A major US study - the Diabetes Prevention Trial-Type 1 Diabetes - was carried out to determine exactly this issue in the genetically predisposed relatives of diabetic patients.

In this randomised clinical trial, nearly 85,000 close relatives of type 1 diabetics were assessed for risk. Of those at greatest risk, 339 (median age 11.2 years) were either closely watched or given subcutaneous insulin (total dose, 0.25 units/kg of body weight/day), plus a four-day continuous intravenous insulin infusion once a year, for nearly four years.

In both groups, 41 per cent developed the disease, indicating that just-in-case insulin - at least at these dosages - did not delay or prevent the development of type 1 diabetes (N Engl J Med, 2002; 346: 1685-91).