Proponents of the sodium theory of blood pressure propose that high intakes of dietary sodium are a cause of elevated blood pressure. This is suggested to occur through an increase in the fluid retained in extracellular tissue including the vascular systems. However, studies investigating the cause of blood pressure do not show that high intakes of dietary sodium in isolation are a driver of blood pressure increases. This is because in a normal healthy individual, excess sodium can be excreted to maintain electrolyte balance thus returning homeostatic control. In addition, evidence suggests that low intakes of potassium are able to cause elevations in blood pressure. As low potassium diets often contain high concentrations of sodium (and vice versa), it might be that potassium is a confounding variable in the association between sodium and blood pressure. Many studies have investigated the sodium intakes of healthy subjects and found that sodium intake is not associated with blood pressure changes.
For example, in one study1 researchers investigated the effects of sodium in the drinking water of 295 individuals living in the United States. The authors estimated the amounts of sodium in the diets of the subjects, including drinking water, from 24-hour dietary recall questionnaires (a validated and accurate method of assessing nutritional intake). The authors then used medical records to assess the blood pressure and sodium excretion of the subjects. When the authors performed correlation statistics on the data they found that there was no significant association between mean daily sodium intake from food with blood pressure in adults. In addition, there was no significant association between the sodium intake from drinking water and the mean blood pressure in adults. However, they did find an significant association between mean 24-hour sodium excretion and mean blood pressure in adults, and between dietary intake of sodium and mean diastolic blood pressure in children.
It is actually disingenuous to say that high salt diets do not cause any elevations in blood pressure. This is because is experimental models, very high sodium intakes can increase blood pressure in humans transiently. However, in such studies the amount of sodium required to cause even modest increases in blood pressure is twice that found in even the highest sodium diets, a level that is unpalatable to most individuals. Even with such diets sodium homeostasis is achieved in a relatively short time period2 with urinary sodium excretion approaching the salt intake after just 72 hours. Therefore even salt intake at levels considered outside those provided by the diet result in increases in salt excretion and a return to sodium homeostasis in a short period of time. The physiological changes that allow the healthy individual to accomplish sodium homeostasis after a salt load include an increased glomerular filtration rate and a decrease in vascular tone caused by a reduction in sympathetic nervous system activity.
Dr Robert Barrington’s Comments: The evidence that high intakes of sodium are the cause of blood pressure changes is weak. Of course high sodium diets are also low potassium diets and it is therefore difficult to ascribe blame to the intake of sodium. In addition, the modern Western diet is high in sodium, but also possesses other characteristics that are known to elevate blood pressure (such as being low in essential nutrients and antioxidants). Therefore consuming a low sodium diet with the expectation that is will cause reductions in blood pressure is a futile gesture. Reductions in blood pressure are achieved by consumption of a high quality diet that includes the correct balance of all essential nutrients. Focussing blood pressure lowering efforts solely on sodium is shortsighted and a strategy that is doomed to failure.
RdB
