It is claimed that sodium has detrimental effects on blood pressure. High sodium intakes have been suggested to be responsible as a causative agent in high blood pressure, and low sodium diets are one of the primary treatments for hypertension. However, the results of studies assessing the effectiveness of low sodium diets have been controversial and very inconsistent in their findings. Generally, the effectiveness of low sodium diets at reducing blood pressure in those without elevated blood pressure is negligible, and in those with hypertension highly inconsistent. Animal studies investigating the effects of low sodium diets in animal models of hypertension have shown the same inconsistent results, supporting the consensus of data that sodium restricted diets are not an effective treatment for most cases of hypertension. Part of the problem lies with the way that the studies are performed and the fixation on single macronutrient causation for disease, with no consideration for other elements of the diet.
Human studies investigating the effects of sodium on blood pressure must be very carefully controlled so that sodium is the only variable of interest. However, many studies have not adequately controlled for other factors. Changing an individual’s diet to lower the sodium intake may well reduce blood pressure, but it is very difficult to attribute the changes to the sodium if other factors within the diet have also changed. For example, it is common to allocate a diet to an individual and assess its sodium content based on the urinary 24-hour sodium excretion rates1. This is indeed an effective way to estimate sodium intake and has been verified as such. However, it does nothing to quantify the other aspects of the diet. It is therefore very difficult to ascertain which dietary change is causing the effect on blood pressure. Add in the fact that any change in blood pressure is often very small, and you can see that the science behind the use of low sodium diets to treat hypertension is problematic to say the least.
If a low sodium diet is said to reduce blood pressure a high sodium diet should increase it. However feeding animals very high sodium diets does not cause elevations in blood pressure unless other variables are also manipulated. There is also no real known mechanisms by which sodium could cause high blood pressure, and the explanation that is often cited, that of increased extracellular volume caused by an osmotic effect, does not stand up to scrutiny. For example, in some animal models of high blood pressure high sodium diets can cause very modest elevations in blood pressure. If sodium were a cause of blood pressure increases for these animals, they would also experience an increase water weight, and the animals would gain weight. However they do not gain weight and therefore water retention cannot be a cause of weight gain. Following a low sodium diet in the hope of reducing blood pressure will likely be ineffective without other dietary changes, and in particular, improvements in the quality of the diet.
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