NOTE: THESE SUMMARIES ARE OF HISTORICAL INTEREST ONLY AS THE COCHRANE REVIEWS ARE OUTDATED
- Calcium supplements: Thirteen trials
- Magnesium supplements: Twelve trials
- Potassium supplements: Six trials
- Combined calcium, magnesium and potassium supplements: Three trials
- Low salt versus high salt diets: 115 trials
- Dietary salt reduction: Eleven trials
- Longer-term modest salt reduction: Twenty trials
- Non-drug programmes to reduce blood pressure and other risk factors for heart disease: Thirty-nine trials
High blood pressure, or hypertension, is defined as prolonged raised blood pressure, that is, a systolic pressure consistently above 140 millimeters of mercury, or mm Hg, or a diastolic pressure consistently above 90 mm Hg. (Systolic is the blood pressure when the heart is contracting and diastolic is the blood pressure when the heart is at rest.) The condition has no symptoms, but in time it can cause blood vessel changes in the back of the eye (retina) and abnormal thickening of the heart muscle. People with untreated hypertension have a higher risk for heart disease, kidney damage and other medical problems than people with normal blood pressure.
This overview explains the results of eight Cochrane reviews of trials that looked at non-drug approaches to lowering blood pressure. These include potassium, magnesium and calcium supplements as well as lifestyle changes like physical exercise and weight loss. All of the participants in these trials were healthy adults – some with hypertension and some with normal blood pressure. Certain lifestyle changes like the reduction of salt intake did, in fact, lead to small reductions in blood pressure; others had no effect. In some reviews, such as the one that looked at calcium supplements, results did not clearly show a strong benefit because the trials did not have enough participants to come to a trustworthy conclusion. In all eight reviews, the trials followed the participants to determine whether their blood pressure was reduced but not long enough to see whether they achieved the ultimate goal — longer survival or a reduced risk of heart attack, stroke or kidney damage.
Studies of people’s metabolism suggest that calcium may have a role in the regulation of blood pressure. In line with this, some population studies have reported that people with a higher intake of calcium tend to have lower blood pressure. Previous reviews came to conflicting conclusions about whether calcium supplements reduce blood pressure.
This review included 13 trials with a combined total of 485 participants who were randomly assigned to take daily calcium supplements (about 1.2 g/day), placebo or usual care. The people were followed up for between 8 and 15 weeks. On average, people taking calcium supplements achieved slightly lower systolic blood pressure at the end of the trial. However, most trials were of poor quality, so their results are not reliable. The trials were too small and did not last long enough to determine whether extra calcium reduces the risk of death, heart attack or stroke. Calcium supplements usually did not cause any more adverse effects than did placebo. Larger, longer, better quality trials are needed to clarify whether calcium supplementation can lower high blood pressure.
HO Dickinson, DJ Nicolson, JV Cook, F Campbell, FR Beyer, GA Ford, J Mason. Calcium supplementation for the management of primary hypertension in adults. Cochrane Database of Systematic Reviews 2006, Issue 2. Art. No.: CD004639.pub2. DOI: 10.1002/14651858.CD004639.pub
Metabolic and experimental studies suggest that magnesium may be involved in the regulation of blood pressure. Some population studies show that people with high intake of magnesium have normal blood pressure, whereas other studies do not show this effect.
This review includes 12 trials with a combined total of 545 people with high blood pressure who were randomly assigned to start taking daily magnesium supplements, placebo or no treatment. All then had their blood pressure measured 8 weeks to 6 months later. The results of the trials varied considerably: some trials found magnesium lowered blood pressure much more than placebo while others found little difference between magnesium and placebo. On average, people receiving extra magnesium achieved slightly lower measures of diastolic blood pressure at the end of the trials.
None of the trials reported any serious side effects with taking magnesium supplements.
However, most of these trials were of poor quality, so their results are not reliable. The trials were not long enough or large enough to measure whether extra magnesium can reduce the possible consequences of high blood pressure: death, heart attack, stroke or kidney damage. Larger, longer and better quality trials are needed.
HO Dickinson, DJ Nicolson, F Campbell, JV Cook, FR Beyer, GA Ford, J Mason. Magnesium supplementation for the management of primary hypertension in adults. Cochrane Database of Systematic Reviews 2006, Issue 3. Art. No.: CD004640.pub2. DOI: 10.1002/14651858.CD004640.pub2.
Population studies provide contradictory evidence as to whether potassium supplements can lower blood pressure.
This review includes 6 trials with a combined total of 483 participants who were followed for 8 to 16 weeks. Only two of these trials were considered to be of high quality. The results of all 6 trials varied considerably: some trials found potassium (about 100mmol/day) lowered blood pressure much more than placebo, while others found little difference between potassium and placebo. Overall, no significant reduction in blood pressure when taking potassium supplements was found.
Since most of the trials were of poor quality, their results are not reliable. Furthermore, the trials were not long enough or large enough to measure whether potassium supplements reduced the risk of death, heart attack, stroke or kidney damage. The trials reporting adverse effects did not find any serious side effects from taking potassium supplements. More trials enrolling a large number of participants with long periods of follow up are necessary to know whether potassium supplements can lower high blood pressure.
HO Dickinson, DJ Nicolson, F Campbell, FR Beyer, J Mason. Potassium supplementation for the management of primary hypertension in adults. Cochrane Database of Systematic Reviews 2006, Issue 3. Art. No.: CD004641.pub2. DOI: 10.1002/14651858.CD004641.pub2.
Previous research suggests that increasing dietary intakes of calcium, potassium or magnesium separately may reduce blood pressure to a small degree over the short term. It is unclear whether increasing intake of a combination of these minerals produces a larger reduction in blood pressure.
This review focuses on the effects of concurrently changing any combination of calcium, magnesium, or potassium levels in the diet of adults who have high blood pressure. Only three trials with a combined total of 277 participants were found. The only combination that was assessed by all three trials was potassium and magnesium, which did not produce reductions in blood pressure. One trial assessed calcium with magnesium and calcium with potassium and found that neither combination had very much effect on blood pressure.
None of the trials were of high quality, so the results are not definitive. Very few mild adverse effects were reported.
FR Beyer, HO Dickinson, DJ Nicolson, GA Ford, J Mason. Combined calcium, magnesium and potassium supplementation for the management of primary hypertension in adults. Cochrane Database of Systematic Reviews 2006, Issue 3. Art. No.: CD004805.pub2. DOI: 10.1002/14651858.CD004805.pub2
Reducing salt intake will not only decrease blood pressure, but also has effects on the body’s hormones (renin, aldosterone, catecholamines) and blood fats including cholesterol and triglyceride (lipids). This has led some researchers to suggest that studies lasting many years should be conducted to determine risks and benefits before advising the public to lower salt intake.
This review includes 115 trials, half of which had participants with normal blood pressure. The rest of the trials involved people with high blood pressure. Those with normal blood pressure who went on a low salt diet for up to a month showed a slight decrease in blood pressure. Those with high blood pressure who went on a low salt diet showed reductions of -4 mm Hg in systolic blood pressure and -2 mm Hg in diastolic blood pressure. But these studies most often lasted only 8 to 28 days and it is not known whether these small reductions were maintained after the studies ended. The reviewers concluded that low salt diets lead to small reductions in blood pressure but overall harms or benefits are not known due to the short duration of the studies. The researchers also found that it was very hard for participants to keep to a low salt diet.
Most of the people who took part in the studies were Caucasians, but in the small number of non-Caucasians (mostly African) the blood pressure reduction was, if anything, greater. More research on salt intake is required, particularly in non-Caucasian people.
G Jürgens, NA Graudal. Effects of low sodium diet versus high sodium diet on blood pressure, renin, aldosterone, catecholamines, cholesterols, and triglyceride. Cochrane Database of Systematic Reviews 2004, Issue 1. Art. No.: CD004022.pub2. DOI: 10.1002/14651858.CD004022.pub2.
This review set out to assess the long-term effects of advice to cut down on salt intake. Specifically, the reviewers were looking for a reduction in deaths, cardiovascular disease and blood pressure. Included are 11 trials with a combined total of more than 3500 adult participants who had normal blood pressure, untreated high blood pressure or treated high blood pressure. They were followed up for from 6 months to 7 years. One large, high quality trial included intensive behavioural techniques to encourage participants to reduce their salt intake.
Intensive support and encouragement to reduce salt intake did lead to reduction in salt intake. It also lowered blood pressure but only by a small amount (about 1 mmHg for systolic blood pressure, less for diastolic) after more than a year. However, the reduction in blood pressure appeared larger for the people with higher blood pressure. The trials did not determine whether the reductions in blood pressure had any effect on health or deaths. The reviewers also found that it was very hard for the people in the trials to keep to a low salt diet.
L Hooper, C Bartlett, G Davey Smith, S Ebrahim. Advice to reduce dietary salt for prevention of cardiovascular disease. Cochrane Database of Systematic Reviews 2004, Issue 1. Art. No.: CD003656.pub2. DOI: 10.1002/14651858.CD003656.pub2.
The authors of this review criticised the above review by L Hooper and colleagues because they included studies that lasted only a few days, involved minor reductions in salt intake and, in some cases, used a technique called salt loading. This involves increasing salt intake in participants before taking their blood pressure and then lowering salt intake to determine whether blood pressure went down over the next few days. The authors say that these short-term experiments are not appropriate to inform public health policy which generally aims toward a modest reduction in salt intake over a prolonged period of time.
This review includes trials that lasted 4 or more weeks. All trials tried to determine whether the size of the reduction in salt intake would correlate with the size of the drop in blood pressure. There were 20 trials with 802 participants who had high blood pressure and 11 trials with 2220 participants who had normal blood pressure. The combined results of these trials showed that reductions in salt intake lowered blood pressure both in those with high blood pressure and those with normal blood pressure. For example, those with high blood pressure showed a mean reduction of –5 mm Hg for systolic blood pressure and –3 mm Hg for diastolic blood pressure. And those with normal blood pressure showed reductions of –2 mm Hg and –1 mm Hg, respectively.
The authors of this review say their results “demonstrate that a modest reduction in salt intake for a duration of 4 or more weeks has a significant and, from a population viewpoint, important effect on blood pressure in both individuals with normal and elevated blood pressure.” They note that the current recommendations to reduce salt intake to 5 grams per day (about 2 teaspoons) will lower blood pressure, but a further reduction to 3 grams per day will lower blood pressure more.
Though some of the trials in this review followed participants between 1 and 3 years, the duration for the majority was less than 8 weeks. The trials were not designed to determine whether salt reduction has any health benefits or risks beyond reduction in blood pressure. Yet the authors conclude, “These results support other evidence for a modest and long-term reduction in population salt intake. If this occurred it would result in a lower population blood pressure, and a reduction in strokes, heart attacks and heart failure”.
FJ He, GA MacGregor. Effect of longer-term modest salt reduction on blood pressure. Cochrane Database of Systematic Reviews 2004, Issue 1. Art. No.: CD004937. DOI: 10.1002/14651858.CD004937.
In many countries, there is enthusiasm for healthy heart programmes that use one-on-one counseling or educational methods to encourage people to reduce their risk for developing heart disease. These risk factors include high blood pressure, high cholesterol, excessive salt intake, being overweight, a high-fat diet, smoking, diabetes and a sedentary lifestyle.
A systematic review of all relevant trials found that the programmes do help people make small reductions in blood pressure, cholesterol, salt intake, weight loss, etc. Contrary to expectations, these lifestyle changes had little or no impact on the risk of heart attack or death. The people least likely to benefit from multiple risk factor interventions were those at relatively low risk for heart disease. This review is based on the findings from 39 trials that were conducted in several countries over the course of three decades. The trial participants had been randomly assigned to either an intensive program to reduce their risk factors or to continue seeing their doctors for usual care.
The authors of this review discourage more research on the topic: “Our methods of attempting behaviour change in the general population are very limited. Different approaches to behaviour change are needed and should be tested empirically before being widely promoted. For example, the availability of foods and better access to recreational and sporting facilities may have a greater impact on dietary and exercise patterns respectively, than health professional advice”.
S Ebrahim, A Beswick, M Burke, G Davey Smith. Multiple risk factor interventions for primary prevention of coronary heart disease.Cochrane Database of Systematic Reviews 2006, Issue 4. Art. No.: CD001561.pub2. DOI: 10.1002/14651858.CD001561.pub2.