Calcium is an important mineral for lowering blood pressure. While around 99 per cent of absorbed calcium goes straight to your bones, the other one per cent plays a vital role in regulating blood pressure, blood clotting, muscle contraction, nerve impulses and energy production.
Good calcium intakes promote the excretion of excess sodium via the kidneys, and regulates the dilation and contraction of blood vessels. In fact, many antihypertensive drugs such as calcium channel blockers work via their effects on the flow of calcium in and out of cells. There is now growing recognition that low intakes of calcium are associated with high blood pressure and an increased risk of stroke, as well as promoting eye complications involving the retina which can occur in hypertension.
A large analysis of 40 studies involving almost 2500 adults found that taking 1200mg calcium per day reduced blood pressure by 1.86/0.99mmHg. In those with relatively low calcium intakes, the benefit was greater with a reduction in blood pressure of 2.63/1.30 mmHg – a small but significant difference. The researchers suggested that adequate calcium intakes should be recommended for the prevention of hypertension.
A gold-standard Cochrane review of 16 trials, involving over 3000 adults found that using calcium supplements or eating calcium fortified food was associated with a blood pressure that was, on average, 1.43/0.98 mmHg lower than in those not using calcium fortification. The benefits were most pronounced in people who were younger than 35 years, in whom additional calcium intakes lowered blood pressure by 2.11/2.61 mmHg compared with 0.96/ 0.59 mmHg in those who were over 35 years. When men and women were considered separately, the benefits were also greater in men, for whom calcium supplements lowered blood pressure by an average of 2.07/1.91 mmHg compared with 1.45/0.92 mmHg in women.
These results did not depend on baseline calcium intakes, but on having an additional intake of at least 1000 mg calcium on top of normal dietary intakes. The researchers stated that the quality of evidence was high, that increasing calcium intakes could play a role in preventing hypertension, and that none of the studies reported any adverse events from higher calcium intakes.
It does seem, however, that if you are taking a calcium channel blocker drug, the additional blood pressure lowering effect of calcium supplements is lost (the medication has already achieved the benefits by boosting the effects of calcium for you).
Calcium deficiency and pre-eclampsia
Poor calcium intake also increases the risk of developing high blood pressure during pregnancy – a serious condition known as pre-eclampsia. According to another gold-standard Cochrane review of 13 studies, involving 15,730 women, taking calcium supplements during pregnancy halved the risk of pre-eclampsia, for example, and reduced the risk of preterm birth.
Where to get calcium in your diet
Calcium is absorbed in the small intestine when vitamin D is present. Dietary sources of calcium include milk and dairy products, eggs, green leafy vegetables -especially broccoli – nuts and seeds, pulses and bread made from fortified flour.
The easiest way to increase your calcium intake is to drink an extra pint of milk per day, which provides around 720mg calcium per pint (600ml). Skimmed milk provides as much calcium as whole milk but without the additional fat or calories – although even full fat milk is technically a low-fat product as the fat content is less than 5%.
Opt for broccoli
The calcium found in milk is in the readily absorbable form of calcium lactate. Interestingly, the bioavailability of calcium in brassica vegetables is even higher so that 61% of calcium found in broccoli is absorbable, compared with only 32% of that in milk. The reason remains unknown. Only 30–40 per cent of the calcium present in other food and drinks is absorbed. Other green leafy vegetables, such as romanesco and kale are also good sources, but not spinach, whose high oxalate content bind it in the intestines to reduce the amount you can absorb, although spinach is good for hypertension for other reasons.
Foods to avoid
Some types of dietary fibre (phytates from wheat in unleavened bread e.g. chapatti) also bind calcium in the bowel to form an insoluble, non-absorbable salt. High-fibre diets, which speed the passage of food through the bowels, will also reduce the amount of calcium absorbed.
Calcium deficiency is common
Lack of calcium is widespread, with many people avoiding dairy products because they are following a low fat diet, or because they have a lactose intolerance. If you don’t have the equivalent of at least a pint of milk per day (including cheese, yoghurt, fromage frais) then calcium supplements are an excellent idea if you have high blood pressure.
Supplement doses for calcium
The EU RDA for calcium is 800 mg per day. The upper safe level for long-term use from supplements is suggested as 1500mg.
Calcium tablets are best taken with meals. They are also better taken with an evening meal as calcium movements and usage in the body is greatest at night, when growth hormone is secreted.
If you are taking a high dose, however, it is usually best to divide it into two or three smaller doses spread throughout the day to improve absorption.
Who should avoid calcium supplements?
People with a tendency to form kidney stones should seek medical advice before taking calcium supplements.
If your blood pressure is borderline or raised, self-monitoring is key to maintaining good control.
Click here for advice on choosing a blood pressure monitor to use at home.
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