The role of sodium in the development of hypertension has been overstated. While sodium does play a role in the regulation of blood pressure in some individuals, those not sensitive to its effects are not affected by dietary levels. Although the percentage of people in the population who are sensitive to sodium induced high blood pressure is small, inclusion of these individuals in studies investigating the role of dietary sodium in blood pressure regulation has caused bias to results. Removal of these salt sensitive individuals negates the effects of sodium on blood pressure. Research suggests that calcium deficiency plays a role in blood pressure elevations, possibly because it is able to regulate vascular tone and smooth muscle function through normalisation of intracellular calcium concentrations. The ability of calcium to regulate blood pressure may explain the epidemiological inverse association between dairy intake and blood pressure.
Researchers1 have investigated the effects of dairy foods on blood pressure and intracellular calcium concentrations, and also reported how changes in intracellular calcium concentrations affect blood pressure. The study was performed in hypertensive subjects and was of a randomised cross-over design. Subjects were fed a high dairy, high fruit and vegetable diet (3 to 4 servings of dairy per day), a high fruit and vegetable diet or an average Western diet as a control. After 5 weeks, in both the treatment groups, systolic and diastolic blood pressure were reduced by ~2mm Hg compared to the control. The high dairy diet also lowered serum 1,25-dihydroxyvitamin D and intracellular calcium, and increased intracellular magnesium, compared to the low dairy diets. Subjects who where sensitive to dairy reducing intracellular calcium, also showed the greatest reductions in diastolic blood pressure when compared to other subjects.
Modest reductions in blood pressure are therefore possible with a diet high in dairy, fruits and vegetables. However, addition of calcium to the fruit and vegetable diet had no additional benefits. Fruits and vegetables are rich sources of potassium and magnesium, which may have contributed to the beneficial effects seen on blood pressure. The high dairy, high fruit and vegetables diet significantly decreased intracellular calcium and increased intracellular magnesium levels, and intracellular calcium correlated with blood pressure changes. Other studies have shown that high calcium intakes favourable lower intracellular calcium levels. The lowering of 1,25-dihydroxyvitamin D may have contributed to this effects though decreased stimulation of calcium influx through calcium channels. Calcium may therefore decrease blood pressure by reducing 1,25-dihydroxyvitamin D levels, which causes a reduced influx of calcium and decreased intracellular calcium concentrations in smooth muscle cells.