Monday October 13 2014
Even unsweetened fruit juice can be high in sugar
"Does drinking fruit juice give you high blood pressure?," the Mail Online asks, as an Australian study found people who reported a daily intake of fruit juice tended to have slightly higher blood pressure. This finding, the researchers argue, is likely down to the high sugar content of fruit juices.
But this and other headlines exaggerated the results of a small, potentially unreliable and unrepresentative study.
In fact, the study showed there was a link between daily fruit juice consumption and central blood pressure in a group of 130 largely healthy 50 to 70 year olds.
People who drank juice daily had a systolic pressure (the upper figure in a blood pressure reading) 3 to 4mmHg higher than those who drank juice rarely or occasionally. But there was no link when measuring blood pressure in the arm using standard measures.
The media reports focus on the possibility that the slight raise in blood pressure could increase a person's risk of a variety of blood pressure-related diseases, such as a heart attack. But it's unclear whether this small difference would have a meaningful impact on health.
As both juice consumption and blood pressure were assessed at the same time, the study does not prove that fruit juice caused the raised blood pressure. There could be other dietary or physical activity factors accounting for this link, or there could be reverse causation (people drank fruit juice because they were worried about their blood pressure).
Overall, this study alone does not provide evidence that fruit juice increases blood pressure or, by proxy, raises the risk of heart attack or angina.
Nonetheless, it is important to note that fruit juice is high in sugar, so it's recommended you drink no more than 150ml a day for the sake of your teeth – and to help keep your calories down.
Where did the story come from?
The study was carried out by researchers from Swinburne University of Technology and Monash University Australia, and was funded by Swisse Wellness, a company that sells vitamins, supplements, "superfoods" and skincare products, but – notably – not fruit juices.
It was published in the peer-reviewed medical journal Appetite.
The UK media usefully noted the large amount of sugar present in fruit juice, but the Daily Express' statement that fruit juice is a "health danger" that is "putting millions at risk" – while attention-grabbing – is highly subjective. It also doesn't reflect the results of the study, which does not prove fruit juice is a danger.
What kind of research was this?
This was a cross-sectional study looking at the link between regular fruit juice consumption and blood pressure.
The authors outline that, "Despite a common perception that fruit juice is healthy, fruit juice contains high amounts of naturally occurring sugar without the fibre content of the whole fruit."
They say, therefore, that regular fruit juice consumption, like soft drink consumption, represents a source of excess sugar in the modern diet.
Excess sugar intake, the team say, is linked to higher blood pressure, obesity and being overweight, and raises the risk of developing a number of associated diseases, such as cardiovascular disease and stroke.
The researchers point to previous research suggesting a link between higher sugar consumption and higher blood pressure, but wanted to investigate the specific role of fruit juice in this relationship.
As this was a cross-sectional study, it cannot prove fruit juice causes higher blood pressure. There may be other explanations and factors at play, such as dietary, physical activity or other lifestyle habits.
It also cannot exclude the possibility of reverse causality, where people with high blood pressure may be consuming more fruit juice because of health concerns, rather than vice versa.
Ultimately, a randomised control trial would be needed to prove that fruit juice – or any other food or drink item representing a source of excess sugar – causes sustained blood pressure increases.
What did the research involve?
The researchers asked a group of 160 adults (aged 50 to 70) about their food and drink habits over the past year. On the same day, the team took their blood pressure using two different measures.
One was the standard blood pressure measurement of the upper arm using a blood pressure cuff (brachial blood pressure), and one estimated pressure in the main blood vessel taking blood from the heart, called central, or aortic, blood pressure.
To do this, a probe was used to measure the waveform of blood in the wrist artery. This information was fed into special software that estimated central pressure.
The main analysis looked for links between fruit juice consumption categories and the one-off blood pressure readings at one or both sites.
Fruit juice consumption was categorised as follows:
- rare – combined those who never drank juice with those consuming no more than three times a month
- occasional – those consuming juice once a week up to five to six times a week
- daily – once or more a day
The researchers used two measures of blood pressure (central and brachial), as there is debate about which is best to use in terms of predicting future disease risk.
To be included in the study, people had to be free of major neurological and psychiatric illness, cardiovascular disease, currently be a non-smoker, and have no history of alcohol or drug abuse.
The main analysis was adjusted to reduce the influences of the following confounders:
- mean arterial pressure
- heart rate
- cholesterol and blood pressure treatments
What were the basic results?
There were no differences found between the standard blood pressure measure in the arm and the different juice groups, but differences were found for central blood pressure.
Those who consumed fruit juice daily, versus rarely or occasionally, had significantly higher central systolic blood pressure (the upper of the two-figure blood pressure measurement – in a measure of 140/80, 140 is the systolic blood pressure).
They also had higher readings for central pulse pressure and other measures looking at the heart rate and blood pressure wave forms (central augmentation pressure, central augmentation index and lower pulse pressure amplification).
Central systolic blood pressure was 3 to 4mmHg higher for those who consumed fruit juice daily rather than rarely or occasionally.
How did the researchers interpret the results?
The researchers concluded that, "more frequent fruit juice consumption was associated with higher central BPs [blood pressures]".
This cross-sectional study found a link between regular fruit juice consumption and slightly increased central blood pressure in a group of 130 largely healthy 50 to 70 year olds. People who drank juice daily had a systolic pressure (the upper figure) 3 to 4mmHg higher than those who drank juice rarely or occasionally.
However, when measuring the blood pressure in the standard way, using an inflatable cuff around the arm, there was no link.
The media reports focus on the possibility that the slight raise in blood pressure could increase a person's risk of a variety of blood pressure-related complications. But it is unclear whether the small difference in systolic pressure would have had clinically significant meaning for the individual.
Similarly, it is not clear why only one of the blood pressure measures was affected and not both, if indeed there was a real link between fruit juice and blood pressure.
Also, the link was only found with systolic blood pressure (arterial pressure when the heart contracts) and not for diastolic (arterial pressure when the heart relaxes), when both figures are equally relevant in terms of the clinical significance of raised blood pressure.
This might be caused by some of the additional limitations in the study, which all introduced error and uncertainty into the findings. These limitations include:
- relying on people's ability to accurately recall their food and drink habits over the last year, which may be inaccurate
- being able to estimate central blood pressure accurately
- only measuring blood pressure once, which is less reliable than measuring it many times on different days to get an average reading
- all participants were 50 to 70 years old, and the effects in other age groups were not tested
- it was not clear how long established the juice drinking habits were – we only know about consumption in the previous year
Furthermore, the analysis made no adjustment for other sources of sugar in the diet. Given that fruit juice was being investigated because it represented a source of additional sugar in the diet, this is an important omission.
Without knowing about other sources of sugar, it is difficult to tell how important fruit juice was in the bigger picture, or how much of a person's overall sugar intake was from juice. The role of other sugary drinks or foods is likely to be very important, but was not accounted for in the analysis.
The lack of information on other dietary and physical activity patterns also makes it difficult to exclude the possibility of reverse causality. Based on this cross-sectional analysis, it could equally be possible that people with raised blood pressure could be drinking more fruit juice in addition to other healthy lifestyle changes, rather than that fruit juice is causing the high blood pressure.
So, on its own, this study does not justify a change in fruit juice drinking habits relating to blood pressure, as the risk is unproven. A more robust study design would be needed to properly prove whether this was the case.
However, it serves to remind us that fruit juice contains a lot of sugar, something many people may not be fully aware of. Some juice drinks can contain as much sugar, and sometimes more, than a can of coke.
Any high-sugar food or drink should be consumed in moderation as part of a wide and varied diet, rich in unprocessed fruit and vegetables, and generally low in sugar.
Being aware of the sugar content of food is one of many simple ways to maintain a healthy weight and minimise your risk of developing weight-related diseases now or in the future.
Find out more about how sources of added sugar can sneak into your diet.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.