Cardiovascular disease is often accompanied by blood pressure increases, which likely contribute to the pathology of the disease. One nutritional strategy that is often suggested to counter these rises is a reduction in sodium intake. The traditional medical view is that sodium causes high blood pressure because it increases the extracellular fluid space. This In turn increases the volume of the blood and this raises the pressure through an increased resistance to flow. However, this hypothesis is provably false as animal models show that blood pressure rises caused by sodium intake are not accompanied by an increase in body weight, showing that water weight is not increased. Further feeding sodium to animals only causes blood pressure changes at levels that would be unpalatable in a human diet. Further, evidence from human studies suggest that only small numbers of individuals experience long term blood pressure increases with increased sodium intakes, so called salt sensitive people.
More recently researchers have identified changes to the endothelium of the vasculature as a primary driver of blood pressure changes. Such endothelial dysfunction has been shown to be caused by poor diet, particularly one low in antioxidants, and is reversed through a diet containing high amounts of antioxidant nutrients. Endothelial dysfunction is a condition whereby the arterial vasculature fails to relax correctly in response to flow, something called flow mediated dilation. This increases the resistance to flow, thus raising diastolic blood pressure particularly. Dilation is normally caused by a signal molecule called nitric oxide, but free radicals generated as a result of a poor quality diet inhibit the nitric oxide synthesising enzyme (nitric oxide synthase) and this decreases the dilatative properties of the arteries. Endothelial dysfunction is now considered a primary cause of blood pressure increases in cardiovascular disease, and restoration of correct flow dynamics is a priority in treatment.
However, sodium also appears to have detrimental effects on endothelial dysfunction, and reductions of sodium intake by 3 grams per day has been shown to improve flow mediated dilation. Further, potassium improves flow mediated dilation suggesting that it has an antagonistic effect of endothelial dysfunction compared to sodium. For example, in one study1, researchers showed that addition of potassium to a high sodium meal attenuated the sodium induced deterioration that occurred in endothelial dysfunction following the meal. Plant based diets are high in potassium and low in sodium, with most fruits and vegetables having a potassium to sodium ratio of around 20:1. In contrast, the Western diet has sodium to potassium ratios that favour sodium, and this may be a causative factor in blood pressure increases seen in Western populations eating the typical Western diet. This also explains why the consumption of plant foods has been shown to improve blood pressure in those with hypertension.