Children who do little exercise risk developing the chronic fatigue syndrome ME in later life, say UK researchers.
Exercising may protect against ME
ME (myalgic encephalomyelitis) is a recognised illness but its cause is not known.
A study by a team at London's Royal Free and University College Medical School found sedentary children were at greatest risk of ME.
The findings, based on a study of more than 16,500 people, are published on the internet at bmj.com.
They researchers followed babies born in April 1970 up to the age of 30.
Overall, 93 reported ever having ME and 48 of the 30-year-olds said they had ME currently.
When the researchers looked at how much exercise the study participants had done as children, they found a striking trend with the chronic fatigue illness.
Those who had been highly active as a child were unlikely to develop ME in later life, while those who went on to develop ME were likely to have done little exercise when they were young.
Lead investigator Russell Viner said: "Contrary to previous suggestions that high levels of exercise increase risk, we found that the most sedentary children were at greatest risk."
Being female and from a higher social class also seemed to increase the chance of the illness.
But there was no association with academic ability, obesity, allergies or birth order.
Similarly, psychological problems in the patient as a child or in their mother, played no role.
This contradicts previous claims that ME may be linked to psychological problems.
Chris Clark from the charity Action for ME, said: "Unfit people of all ages are more likely to become ill with a range of diseases than fit people, and [we] would support general advice to the public that normal sensible exercise is to be encouraged."
He said the study raised very interesting questions about the role of previous medical conditions and the subsequent development of ME.
"This seems potentially important, particularly given the absence of any link with a history of psychological illness," he said.
Dr Nigel Speight, consultant paediatrician at the University Hospital of North Durham, said: "Without knowing more about the individual cases, it's hard to say whether a sedentary childhood is a genuine risk factor or a chance association."
He said it might be that the adults with ME had had mild ME throughout their childhood, which, in turn, might have explained why they were less active.
"It's interesting to note that some of their other findings are against factors which are used to support the psychological view of ME," he said.
ME affects 240,000 people across the UK, of whom an estimated 25,000 are young people and children.
Symptoms include a combination of extreme exhaustion, muscular and joint pain, sensitivity to noise, alcohol, light and changes in temperature, headaches, disturbed sleep patterns, concentration problems, nausea and dizziness, and digestive disturbances.