Friday April 5 2013
Bananas: one of your 5 a Day and a good source of potassium
‘More bananas and fewer crisps can help ward off strokes’, the Daily Mail reports, saying a study has found that people with high potassium intake have a 24% reduced risk of stroke. Researchers are also reported to say that lowering salt intake could increase benefits further.
Advice to switch from eating crisps to eating bananas is sound, but do we really need to boost our potassium intake?
The headlines stem from a well-conducted systematic review of global evidence on the effects of higher potassium concentration on cardiovascular health in healthy adults.
Good quality evidence suggests that boosting potassium intake to the recommended daily levels is associated with a decrease in blood pressure (by a few mmHg) compared to lower intakes. However, this effect was only found for people with high blood pressure.
Other evidence suggested that higher potassium intake could reduce stroke risk by 24%. However, it’s unwise to draw firm conclusions from these studies about how people’s health is affected by increased potassium intake.
A balanced diet featuring lots of fruit, vegetables and protein should give you all the potassium you need, without the need for supplements. In fact, too much potassium can be harmful, particularly for people with kidney disease or those already on certain blood pressure drugs.
Before you start scoffing bananas or popping potassium pills, it may be wise to talk about your blood pressure with your GP.
Where did the story come from?
The focus of this appraisal is on a study into potassium, carried out by researchers from the World Health Organization’s (WHO) Department of Nutrition for Health and Development, Geneva, Switzerland and other institutions in the UK. Funding was provided by various sources, including WHO funds, the Kidney Evaluation Association Japan, and the governments of Japan and the Republic of Korea. The study was published in the peer-reviewed, British Medical Journal.
The news reports are generally representative of this research.
What kind of research was this?
This was a systematic review that aimed to examine the global literature looking at the effects of potassium intake on health.
The researchers claim that historically, humans tended to have a much higher intake of potassium – above 200mmol/day. Now our intake is much less, due to diets high in processed foods and low in fresh fruit and vegetables, they say, with intake in many countries below the WHO-recommended daily intake of 70 to 80mmol/day.
Because previous studies have linked lower potassium intake to increased risk of high blood pressure and stroke, the researchers consider that increasing potassium intake may help to reduce people’s risk of such chronic conditions.
The researchers say that previous reviews have had inconsistent findings. The WHO initiated the current review to systematically gather the results of studies in healthy adults and children without illnesses that could compromise the body’s potassium balance. The WHO did this to inform future guidelines. The researchers wanted to identify randomised controlled trials (RCTs) looking at:
- how increased potassium intake affected blood pressure, death from any cause and cardiovascular disease in apparently healthy adults
- how increased potassium intake affected blood pressure in apparently healthy children
- how increased potassium intake affected blood lipid (fat) concentrations, kidney function and hormones released from the adrenal glands (such as adrenaline) in apparently healthy adults and children
- what level of potassium intake would result in the maximum benefit for reducing blood pressure, and risk of death and cardiovascular diseases
- whether the effects of increased potassium are affected by factors such as people’s health, diet, or by the type of intervention used to help them increase their potassium intake
If insufficient RCTs were identified, the researchers planned to include less robust study designs, including non-randomised trials and observational studies.
What did the research involve?
The researchers used systematic review methods recommended by the Cochrane Collaboration. They searched numerous electronic databases and manually searched reference lists of studies and reviews. They identified randomised and non-randomised trials which had allocated at least one group of participants to increased potassium intake (intervention) and one group to lower potassium intake (control) for at least four weeks. To be included in the analyses, trials had to have measured potassium from urine samples collected every 24-hours (which can be used to estimate potassium intake). The researchers excluded studies involving:
- acutely ill people
- HIV-positive people
- people admitted to hospital
- people whose urinary potassium excretion was impaired due to a medical condition or drug treatment
Researchers were looking for outcomes related to blood pressure, all-cause mortality, all cardiovascular disease, and specifically stroke and coronary heart disease. They also looked at potential adverse effects of changes in concentrations of blood fat (cholesterol and triglycerides), concentrations of catecholamine (hormones such as adrenaline produced by the adrenal glands at the top of the kidneys) and kidney function. In children, the researchers wanted to find out about blood pressure, blood fats or catecholamine concentrations.
The researchers assessed studies for quality and risk of bias. Where possible, they pooled the results in meta-analyses to estimate the effects of higher potassium intake compared to lower.
What were the basic results?
The researchers identified 37 relevant studies, 35 of which were included in the meta-analysis. Of these, 22 were RCTs of adults, 11 were cohort studies of adults, and one was an RCT of children and one a cohort study of children. Due to the limited search results for children, the researchers broadened their inclusion criteria and identified a further RCT, one non-randomised study, and one additional cohort study in children. The two randomised trials in children included a total of 250 boys and girls aged 13-15 years.
Results for adults
The 22 RCTs in adults included 1,606 participants (individual study size 12 to 353 people) and were conducted across countries worldwide. In 20 studies, participants were given potassium supplements (as the intervention), in one study, participants were given potassium supplements and dietary advice or education, and in two studies the intervention was dietary advice or education alone. The cohort studies in adults included 127,038 people.
The researchers found by pooling the results of the RCTs in adults (after excluding those with outlying results) that increased potassium intake reduced systolic blood pressure (the upper figure) by 3.49mmHg (95% confidence interval (CI) 1.82 to 5.15) and diastolic blood pressure (the lower figure) by 1.96mmHg (95% CI 0.86 to 3.06). However, when they carried out sub-group analyses according to baseline blood pressure, they found that these beneficial effects were seen in the 16 studies including adults with high blood pressure (hypertension) at baseline, but not in the three studies including people with normal blood pressure.
When looking at the specific potassium dose used, they found that greatest effect upon blood pressure was obtained when the potassium intervention was between 90 and 120mmol/day (which reduced systolic blood pressure by 7.16mmHg).
When looking at disease risk, they found that potassium intake had no significant effect on risk of any new cardiovascular disease in general, or of coronary heart disease. However, the pooled results of nine cohort studies found that higher intake significantly decreased risk of stroke by 24% (risk ratio 0.76, 95% CI 0.66 to 0.89).
Increased potassium intake had no significant adverse effects on kidney function, blood fats, or catecholamine concentrations in adults.
Results for children
In children, the three controlled trials and one cohort study found non-significant effects of potassium upon blood pressure.
How did the researchers interpret the results?
The researchers say there is high quality evidence that increased potassium intake reduces blood pressure in people with high blood pressure at baseline, without having detrimental effects upon blood fat concentrations, catecholamine concentrations, or kidney function in adults. Evidence from observational studies suggests higher potassium intake is associated with a 24% lower risk of stroke.
They conclude that increased potassium intake is ‘potentially beneficial’ to most people who have normal kidney function for the prevention and control of high blood pressure and stroke.
This is a well-conducted systematic review, in which researchers scanned the global literature to identify all relevant studies examining the effect of higher potassium concentration in adults and children upon blood pressure and other cardiovascular health outcomes. Previous studies in this area have given inconclusive results.
This review has found evidence that higher potassium intakes are associated with a decrease in blood pressure (on average about 2 to 4mmHg) when taken by people who have high blood pressure. However, it is not certain how beneficial these relatively small changes would be to people, as it is not possible to say if this would have brought the person’s blood pressure to within normal range, or reduced their risk of other adverse health outcomes.
The evidence for a 24% reduction in stroke risk with higher intake comes from nine observational studies, rather than RCTs, and as such this is lower quality evidence. As no significant benefits were found for cardiovascular disease as a whole, or heart disease specifically, it is difficult to firmly conclude what direct impact increased potassium has upon cardiovascular disease risk.
Because of the limited number of studies in children found, this review can’t make any conclusions about the impact of increased potassium intake for children either.
Also, as the researchers importantly note, their results cannot be applied to people with impaired kidney function or who are taking medication that affects their ability to control potassium. Nor should the findings be applied to pregnant or breastfeeding women who have a slightly higher daily potassium requirement. The review is not able to say which specific type of potassium supplement may be beneficial. For example, some studies used potassium bicarbonate, others potassium chloride and others potassium citrate.
This study supports the recommended daily amount of potassium (3,500mg). People should be able to obtain all the daily potassium they need by eating a balanced diet with lots of fruit, vegetables and protein, without the need for supplements. For more information, read about the Eatwell plate.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.