The mainstream medical establishment still pushes the paradigm that salt (sodium chloride) is a cause of high blood pressure. However, for the general population, those with good health, this is not evidenced in the scientific literature. Healthy individuals who increase their salt intake are able to excreted larger amounts of salt and this maintains the balance of sodium in the tissues. Those who believe the salt hypertension paradigm often claim that high salt diets can cause high blood pressure because they cause an increase in extracellular fluid retention, and this increases the volume of the vasculature. However, animal studies show that this is not true. Monkies for example have to be fed 3 % salt diets, a huge amount of salt, for over 100 days, before they display any elevations to blood pressure. However during these elevations the water weights of the monkeys do not increase and so the reason for the rise in blood pressure cannot be retention of fluids in the vasculature of the animals (here).
So animals have to be fed very large quantities of sodium over long periods of time before they develop changes to blood pressure and these changes are not due to changes in water weight. At these sorts of levels the sodium is no longer acting nutritionally, and so using this type of study to evidence effects for sodium in human diets is not valid. In addition, human epidemiological studies are often used to evidence the association between high sodium intakes and high blood pressure. There is no argument that such studies show an association, but are they really measuring a cause and effect or is there another factor that accounts for the association? Very likely the association is caused because diets high in sodium are also poor quality diets. Low quality diets can increase blood pressure because they lead to the formation of oxidative stress and this causes dysfunction in the endothelial lining of the vasculature. Improvements to the diet can lower sodium intake but the increase in the quality may be the beneficial factor.
For example in one study researchers investigated the association between dietary intake and blood pressure in a large group of individuals at high risk of cardiovascular disease1. The results of the study showed that a sodium intake of less than 2300 mg per day was associated with a lower risk of cardiovascular disease compared to a sodium intake above this value. Changes in sodium intake above or below this level over time increased or decreased mortality, respectively. The conclusion of the researchers was that decreasing sodium intake below 2300 mg per day was associated with a reduced risk of all cause mortality, whereas increasing it above 2300 mg per day was associated with an increase in all cause mortality. However, they took no account of other dietary factors or the quality of the diet alone. The conclusion should really have been that changes to diet, which may include changes to sodium levels, are associated with mortality changes. But this may have little to do with sodium, and is simply a reflection of diet quality.
Dr Robert Barrington’s nutritional comments: There are a number of interesting factors to consider in this study. Firstly, the subjects were selected because they were at high risk of cardiovascular disease. Evidence suggests that certain people are salt sensitive and these people do respond to changes in dietary sodium with elevations in blood pressure. Selecting those subjects with cardiovascular disease, and therefore high blood pressure, would select a greater proportion of salt sensitive individuals, and this puts a bias into the results. Secondly, as sodium levels decline, potassium levels tend to increase, because the types of foods with low sodium contents tend to have high potassium contents. This is true for fruits and vegetables for example. Research suggests that more important to health than the sodium content of the diet, is the sodium to potassium ratio. Those subjects that decreased sodium intakes, may also have inadvertently raised potassium levels and thus improved their health with a higher quality diet.
RdB
