lower salt intake had a 2.1 mmHg lower systolic blood pressure (P<0.01). The study was discontinued at six months.
Many of these babies were restudied 15 years later (2). There remained a significant difference in blood pressure, when adjusted, between those babies who in the first six months of life had had a reduced salt intake compared to those that had not, suggesting that there was a programming effect of salt intake in early life, which fits with several studies in animals (Fig 1).
Figure 1. Difference in systolic blood pressure in newborn babies, randomised to either a normal salt intake or a moderate reduction in salt intake over the first six months of life. At six months, the study was discontinued, with all participants resuming a normal salt intake. Fifteen years later, a subgroup of those in the study had blood pressure re-measured. (Adapted from Ref. 1,2).
The other intervention study which successfully managed to reduce salt intake is a study of two villages in Portugal (3). One village was given information on how to reduce salt intake, and given processed foods with less salt. Salt intake was reduced by just under half, as judged by 24 hour urinary sodium excretion. After one year, there was a difference in blood pressure and at two years both systolic and diastolic pressure showed highly significant falls compared to the control village where no reduction in salt intake was made (Fig 2).
Two other intervention studies are often quoted as being negative (4, 5). Neither were successful in reducing salt intake and it is not surprising that there was no change in blood pressure in these studies between the community that was instructed on reducing salt, but failed to do so, and those that were not given such instructions. The only conclusion from these latter studies is that in developed countries where 80% of salt consumed is hidden in food it is very difficult to reduce salt intake unless processed and other foods with much less salt are provided.
Figure 2. Blood pressure changes with time in two Portuguese villages, one of which was advised on how to reduce salt intake and given processed foods with a reduced salt content, the other had similar measurements of blood pressure but no advice on diet. Note the significant differences in blood pressure at 1 year and continuing differences at 2 years. (Adapted from Ref. 3).
References