Fruit juice is broadly good for your health but an Australian study has shown how for many people it can be upsetting their belly.
An Adelaide-based study has identified a high rate of people known as "fructose malabsorbers" - that is their body has difficulty processing the natural sugar contained in the juice.
Professor Ross Butler said these people can experience some unpleasant side-effects if they drink too much a high fructose-content juice, such as apple juice.
"It may be abdominal pain, it may be bloating, it may be flatulence, it may be diarrhoea...in this case you'd think it would be more like diarrhoea symptoms," said Prof Butler, who is chair of paediatric research at the University of South Australia's Sansom Institute.
"What we don't have at the moment is really good way of gauging an individual's threshold for it."
The study, involving the city's Women's & Children's Hospital, took in 1,000 children and adults who had visited their GP to report digestive problems.
It found 80 per cent of the children aged under one tested positive for fructose malabsorption.
Fructose was the cause of intestinal woes for 40 per cent of those aged one to six years.
Prof Butler said the reducing incidence suggested you could grow out of it, but for many the problem was life-long.
Just over 30 per cent of teens and adults in the study were found to also have a problem digesting fructose.
Prof Butler said the strength of the finding indicated that fructose malabsorption could be a major contributor to irritable bowel syndrome, a problem which affects one in five Australians.
It was not yet known if the rising use of fructose as a natural alternative to ordinary sugar in food products could trigger the same digestive problems, he said.
Prof Butler also said the message for parents was not to avoid giving juice to their children, but instead opt for a low fructose content juice if they encountered a problem.
Juice also contains glucose - sugar in a form the body is much more familiar with and much less likely to cause a problems.
"Different fruit juices could resolve the problem and different juices could trigger it," he said.
"Apple juice is something like two to one (fructose to glucose) while grape juice is closer to (being equal).
University of South Australia PhD candidate Hilary Jones will present the research at an expert summit for gastroenterologists in Sydney this week.