Fructose, the sugar found in fruit, may increase cravings for high-calorie foods, according to researchers.
In a small experiment, reported in the journal PNAS, 24 volunteers consumed a sugary drink sweetened with fructose on one day and glucose on another day.
Compared with glucose, the fructose drink led to more hunger and desire for treats such as biscuits and sweets.
The findings suggest different sugars may affect us differently. Nutrition experts say more studies are needed.
They say we should all think carefully about how much sugar we eat. But whole fruit is good for us and contains much more than simply fructose.
Fruit contains fibre, vitamins and minerals and is a healthy alternative to foods high in added sugars and fat.
Pure fruit juices contain a lot of sugar so a small 150ml glass per day is ample, says the British Dietetic Association.
Sugar appears in food under different names - maltose, glucose, fructose, sucrose, lactose, dextrose, honey, syrups, treacle, sugar cane and sugar beet.
The World Health Organization says eating a small amount each day - around six teaspoons - is fine.
But most adults and children in the UK eat too much sugar.
The sugary foods we should cut down on, say experts, are sweets, cakes, biscuits, chocolate, and some fizzy drinks and juice drinks.
In an effort to learn more about different sugars and how they might affect us, researchers at the University of Southern California in the US conducted an experiment with volunteers.
The 24 participants were given a sweetened, cherry-flavoured drink but were not told what was in it - fructose or glucose.
Shortly after, they were asked to rate how hungry they were.
And they underwent brain scans while being shown pictures of tempting food - biscuits, sweets, burgers and pizza - as well as some "neutral" photographs of buildings.
Some days later, the same volunteers came in again and repeated the experiment. The only difference was the type of sweetener added to their cherry drink.
The brain scans showed that people responded more strongly to photos of food if they had been drinking fructose, rather than glucose.
People also reported more food cravings for treats shortly after consuming fructose.
Although fructose and glucose contain the same energy or calories, the body breaks these sugars down in different ways.
The researchers believe this might explain their findings.
Fructose used in sweetened foods has received lots of bad press in recent years, with many blaming it for rising rates of obesity.
But evidence for this is lacking.
Others point out that since fructose is sweeter than table sugar, less is needed to achieve the same sweetness, offering calorie savings.
Priya Tew from the BDA explained that the picture was not clear-cut: "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."
Expert scientific advisers to the English government have been reviewing all the available evidence on sugars and health and will soon issue updated advice.
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