The study investigates the effects of caffeine on the timing of the onset of hypoglycaemia during moderate exercise in people with type 1 diabetes. Blood glucose is largely dependent on the amount of insulin, the most important hormone for lowering blood glucose. Since people with type 1 diabetes do not produce insulin, it has to be supplemented subcutaneously. Hereby, the duration of action can be well over 24 hours, depending on the preparation used. Since insulin-independent glucose-uptake into muscle cells occurs during exercise, people with type 1 diabetes run the risk of hypoglycaemia during exercise unless they reduce the insulin dose early and/or consume carbohydrates before exercise. Caffeine has multiple physiological effects. Among others, caffeine increases glucose production in the liver and inhibits the uptake of glucose into the muscle. From this and from previous study results, we assume that caffeine before exercise delays the time of hypoglycaemia and can therefore prevent hypoglycaemia. To test this hypothesis, study participants are given a drink containing either caffeine and glucose, glucose only or sweetener only on three different days in random order.  After taking the drink, the changes in blood glucose are monitored during a defined sport session on a ergometer bicycle and the time of occurrence of hypoglycaemia is determined.