Scientists previously believed that people with type 1 diabetes lost all beta cells of the pancreas, which normally produce insulin to regulate sugar levels.
Ultra-sensitive technology has now allowed researchers to detect minute changes in insulin levels after a meal in patients with the disease. It shows they still have some healthy, functioning beta cells remaining.
The study, led by the University of Exeter Medical School and backed by the National Institute for Health Research and Diabetes UK, tested 74 patients with type 1 diabetes for levels of C-peptide, part of the insulin molecule.
The researchers found that 73% were capable of producing very low levels of C-peptide, which changed in response to a meal.
Study lead Dr Richard Oram of the University of Exeter Medical School said: 'It's extremely interesting that low levels of insulin are produced in most people with type 1 diabetes, even if they've had it for 50 years.
'The fact that insulin levels go up after a meal indicates these remaining beta cells can respond to a meal in the normal way - it seems they are either immune to attack, or they are regenerating.'
Matthew Hobbs, head of research for Diabetes UK, said: 'This research shows that some of a person's own beta cells remain and therefore it may be possible to regenerate these cells in the future.'