What is high blood pressure (hypertension)?
Blood pressure is a measure of the force that your heart uses to pump blood around your body. It is usually given as two figures: the pressure when your heart pushes blood out (systolic pressure), and the pressure when your heart rests between beats (diastolic pressure). Blood pressure is considered to be high when systolic pressure is over 140 and/or diastolic pressure is over 90, often written as '140 over 90' and measured in millimetres of mercury (mm Hg). The risk of developing high blood pressure increases as you get older.
High blood pressure is one of the main causes of early death and disability around the world. It can increase people's risk of developing serious long-term health problems, such as heart attack or stroke. Lowering blood pressure in people with hypertension reduces the number of people who develop diseases of the heart and blood vessels (cardiovascular disease), which leads to fewer deaths and cardiovascular problems.
Weight and high blood pressure
High blood pressure is often related to unhealthy lifestyle habits, such as smoking, drinking too much alcohol, being overweight and not exercising enough. All treatment guidelines recommend keeping to a healthy weight and losing weight when needed. Some people choose to lose weight by following a diet, for example, by eating less fat, or by limiting the number of calories they eat.
Why we did this Cochrane Review
We wanted to find out if following a diet to lose weight has long-lasting effects on blood pressure, and whether it could reduce the unwanted effects of high blood pressure on people's health.
What did we do?
We searched for studies that compared the effects of following a weight-loss diet with not following a diet, in people with high blood pressure. We were interested in the effects of the diet on blood pressure and body weight. We also wanted to know how many people experienced any unwanted effects, how many people developed cardiovascular disease, and how many people died.
We looked for randomised controlled studies, in which the treatments people received were decided at random. This type of study usually gives the most reliable evidence about the effects of a treatment.
We assessed the reliability of the evidence we found. We considered factors such as: how the studies were conducted, how many people they involved, and whether their findings were consistent across studies.
Search date: we included evidence published up to April 2020.
What we found
We found eight studies in 2100 people with high blood pressure (average age 45 to 66 years). The studies were conducted in the USA (4 studies) and Europe (4 studies), and lasted 6 months to 36 months.
None of the studies reported useful information about any unwanted effects of following a weight-loss diet.
What are the results of our review?
A weight-loss diet probably enabled people to lose weight (5 studies, 888 people) and may have lowered their blood pressure (3 studies; 731 people), compared with people who did not follow a diet.
We did not find enough evidence about whether following a diet affected the number of people who died or developed cardiovascular disease. Three studies reported that no-one died during the study; only one study looked at how many people developed a cardiovascular disease.
How reliable are these results?
We are moderately confident that people with high blood pressure lose weight after following a weight-loss diet; however, these results might change if more evidence becomes available. We are less confident about whether a weight-loss diet lowers blood pressure, because these results are based on a small number of studies; this result is likely to change if more evidence becomes available.
Although people with high blood pressure lost weight and had lower blood pressure after following a weight-loss diet, compared with people who did not follow the diet, we did not find enough reliable evidence to be certain about this result. We are uncertain whether following a weight-loss diet could reduce cardiovascular disease because we did not find enough studies that looked at this.
In this second update, the conclusions remain unchanged, as we found no new trials. In people with primary hypertension, weight-loss diets reduced body weight and blood pressure, but the magnitude of the effects are uncertain due to the small number of participants and studies included in the analyses. Whether weight loss reduces mortality and morbidity is unknown. No useful information on adverse effects was reported in the relevant trials.
All major guidelines for antihypertensive therapy recommend weight loss. Dietary interventions that aim to reduce body weight might therefore be a useful intervention to reduce blood pressure and adverse cardiovascular events associated with hypertension.
To assess the long-term effects of weight-reducing diets in people with hypertension on all-cause mortality, cardiovascular morbidity, and adverse events (including total serious adverse events, withdrawal due to adverse events, and total non-serious adverse events).
To assess the long-term effects of weight-reducing diets in people with hypertension on change from baseline in systolic blood pressure, change from baseline in diastolic blood pressure, and body weight reduction.
For this updated review, the Cochrane Hypertension Information Specialist searched the following databases for randomised controlled trials up to April 2020: the Cochrane Hypertension Specialised Register, CENTRAL (2020, Issue 3), Ovid MEDLINE, Ovid Embase, and ClinicalTrials.gov. We also contacted authors of relevant papers about further published and unpublished work. The searches had no language restrictions.
We included randomised controlled trials (RCTs) of at least 24 weeks' duration that compared weight-reducing dietary interventions to no dietary intervention in adults with primary hypertension.
Two review authors independently assessed risks of bias and extracted data. Where appropriate and in the absence of significant heterogeneity between studies (P > 0.1), we pooled studies using a fixed-effect meta-analysis. In case of moderate or larger heterogeneity as measured by Higgins I2, we used a random-effects model.
This second review update did not reveal any new trials, so the number of included trials remains the same: eight RCTs involving a total of 2100 participants with high blood pressure and a mean age of 45 to 66 years. Mean treatment duration was 6 to 36 months. We judged the risks of bias as unclear or high for all but two trials. No study included mortality as a predefined outcome. One RCT evaluated the effects of dietary weight loss on a combined endpoint consisting of the necessity of reinstating antihypertensive therapy and severe cardiovascular complications. In this RCT, weight-reducing diet lowered the endpoint compared to no diet: hazard ratio 0.70 (95% confidence interval (CI) 0.57 to 0.87). None of the trials evaluated adverse events as designated in our protocol. The certainty of the evidence was low for a blood pressure reduction in participants assigned to weight-loss diets as compared to controls: systolic blood pressure: mean difference (MD) −4.5 mm Hg (95% CI −7.2 to −1.8 mm Hg) (3 studies, 731 participants), and diastolic blood pressure: MD −3.2 mm Hg (95% CI −4.8 to −1.5 mm Hg) (3 studies, 731 participants). We judged the certainty of the evidence to be high for weight reduction in dietary weight loss groups as compared to controls: MD −4.0 kg (95% CI −4.8 to −3.2) (5 trials, 880 participants). Two trials used withdrawal of antihypertensive medication as their primary outcome. Even though we did not consider this a relevant outcome for our review, the results of these RCTs strengthen the finding of a reduction of blood pressure by dietary weight-loss interventions.