The current mainstream opinion is that high salt (sodium) diets are a cause of high blood pressure. However, this opinion is controversial because results from studies looking at high sodium diets are not consistent. While some studies do show some evidence of the ability of sodium to raise blood pressure, others clearly show no detrimental effects. Animal studies are useful in studying the physiological changes induced by diets as they can be rigorously controlled to a degree far higher than with humans, and doses can be provided that would be unpalatable to humans. For example, in one study1 monkeys were fed high sodium diets of varying degrees that made up 0, 3, 4, 5 and 6 % of the wet weight of the diet for 45, 139, 37 and 90 days, respectively. The salt content of the diet was therefore extremely high. The results showed that salt causes increases in blood pressure, but the monkeys took around 103 days to to show consistent increases in blood pressure with a salt intake of 3 %. These results reflects results seen previously in both rats and dogs.
The data in this study is interesting because it hints at the reason that salt may cause increases in blood pressure. Theory suggest that because high sodium intakes cause an expansion in extracellular fluid volume, including plasma volume, and also causes a greater peripheral resistance through a greater concentrations of sodium in vascular tissue. The main problem with this theory is that sodium should cause very rapid rises in body stores of sodium, but the length of time it takes for blood pressure changes to occur are too long. In addition, if sodium caused a rise in extracellular fluid, this would increase waters weight and the animals should gain weight from fluid retention, however this does not happen. Therefore this theory falls short of offering a credible explanation of how sodium can cause increases in blood pressure. It is interesting that proponents of the salt theory of hypertension conveniently forget that the body is able to regulate sodium intake through increased expulsion from the kidney. In fact the authors of the study admitted this fact and could not explain the mechanisms by which sodium caused the blood pressure increases.
So while the inconsistent results of studies shows that the interaction of sodium is far more complex that a simple cause and effect, there is also reason to suspect that there is no smoke without fire. Sodium clearly does cause some changes that initiate rises in blood pressure. Perhaps other factors are involved and these modulate the effects of sodium and this in turn produces varying degrees of change? In nutrition is is very difficult to alter one variable without affecting another and this sentiment brings us to the crux of the matter. High sodium diets are also diets relatively low in the other macrominerals magnesium, potassium and calcium. These macrominerals are involved in blood pressure regulation, and high sodium diets may over time cause imbalances in mineral homeostasis which eventually result in metabolic dysfunction. The role of calcium, potassium and magnesium in lowering blood pressure has been evidenced. High sodium diets may therefore alter blood pressure through changes in the ratios of macrominerals.
Dr Robert Barrington’s Nutritional Recommendation: The typical Western diet contains too much sodium, and too little potassium, magnesium and calcium. This is because the diet is rich in processed and preserved foods and low in natural and fresh plant material. Plant foods contain a high potassium to sodium ratio which is correct for human nutrition. In addition green leafy vegetables are good sources of magnesium and grains are rich sources of calcium and magnesium. Eating a more traditional diet based on plant foods therefore lowers sodium intakes naturally by supplying the macrominerals in the correct ratios.
RdB
