Wednesday May 6 2015
Fruit may not suppress your appetite, but it's nutritious
Are you tempted to avoid eating fruit "because the sugar in it triggers cravings," as the Mail Online suggests today? If you are, it's worth having a look at some of the reassuring facts that can keep you chomping cherries and guzzling grapefruit.
The news – covered by the Mail and the BBC – stems from a laboratory study of just 24 young, healthy volunteers. These volunteers were given cherry-flavoured drinks containing either fructose ("fruit sugar", not real fruit juice) or glucose.
Brains scans showed that those who had a fructose drink had more brain activity when shown pictures of high-calorie food than when they were given glucose. They also said they were hungrier when they saw pictures of the food in question.
The researchers suggest that in real life, people might be more likely to seek out food and eat more after having fructose. But as the study didn't test this directly, we can't conclusively say this is true. Also, the results from this small sample of young people may not be representative of the population at large.
We know that eating or drinking fructose in isolation, as in this study, is very different from eating it as part of whole fruit, where other nutrients and fibre interact and affect how it is digested. It's worth bearing such facts in mind when reading media stories casting doubt on generally healthy foods.
Where did the story come from?
The study was carried out by researchers from the University of Southern California, and was funded by a Doris Duke Charitable Foundation Grant, the American Heart Association, and the Southern California Clinical and Translational Science Institute.
It was published in the peer-reviewed science journal, Proceedings of the National Academies of Science of the USA.
Generally, the media reported the story accurately. The BBC reminded readers that drinking fructose in a drink, as in the experiment, is not the same as eating it from whole fruit.
However, the Mail's suggestion that advice to get your 5 A DAY of fruit and veg should carry "a word of warning – to steer clear of the sugar, fructose" takes the implications of the research too far.
What kind of research was this?
This was a small, double-blind randomised controlled trial (RCT) looking at the effect of the sugars fructose and glucose on hunger and desire for food. The study team say fructose, a fruit sugar, may not suppress appetite as strongly as glucose, a different form of sugar.
The researchers wanted to measure the effect of fructose and glucose on brain, hormone and appetite responses to food cues. They also wanted to see whether it had any effect on "food-seeking behaviour".
An RCT is an effective way of proving cause and effect. However, this particular one didn't involve many people, so may not be applicable to the UK population at large.
What did the research involve?
Researchers recruited 24 healthy volunteers. These volunteers had two separate brain scans to measure their brain activity after drinking a cherry-flavoured drink containing either fructose or glucose.
The brain scans, called functional (f)MRI, happened as participants looked at sets of images of high-calorie foods and non-food items. After each set of images, participants rated their hunger and desire for food.
The volunteers also completed a task where they chose between immediate food rewards and monetary reward in a month's time.
The researchers measured the volunteer's levels of the hormones insulin and glucagon (both involved in food metabolism) before having the sugary drink, and again 30 and 60 minutes after the drink. As a control group, 18 volunteers were also given a cherry drink without fructose or glucose.
The study used a crossover design, meaning a volunteer would be tested and scanned after drinking a glucose drink on one day and a fructose drink on a different day.
The study was double blind, meaning neither the volunteers nor those analysing the brain scan results knew which drink the participants had been given. This helps to reduce the chance of bias and confounding factors influencing the results.
What were the basic results?
Compared with drinking glucose, drinking fructose resulted in smaller increases in blood insulin hormone levels.
It also resulted in greater brain reactivity to food cues in areas of the brain called the visual cortex and left orbital frontal cortex, which is thought to be involved in attention and reward processing.
Fructose also led to greater ratings of hunger and desire for food, and a greater willingness to give up long-term monetary rewards to obtain immediate high-calorie foods, compared with glucose.
How did the researchers interpret the results?
The researchers concluded: "These findings suggest that ingestion of fructose relative to glucose results in greater activation of brain regions involved in attention and reward processing, and may promote feeding behaviour."
Can we conclude from this evidence that fruit makes you hungry, as the Mail Online suggested? No.
This small double-blind RCT showed that young adults who consumed a fructose-sweetened drink had more brain activity in attention and reward centres in response to pictures of food, compared with the same drink sweetened with glucose. There were signs this also affected food-seeking behaviour in a somewhat artificial laboratory test.
The research team took this to mean that the volunteers might be more likely to seek out food and eat more in a real-life scenario. But the study didn't test this directly, so it remains unproven.
Numerous factors influence what and how much you eat in the real world, such as food availability, whether you have company, boredom and the time of day. From this study alone, we can't tell how important fructose is in influencing how much people eat, or what they eat.
The randomisation and double blinding in the study mean that bias and confounding factors are not likely to affect the results. But the study was small, only involving 24 healthy adults aged around 21.
It doesn't tell us much about how fructose affects people's appetite or food-seeking behaviour in other groups – for example, the over-60s or people with chronic diseases.
The best way to find out is to do a larger study using a more diverse group that is more representative of the UK population.
This RCT tested fructose and glucose in isolation in a drink. But some of the media coverage gave this the label "fruit sugar", perhaps giving the impression that fruit should be avoided if you want to reduce your hunger and, as an inevitable result, your calorie intake.
However, sugar in fruit is digested by the body in a different way from fructose in a drink. For example, having a whole apple is better for you than having juice made from the same apple. The whole fruit contains vitamins, minerals and fibre, and allows your body to absorb the sugar slower than in a "free" juice form.
As Priya Tew from the British Dental Association explained to the BBC: "Eating fructose and glucose in isolation is very different to eating them within the context of a food, where we have other nutrients that interact and can affect digestion.
"For example, fructose in fruit is tied up within the cellular structure of that fruit, and the fibre content slows down the release of the fructose into the bloodstream. Fruit also has a high water content and takes a while for us to chew and digest, so the fructose is not instantly released."
Find out about 5 A DAY portion sizes.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.