Some evidence suggests that calcium lowers blood pressure because calcium ions lower vascular tone. Other evidence suggests calcium lowers blood pressure because it causes weight loss.
The possibility that calcium lowers blood pressure has been reported in the nutritional literature. The association between hard drinking water and lower rates of cardiovascular mortality have been suggested to result from the fact that calcium lowers blood pressure. Theoretically, calcium lowers blood pressure because the ionised calcium affects vascular tone, relaxing arteries and reducing resistance to flow. In animal experiments, alterations to normal calcium homeostasis has been reported in hypertensive animals, and in humans inverse associations between calcium intake and blood pressure have been reported. However, some studies have failed to detect effects of dietary calcium on blood pressure. This may relate to the complexity of the calcium regulatory systems in humans. Low calcium intakes can be compensated for to some extent by increasing parathyroid hormone release, which act to raise plasma calcium through increased absorption, reabsorption and resorption from the intestine, kidney and bone, respectively. The vitamin D metabolite 1, 25-dihydroxyvitamin D also increased intestinal absorption of calcium.
Vitamin D and parathyroid hormone may therefore be able to compensate and prevent changes in blood pressure with fluctuating dietary intakes. If calcium lowers blood pressure only in those individuals who have failures in these regulatory mechanisms, this may explain the inconsistent results of studies. For example, low intakes of vitamin D could prevent adequate absorption of calcium and thus create a calcium insufficiency despite adequate calcium in the diet. In this regard vitamin D intake might be expected to be inversely associated with blood pressure. Some evidence suggests that this is indeed the case. For example, in one study1 researchers investigated the association between vitamin D intake and blood pressure in women. Although there was no significant association between calcium and blood pressure the intake of vitamin D was associated with reductions in systolic blood pressure in young women. In older women, those with the lowest intakes of vitamin D and calcium had the highest systolic blood pressure.
Therefore vitamin D status may be related to blood pressure through its ability to affect plasma calcium levels. Low dietary intakes of calcium do not necessarily cause reduced plasma levels of calcium, if regulatory mechanisms can efficiently modulate excretion and absorption rates. Where regulatory mechanisms can compensate for low intakes, vitamin D status may better predict blood pressure risk. Chronic low levels of calcium are known to stimulate increases in parathyroid hormones, and this in term increases conversion of the vitamin D metabolite 25-hydroxyvitamin D to 1, 25-dihydroxyvitamin through increased expression of the 1-α-hydroxylase enzyme. As parathyroid levels rise, 1, 25-dihydroxyvitamin D rises concomitantly, causing an increase in cellular calcium concentrations. Low plasma calcium with high cellular calcium is termed the calcium paradox disease. The high concentration of intracellular calcium ions may stimulate de novo lipogenesis resulting in possible insulin resistance and weight gain.
That calcium and dairy consumption have been shown to cause weight loss (here, here and here) is interesting because reduced body weight is associated with a reduction in blood pressure. If calcium lowers blood pressure, some of the beneficial effects may therefore come from this weight loss effect. Perhaps calcium lowers blood pressure because it lowers parathyroid hormone levels, which in turn decreases plasma levels of 1, 25-dihydroxyvitamin D and this decreases levels of intracellular calcium? The inverse association between plasma vitamin D status and blood sugar disorders add weight to the hypothesis that calcium lowers blood pressure through body weight changes. That high plasma levels of 1, 25-dihydroxyvitamin D are possible with normal plasma levels of 25-hydroxyvitamin D has been reported2. Elevated 1, 25-dihydroxyvitamin D is associated with low bone mineral status indicating a calcium deficient state. If calcium lowers blood pressure by reversing a deficient state then this explains the inconsistent findings in blood pressure studies.
RdB
